Mesenchymal Tumors
INTRODUCTION
Mesenchymal tumors of the gastrointestinal (GI) tract can be classified as shown in Table 7-1. Some are unique to the GI tract (e.g., gastrointestinal stromal tumor [GIST], inflammatory fibroid polyp [IFP], etc.), but most have identical counterparts in soft tissues. GIST, the most common of the GI mesenchymal tumors, has been in the spotlight since the “KIT revolution” provided a firm framework for its diagnosis and treatment. Advances in diagnostic, molecular, prognostic, and therapeutic aspects of these tumors have been exponential. Meanwhile, the list of other mesenchymal tumors involving the GI tract continues to expand.
The increase in screening colonoscopy has seen the emergence of several new mesenchymal polypoid lesions including benign fibroblastic polyp, perineurioma, benign epithelioid peripheral nerve sheath tumor, and Schwann cell “hamartoma.” In addition, reactive nodular fibrous proliferation and heterotopic mesenteric ossification have emerged as new reactive entities.
Table 7-1 Classification of Gastrointestinal Mesenchymal Tumors | |
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A number of rare mesenchymal tumors are starting to be seen as GI primaries (e.g., PEComa, clear cell sarcoma, and calcifying fibrous tumor) sometimes mimicking GISTs. Molecular testing has revealed some “CD117-negative GISTs” to be synovial sarcomas and some “metastatic melanomas” to be clear cell sarcomas. The ever-changing landscape of mesenchymal tumors and the rarity of most lesions pose significant challenges to surgical pathologist. This chapter aims to provide a current view of mesenchymal tumors, offering practical approaches to diagnostic challenges and highlighting their clinical implications.
GASTROINTESTINAL STROMAL TUMORS
GISTs have evolved from poorly defined, histogenetically obscure GI mesenchymal tumors to well-defined oncogenic entities with distinctive clinical, morphologic, ultrastructural, histiogenetic, and molecular features, for which targeted therapy is available.
Histogenesis
Many tumors currently defined as GISTs were, in the past, thought to be smooth muscle or neural tumors due to their morphologic resemblance to these lesions.1, 2, 3, 4 When immunohistochemistry became available, substantial proportions were found to have neither typical smooth muscle nor neural differentiation. The noncommittal term “GI stromal tumor” to accommodate this group of tumors was coined.5 The major conceptual breakthrough came in 1998 with the discovery of activating mutations in the receptor tyrosine kinase c-kit gene6 and overexpression of its product CD1177, 8 in the vast majority of these tumors. GISTs are currently believed to arise from or differentiate along the lines of interstitial cells of Cajal (ICC) (the “pacemaker” cells of the GI tract), which they resemble at a morphologic, ultrastructural, and immunohistochemical (CD117+, DOG1+) level.7, 8
Demography and Clinical Aspects
Although rare, GISTs are the most common mesenchymal tumor of the GI tract. Population-based studies from the United States, Sweden, Iceland, the Netherlands, and Spain indicate that GIST has an annual incidence in the range of 6.5-14.5/million.9, 10, 11, 12, 13 Up to 4,500 to 6,000 new cases are diagnosed per year in the United States.4 GISTs occur mainly in adults with a median age of 55 to 60 years (over 90% occur after 40 years). The age-adjusted incidence for blacks is almost twice that of whites. GISTs show a slight male predominance in adults, while in the pediatric population females are more frequently affected.12, 14 GISTs arise most commonly in the stomach (60%) and jejunum and ileum (30%) and less frequently in the duodenum (5%), colorectum (<5%), and esophagus (<1%). Rare sites include the omentum, mesentery, and retroperitoneum.15 Case reports of GIST involving the gallbladder, liver, pancreas, and urinary bladder are on record.16, 17, 18, 19
GI bleeding (acute or insidious) is the most common presentation but GISTs may also present with bloating, early satiety, abdominal pain, palpable mass, obstructive symptoms, tumor rupture (rare), or incidentally at endoscopy or surgery. Most symptomatic patients present with GISTs that are larger than 5 cm in maximum dimension.
Approximately, 20% to 25% of gastric GISTs and 40% to 50% of small intestinal GISTs behave in a clinically malignant fashion. Metastases usually involve the abdominal cavity and liver. Rarely, the musculoskeletal system or skin may be involved, while metastasis to lymph nodes or lungs is extremely rare. Since metastasis can occur 10-15 years after initial presentation, long-term follow-up is required.
Complete surgical excision is the treatment of choice, but patients with unresectable or metastatic disease are treated with c-kit/PDGFRA tyrosine kinase inhibitors such as imatinib. Most patients show a partial or complete remission, but the development of resistance (due to secondary mutations or clonal selection) has limited the long-term efficacy of this therapy. In such cases, second-line tyrosine kinase inhibitors such as sunatinib may be used.15, 20
Diagnostic Procedures
The majority of GISTs are diagnosed only after pathologic examination of the resected specimen. However, most gastric, duodenal, and colonic GISTs undergo endosocopy and barium studies as initial investigations.21, 22 Endoscopically, GISTs may vary from dome-shaped, submucosal tumors to large, fungating or annular tumors. Some show one or more deep central ulcers, which may be the site of hemorrhage23 (Fig. 7-1). Since the overlying mucosa is intact in most cases, endoscopy is usually nondiagnostic. Ulcerated lesions may have a higher diagnostic yield. In the latter, biopsies should be taken from the margins of the ulcer and from the surface of depressed lesions.22 Exuberant fibroblastic and granulation tissue may be sampled and should not be misinterpreted as a stromal tumor (occasional CD34 and CD117 immunore-activity presents a potential pitfall). All endoscopic lesions should be biopsied since carcinoid tumors, and even carcinomas, may endoscopically resemble GIST if they have a prominent submucosal component. Endoscopic ultrasound (EUS) is emerging as a useful diagnostic modality.21, 22
Mesenchymal tumors display similar echogenicity to the surrounding muscle layer, which helps to narrow the differential diagnosis. EUS features of GIST including size >4 cm, irregular extraluminal borders, echogenic foci >3 mm, and cystic spaces >4 mm were identified as factors that correlated with its malignant behavior.24 EUS-guided fine-needle aspiration biopsy is a safe and reliable method for obtaining tissue for cytologic assessment (immunostains can be performed on cytologic material or cell blocks). However, this procedure is currently available only in a few specialized centers and is not practical in most clinical settings. Also, GIST lesions can be highly vascularized and this may present an unacceptable risk for biopsy.
Preoperative computed tomography (CT) scan is performed to assess the resectability of large tumors.
Tumor size, adjacent organ involvement, metastatic spread to the liver omentum or peritoneum, and ascites are also determined. Contrast CT shows peripheral enhancement in most GISTs, often with central areas of low attenuation representing hemorrhage, necrosis, or cystic degeneration.21 Magnetic resonance imaging (MRI) is inferior to CT for visualizing gastric GIST but is useful in the assessment of hepatic metastases. If available, fluorine-18 deoxy-glucose positron emission tomography (18FDG-PET), in combination with CT scan, may help to delineate lesions more precisely, especially in earlier stage disease and also smaller lesions (<1 cm in size). GIST lesions are quite FDG-avid, and this type of functional imaging is currently used to monitor response to therapy in these patients. Percutaneous or laparoscopic biopsies of localized tumors should be avoided due to the risk of tumor spillage and dissemination.22 However, metastatic tumors are frequently subjected to this procedure.
Tumor size, adjacent organ involvement, metastatic spread to the liver omentum or peritoneum, and ascites are also determined. Contrast CT shows peripheral enhancement in most GISTs, often with central areas of low attenuation representing hemorrhage, necrosis, or cystic degeneration.21 Magnetic resonance imaging (MRI) is inferior to CT for visualizing gastric GIST but is useful in the assessment of hepatic metastases. If available, fluorine-18 deoxy-glucose positron emission tomography (18FDG-PET), in combination with CT scan, may help to delineate lesions more precisely, especially in earlier stage disease and also smaller lesions (<1 cm in size). GIST lesions are quite FDG-avid, and this type of functional imaging is currently used to monitor response to therapy in these patients. Percutaneous or laparoscopic biopsies of localized tumors should be avoided due to the risk of tumor spillage and dissemination.22 However, metastatic tumors are frequently subjected to this procedure.
 Figure 7-1. Gastric GIST. A: Endoscopic appearance of the GIST (foreground) largely covered by intact mucosa but showing a central ulcer from which it may bleed. A normal pylorus is in the background. B: Superficial ulcerated part of the stromal tumor (left) with its cut surface (right). C: Whole-mount section of a gastric GIST showing the central ulcer. Tumor in the ulcer base is probably the most accessible to biopsy for the bold. The margin of the ulcer may also yield diagnostic material. D: In GIST with an intact mucosa, deep biopsies can occasionally demonstrate these tumors especially if they infiltrate the lamina propria (but in many cases the biopsy is negative unless other techniques [needle/core biopsies] are used). |
Targeted Therapy with Imatinib
With the advent of molecularly targeted therapies and detailed identification of molecular signatures in GIST, a group of investigators from academia and industry developed and characterized signal transduction inhibitor (STI-571) or imatinib mesylate as a potent tyrosine kinase inhibitor of both c-kit and PDGF receptor (PDGFR).24a Since then, there has been an extraordinary expansion in the clinical development of imatinib and similar class of inhibitors for molecularly targeted therapy of GIST.
Imatinib significantly improves clinical outcomes in patients with metastatic disease and in the postsurgical adjuvant setting. In addition, imatinib is often used in the preoperative setting to reduce tumor bulk to improve resectability and/or to avoid a more extensive surgical procedure with associated morbidity.24b KIT and PDGFRA mutational status is helpful in predicting response to imatinib and determining
the optimal dose of therapy (see Molecular Features). Primary resistance to imatinib is observed in about 15% GIST patients, but more than 80% will ultimately develop secondary resistance driven by additional KIT mutations.24c Sunitinib maleate has been used as a second-line agent in imatinib-resistant patients. Sunitinib has also been used as first-line treatment in GISTs that are wild type for KIT and PDGFRA or that have mutations that are known to confer imatinib resistance. Although imatinib is quite well tolerated, long-term intake of sunitinib is complicated by significant adverse effects (fatigue, diarrhea, hand-foot syndrome, hypertension, and myelosuppression) in nearly 20% of patients.24d Several other drugs are being tested for patients progressing on imatinib and sunitinib (e.g., regorafenib, sorafenib, dasatinib, nilotinib),24c, 24d with regorafenib showing significant survival benefit in a randomized, placebo-controlled phase 3 trial.24c
the optimal dose of therapy (see Molecular Features). Primary resistance to imatinib is observed in about 15% GIST patients, but more than 80% will ultimately develop secondary resistance driven by additional KIT mutations.24c Sunitinib maleate has been used as a second-line agent in imatinib-resistant patients. Sunitinib has also been used as first-line treatment in GISTs that are wild type for KIT and PDGFRA or that have mutations that are known to confer imatinib resistance. Although imatinib is quite well tolerated, long-term intake of sunitinib is complicated by significant adverse effects (fatigue, diarrhea, hand-foot syndrome, hypertension, and myelosuppression) in nearly 20% of patients.24d Several other drugs are being tested for patients progressing on imatinib and sunitinib (e.g., regorafenib, sorafenib, dasatinib, nilotinib),24c, 24d with regorafenib showing significant survival benefit in a randomized, placebo-controlled phase 3 trial.24c
Surgery for GIST
Surgery remains the primary mode of therapy for GISTs. Its goal is complete resection with negative gross and microscopic margins avoiding breach of the pseudocapsule/serosa.20 Surgical specimens include gastric wedge resections, segmental resections of small or large bowel, or en-bloc resections. The latter procedure is performed when tumor is adherent to surrounding organs, tissues, or both to avoid disruption of the tumor with associated intraabdominal spillage and peritoneal seeding. Laparoscopic surgery for tumors <5 cm appears to be safe in skilled hands if care is taken to avoid tumor spillage. Enucleation surgery is not recommended. Total gastrectomy and formal lymph node dissections offer no apparent survival benefit and are generally not performed.21, 22 Should small polypoid GISTs ever be removed by endoscopic polypectomy, further surgical resection of the appropriate area is required as it is impossible to include the muscularis propria and associated myenteric plexus, from where these tumors arise, in the polypectomy short of perforating the bowel.
Frozen Section Diagnosis
Frozen sections are usually not requested for suspected GISTs that are resectable. However, when the radiology is inconclusive or the disease advanced, the surgeon may want to know the tumor type intraoperatively. Frozen sections are also frequently requested on small, serosal GISTs incidentally found during surgery for other reasons. The diagnosis of “spindle cell tumor” can usually be made and is not a major diagnostic problem. On frozen sections, it is sometimes very difficult to distinguish sheets of adenocarcinoma cells from those of epithelioid stromal tumors, large noncleaved lymphomas, and even carcinoid tumors. Tumors should not be shelled out because of the risk of local recurrence. The differential diagnosis of tumors that have an organoid pattern and form definite nests excludes lymphomas and most carcinomas. It includes epithelioid GISTs, glomus tumors, carcinoids, paragangliomas, ectopic pancreas, rarely metastases, and, in theory, pyloric gland tumors, although the last are unlikely to cause problems. Fortunately, all of these tumors require primary resection. However, gastric carcinomas will require total or partial gastrectomy with regional lymph node dissection compared to the more conservative surgery performed on GIST (see above), while in lymphoma sampling of periaortic lymph nodes may be performed.
Gross Examination and Appearances
Gross examination should include careful inspection of the serosa for peritoneal breach. The mucosal surface should be examined for ulceration or tumor involvement. The maximum tumor dimension should be accurately recorded as this is a critical prognostic determinant. The serosa and resection margins should be inked and adequately sampled. Inspection of the cut surface will show most GISTs to be centered in the muscularis propria although they may extend inward toward the submucosa, outward toward the serosa, or both (Fig. 7-2). A few have a characteristic dumbbell shape with tumor on either side of the muscularis propria. Some project into the lumen while others hang off the serosal surface. GISTs occasionally appear as omental or mesenteric nodules. GISTs vary in size from <1 cm to over 35 cm, with a median size of approximately 5 cm.
Small incidental GISTs have a predilection for the proximal stomach and gastroesophageal junction. Most GISTs are well circumscribed and nonencapsulated (Fig. 7-2) but some compress surrounding muscle and connective tissue creating the impression of a capsule. GISTs generally lack the whorled appearance of leiomyomas and have a more homogenous, often lobu-lated, cut surface (Fig. 7-2B). They vary from tan to red to brown. Cystic degeneration and hemorrhage are frequent (particularly in large tumors, Fig. 7-2D-G), and necrosis may be present. Degenerative features do not indicate malignancy but are seen primarily in larger tumors, which are more likely to behave aggressively.
Most GISTs are solitary, but multifocal GISTs may be seen in association with tumor syndromes (see GIST syndromes) or sporadically.2, 3, 4, 15, 23 At the time of gross examination of a suspected GIST, it is useful to freeze some fresh tissue in case PDGFRA/KIT mutational studies or molecular studies (to rule out certain GIST mimics—see later) are needed (see later); however, paraffin sections suffice in most cases.
Microscopic Appearances
Histologically, GISTs may display spindle (70%), epithelioid (20%), mixed spindle and epithelioid (10%), and rarely pleomorphic morphology.1, 2, 3, 4, 25, 26, 27 Detail of each morphologic subtype is provided in Figures 7-3, 7-4 and 7-5.
Epithelioid morphology is seen in 40% of gastric GISTs but is rare in small intestinal GISTs (5%). Most GISTs are cytologically bland although focal cytologic atypia is not uncommon. Diffuse pleomorphism is distinctly unusual and should prompt consideration of alternative diagnoses (e.g., sarcoma, melanoma, or sarcomatoid carcinoma). Examples of pleomorphic GIST are shown in Figure 7-5. The Armed Forces Institute of Pathology (AFIP) series of 1,765 gastric GISTs, the largest single site series to date, described eight distinctive histologic variants (four spindled and four epithelioid) with consistent and reproducible morphology3 (Fig. 7-6). Small intestinal GISTs could not be similarly subclassified.2
Epithelioid morphology is seen in 40% of gastric GISTs but is rare in small intestinal GISTs (5%). Most GISTs are cytologically bland although focal cytologic atypia is not uncommon. Diffuse pleomorphism is distinctly unusual and should prompt consideration of alternative diagnoses (e.g., sarcoma, melanoma, or sarcomatoid carcinoma). Examples of pleomorphic GIST are shown in Figure 7-5. The Armed Forces Institute of Pathology (AFIP) series of 1,765 gastric GISTs, the largest single site series to date, described eight distinctive histologic variants (four spindled and four epithelioid) with consistent and reproducible morphology3 (Fig. 7-6). Small intestinal GISTs could not be similarly subclassified.2
 Figure 7-2. Gross appearances of GIST. A: Polypoid small intestinal GIST with central ulceration. B: Lobu-lated, homogenous cut surface of a small intestinal GIST. C: Small subserosal GIST of the small bowel. D: Foci of hemorrhage in a polypoid small bowel tumor. E: Gastric epithelioid GIST with diffuse hemorrhage. F: Focal areas of hemorrhage and necrosis in a jejunal stromal tumor. G: Cavitation in a small bowel stromal tumor with extension well beyond the muscularis propria. |
 Figure 7-3. Spindle cell GIST. A: GISTs are generally well circumscribed and mostly centered in the muscularis propria. Cellularity varies from moderate (B), to marked (C), to low (D). The tumor cells are often arranged in fascicles or bundles (E,F) separated by delicate fibrous septa (F) or a collagenous (G) to myxoid stroma. Nuclear pallisading can be striking (H,I)—such prominent pallisading is rarely seen in gastrointestinal schwannomas and when present favors a GIST. J,K: Cytoplasmic vacuolation is a frequent finding. L: Distinctive extracellular aggregates of hyaline collagen, named skenoid fibers due to their ultrastructural lamellar concentric appearance, are seen almost exclusively in small intestinal GISTs and appear to be a favorable prognostic feature. They may also occasionally be seen in other tumors such as fibromatosis and inflammatory myofibroblastic tumors (see Fig. 7-22). M,N: The tumor cells have uniform elongated nuclei with blunt (M) or pointed (N) ends, delicate chromatin and inconspicuous nucleoli. The cytoplasm is eosinophilic, fibrillary, and generally paler than that of smooth muscle cells. The mitotic rate is variable but is low most tumors. O: Focal nuclear pleomorphism is not uncommon. P: Some cases show prominent dystrophic calcification. Q: Blood vessels can be prominent, resembling those of solitary fibrous tumor, and often hyalinized. R: Infiltration of surrounding tissues or lamina propria infiltration may indicate aggressive behavior. |
 Figure 7-4. Epithelioid GIST. Most have nested (A) or diffuse (B) arrangements of closely packed, round to polygonal cells with abundant eosinophilic (C) or clear (D) cytoplasm and round central nuclei. Cell borders may be prominent (D). Some are diffusely pleomorphic and dyshesive (E). Floret-type multinucleated giant cells (F) and focal nuclear pleomorphism (G) are frequently encountered in gastric epithelioid GIST and have no prognostic significance. |
Unusual morphologic variants. GISTs may rarely display unusual features that can lead to diagnostic confusion (Fig. 7-7). GISTs may rarely show epithelioid differentiation to the point of gland formation,28 prominent signet ring cell morphology (often merging with areas of more conventional GIST),29 rhabdoid morphology including paranuclear whorls of vimentin filaments and often PDGFRA mutations30
and rhabdomyomatous morphology.17 Other variants include paraganglioma-like GISTs,3 GISTs with prominent osetoclastic-like giant cells,31 mesothelioma-like GIST (with epithelioid cord-like areas and pseudoglandular formations in a myxoid stroma), oncocytic variants (with abundant mitochondria), small cell variants (with crowded angulated nuclei), and cytotoxic T-lymphocyte-rich GIST (infiltrated by cytotoxic T cells).
and rhabdomyomatous morphology.17 Other variants include paraganglioma-like GISTs,3 GISTs with prominent osetoclastic-like giant cells,31 mesothelioma-like GIST (with epithelioid cord-like areas and pseudoglandular formations in a myxoid stroma), oncocytic variants (with abundant mitochondria), small cell variants (with crowded angulated nuclei), and cytotoxic T-lymphocyte-rich GIST (infiltrated by cytotoxic T cells).
 Figure 7-5. Pleomorphic GIST. Diffuse pleomorphism is uncommon in GIST. A-D: This pleomorphic, CD117-negative, gastric GIST could be mistaken for a storiform malignant fibrous histiocytoma/pleomorphic undifferentiated sarcoma. The demonstration of an exon 18 PDGFRA mutation (D809V) allowed for the correct diagnosis. E,F: A second sarcomatoid CD117-negative GIST associated with a PDGFRA mutation. |
 Figure 7-6. Morphologic spectrum of gastric GIST. In the AFIP series of 1765 gastric GISTs, 70% could be classified into one of the eight histologic subtypes (four spindle and four epithelioid), the remainder contained mixtures. The subtypes were as follows: A,B: Sclerosing spindle cell GIST, which is paucicellular with abundant extracellular collagen and shows no nuclear atypia and low mitotic activity. C,D: Pallisading vacuolated spindle cell GIST, which is more cellular with plump, often vacuolated spindle cells with nuclear pallisading, limited atypia, and low mitotic activity. E,F: Hypercellular spindle cell GIST with densely packed spindle cells in sheets with limited atypia, pallisading, perinuclear vacuolation, and a mitotic rate rarely exceeding 15/50 HPF. G,H: Sarcomatous spindle cell GIST with spindled to oval cells showing diffuse atypia often in bundles separated by a myxoid stroma and with a mitotic rate of >20/50 HPF. I,J: Sclerosing epithelioid GIST with a syncytial pattern composed of cohesive, uniform cells with indistinct cell borders, a low mitotic rate, and a collagenous stroma. K,L: Epithelioid GIST with a dyscohesive pattern featuring large polygonal cells with abundant cytoplasm, distinct borders, scant stroma, occasional focal atypia, and a low mitotic rate. M,N: Hypercellular epithelioid GIST with diffuse sheets of small epithelioid cells, with a higher nuclear to cytoplasmic ratio than the above, and a mitotic rate, which rarely exceeds 10/50 HPF. O: Sarcomatous epithelioid GIST with back-to-back cells, a high nuclear to cytoplasmic ratio, moderate diffuse atypia, prominent nucleoli, and a high mitotic rate. |
 Figure 7-7. Unusual morphologic variants of GIST. A: Gastric GIST with epithelioid differentiation to the point of gland formation. Elsewhere this tumor showed more conventional spindled and epithelioid morphology. All components were immunoreactive for CD117. B: GIST with signet ring cell morphology due to prominent cytoplasmic vacuolation. C: Epithelioid GIST with rhabdoid morphology associated with a D842V exon 18 PDGFRA mutation. D-F: GIST with osteoclast-like giant cells. The latter were immunoreactive for CD68 (F) and negative for CD117 indicating their reactive nature. G: GIST with a prominent osteochondromatous matrix associated with a novel exon 14 PDGFRA mutation (K646E). H,I: GIST with prominent osteoid production with areas of mineralization. |
Posttreatment changes. Histologic changes described postimatinib include those associated with tumor cell necrosis as well as phenotypic changes in residual tumor cells. Most GISTs show some histopathologic response to imatinib, but this is usually incomplete and does not correlate with clinical or radiologic response. Even tumors with a very good histologic response (i.e., >90% necrosis or hyalinization or both) show focal residual viable tumor in most cases if carefully looked for. Immunohistochemistry for CD117 or DOG1 may help to highlight such foci, although CD117 immunoreactivity, in particular, may be lost. DOG1 immunoreactivity tends to be better retained.32
The histologic response often varies between and within individual nodules. Some tumors have a distinctive myxohyaline stroma, while others show extensive cystic change or hemorrhage. A small minority of tumors may show phenotypic alterations in residual tumor including switch from a spindled to epithelioid phenotype, small cell phenotype, acquisition of myoid phenotype (including desmin expression and ultrastructural features), and loss of CD117 and CD34 immunoreactivity.
Diagnostic problems may arise when secondary tumors with unusual morphology and unexpected immunohistochemical profiles arise at new locations. Mutational analysis may be extremely helpful in such circumstances.33, 34, 35 The pathology report should include an estimate of the proportion of necrotic/hyalinized tumor as an indication of pathologic response. A grading system for pathologic response has been proposed, but this is not widely used outside the research setting.36
Immunohistochemistry
Approximately, 95% of GISTs are immunoreactive for CD117. Staining is usually strong and diffuse, most often cytoplasmic but can also be membranous or may involve the paranuclear “Golgi zone” with dot-like immunoreactivity (Fig. 7-8A-D). Membrane staining is more often appreciated in epithelioid GISTs, while staining in spindle cell GISTs is typically pancytoplasmic; membrane staining may difficult to appreciate in spindled cells due to their narrower width. Epithelioid GISTs often show less uniform staining and are sometimes only weakly positive or negative (Fig. 7-8E).2, 3
DOG1 (discovered on GIST 1) is a highly sensitive GIST marker, staining approximately 95% of these tumors. Originally identified in gene expression profiling studies, DOG1 is a calcium-activated chloride channel protein, also known as anoctamin 1 or TMEM16A. DOG1 shows a cytoplasmic and/or membranous pattern of staining similar to that seen with CD117.37, 38 DOG1 appears to be a more specific marker of GIST than is CD117, although a handful of tumors other than GIST may occasionally express this protein (see below).
A number of alternative immunohistochemical markers for GIST have been evaluated, including nestin, protein kinase C theta, and PDGFRA. While nestin and protein kinase C theta have a high sensitivity, their expression by a wide range of histologic mimics of GIST limits their utility.32 PDGFRA has been reported by some to be highly sensitive and generally specific for CD117-negative GISTs.39, 40 However, the unpublished experience of most large GIST centers (including our own) is that reliability and reproducibility of commercial sources of this antibody are poor and therefore not suitable in the diagnostic setting.41
Other commonly expressed but nonspecific markers include CD34, nestin, Bcl2, and h-caldesmon. Smooth muscle actin (SMA) is expressed in a minority of cases (30%-40%), while S100 immunoreactivity is rare (5%-10%).1, 4, 42, 43, 44, 45, 46 GISTs may occasionally show immunoreactivity for CK18 (4%-8%)25, 47 and to a lesser extent CK8,15 but are nonreactive for AE1/AE3.48, 49 Expression of desmin is uncommon (<2%) and usually encountered in gastric epithelioid GISTs where expression is focal. Desmin may also be expressed following treatment with imatinib.35 While interpretation of CD117 immunoreactivity is usually straightforward, significant challenges may arise particularly in small biopsy specimens. These challenges are outlined below together with practical measures to overcome them.
False-positive immunoreactivity. Overdiagnosis of GIST through misinterpretation of CD117 immunohistochemistry is more frequent than underdiagnosis due to failure to perform or correctly interpret CD117 immunohistochemistry. False-positive immunoreactivity is largely related to technical factors such as antibody dilutions, particular sources of commercial antibodies, and overvigorous antigen retrieval techniques, in particular heat-induced epitope retrieval (HIER).1, 42, 50, 51, 52 A prime example is the early report of CD117 immunoreactivity (antibody source and dilution related) in desmoid tumors,53 which under optimum technical conditions are CD117-negative tumors.
The use of HIER in CD117 immunohistochemistry is controversial and conflicting recommendations have been made regarding its use.3, 10, 54 Irrespective of the method used, careful attention to positive controls (particularly ICC but also mast cells) and negative controls (especially smooth muscle and fibroblasts) is critical when establishing immunohistochemistry protocols (Fig. 7-8F-I). For this purpose, we select blocks, wherever possible, that include muscularis propria and myenteric plexus and titrate out until any smooth muscle or fibroblast immunoreactivity has been quenched.
The use of HIER in CD117 immunohistochemistry is controversial and conflicting recommendations have been made regarding its use.3, 10, 54 Irrespective of the method used, careful attention to positive controls (particularly ICC but also mast cells) and negative controls (especially smooth muscle and fibroblasts) is critical when establishing immunohistochemistry protocols (Fig. 7-8F-I). For this purpose, we select blocks, wherever possible, that include muscularis propria and myenteric plexus and titrate out until any smooth muscle or fibroblast immunoreactivity has been quenched.
 Figure 7-8. CD117 immunohistochemistry in GIST. Most GISTs show strong and diffuse CD117 immunoreactivity (A), most often cytoplasmic (B) but sometimes membranous (C) or in the paranuclear “Golgi zone” (D). Epithelioid GISTs often show less uniform staining (E) and are sometimes only weakly positive or negative. F-I: Controls are critical when interpreting immunohistochemical stains. Interstitial cells of Cajal (F) are good controls but mast cells less good as they are so immunoreactive and mast cells (G) serve as good positive controls, while smooth muscle (H) and fibroblasts (I) serve as negative controls. These are generally present in blocks containing muscularis propria. |
In practice, false-positive immunoreactivity is rarely a problem with resected tumors, where blocks containing in-built controls can be selected. The greatest difficulties arise with limited biopsy specimens, usually from metastatic sites. Here, the pattern of CD117 staining within cells, the distribution of staining in the biopsy, and profile of other markers (e.g., DOG1) need to be taken into account. Non-GISTs rarely show either membranous staining or paranuclear membrane accentuation. Rather, the staining is typically cytoplasmic and granular and usually focal and weak. If present, the staining of control material (usually mast cells) is much stronger. Nonspecific staining toward the edge of the biopsy specimen (fringe artifact) should be ignored and not interpreted as focal positive staining.55 Other factors that weigh into the interpretation include tumor location, morphology, and the results of other immunostains (see above). If doubt regarding CD117 immunoreactivity persists, referral to a center with special expertise and/or mutational analyses and/or rebiopsy may help to resolve the diagnosis.20, 55, 56
CD117-negative GISTs. A small subset of tumors (approximately 5%) qualify as GISTs by all other criteria (including demographics, clinical presentation, anatomic location, morphology and immunohistochemical profile) but are negative for CD117 on immunohistochemistry. Lack of immunoreactivity may be due to technical factors (suboptimal fixation, excessive heat during section drying, or very prolonged storage of slides), sampling error (small biopsies from GISTs with only focal CD117 overexpression), loss of KIT overexpression following clonal evolution (e.g., after imatinib therapy), or genuine lack of CD117 overexpression.56 Up to 50% of CD117-negative GISTs are immunoreactive for DOG1. A study of 1,040 GIST cases found only 2.6% of GISTs to be both CD117 and DOG1 negative. The true figure is likely to be even lower since two-thirds of CD117/DOG1- GIST that were tested for KIT and PDGFRA mutations were negative, challenging the original diagnosis. Two other studies have reported CD117-/DOG1- cases to comprise 0.9% and 1.6% of all GISTs.57, 58
Up to 90% of CD117-negative tumors have mutations in either PDGFRA (35%-80%) or KIT (15%-20%).4, 42, 59, 60, 61 Mutational analysis is therefore advocated where feasible in all CD117 tumors that otherwise have the typical features of GISTs.42, 61 Turnaround time for this has improved dramatically beyond the point where it was easier to determine clinical response to imatinib than to wait for mutational analysis reports in overtly malignant tumors.
CD117- and DOG1-Positive Tumors Other Than GISTs
A number of tumors other than GISTs can express CD117 (Table 7-2). Fortunately, most do not overlap morphologically with GISTs.52 Those may include melanoma,62 angiosarcoma,63 endometrial stromal sarcoma,64 PEComa,65 and Kaposi’s sarcoma.63 A small percentage (2%-4%) of colorectal adenocarcinomas may overexpress CD11766, 67, 68. The figure of CD117 positivity is higher (10%-15%) for non-small cell carcinomas of the lung.69 As melanoma can occur as both primary and polypoid metastases throughout the GI tract, this is the main issue with CD117 immunoreactive tumors. At least one melanoma marker (e.g., S100) should always be in a panel, and any immunoreactivity should lead to staining for additional markers, for example, HMB-45, MART-1, or tyrosinase.
Table 7-2 Potential GIST Mimics that May Express CD117 and/or DOG1 | |
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Potential histologic mimics of GIST that may express DOG1 include Ewing’s sarcoma, glomus tumor, synovial sarcoma, angiosarcoma, leiomyosarcoma, peritoneal leiomyomatosis, uterine-type retroperitoneal leiomyoma, and desmoplastic melanoma. To date, there are only four GIST mimics that can express both CD117 and DOG1 (namely angiosarcoma, synovial sarcoma, Ewing’s sarcoma, and melanoma). In all four tumors, staining is focal with at least one of the markers. Thus, GIST remains the only mesenchymal neoplasm that can show diffuse immunoreactivity for both markers.32 Therefore, in a morphologically typical GIST that shows diffuse immunoreactivity for CD117 and DOG1, no further immunohistochemical stains should be necessary.
Molecular Features and Mutational Analysis
Approximately, 80% of GISTs have mutations in KIT, while about 5% to 7% have PDGFRA mutations; KIT and PDGFRA mutations are mutually exclusive. KIT mutations lead to constitutive upregulation of the protein tyrosine kinase, through ligand independent dimerization.15, 70, 71 Most KIT mutations occur in exon 11 (60%-70%), followed by exon 9 (10%-15%; almost exclusively small intestinal GIST), exon 13 (1%-4%), and exon 17 (<1%).4, 42, 72 Very rarely, mutations have been identified in exons 8, 12, 14, and 18 in primary GIST.73, 74 Exon 11 and 9 mutations cause dysregulation through alterations of the juxtamembrane autoinhibitory region of c-kit, while exon 13 and 17 mutations activate c-kit’s enzyme pocket directly. PDGFRA mutations are strongly associated with gastric location and epithelioid phenotype.2, 3, 59, 60, 75, 76 The most common is the D842V point mutation on occurring on exon 18, which comprises over 80% PDGFRA mutations.73
The nature and site of KIT and PDGFRA mutations predict response to imatinib mesylate. KIT mutations involving exon 11 are associated with a far better response to imatinib mesylate therapy than those involving exon 9.42 The PDGFRA exon 18 mutation (D842V) is associated with a poor response to imatinib, while GISTs with other PDGFRA mutations are potentially imatinib sensitive.61, 73, 75 GISTs lacking either KIT or PDGFRA are poorly responsive.42 The occasional presence of KIT mutations in melanoma77 may represent a potential diagnostic pitfall. KIT mutations may also be found in seminomas and systemic mastocytosis, they fortunately do not occur in tumors overlapping morphologically with GIST. Mutational studies performed on frozen tumor provide the best diagnostic yield. Although reasonable yields may also be obtained from DNA extracted from paraffin sections, this diminishes considerably with old archival material.71 Areas of the slide containing tumor should be selected (using microdissection if necessary) to ensure a high proportion of tumor cells in the sample. Failure to do so may result in a false-negative result due to a high background of wild-type DNA.
Mutational analysis has considerable utility in a number of situations. It is particularly useful in the workup of suspected GISTs that are CD117 negative, since the vast majority have mutations in either KIT or PDGFRA genes.4, 42, 61 It may also be helpful in predicting response to imatinib (see above) and determining the optimal dose of therapy. For example, KIT exon 9 mutations are associated with a significantly longer disease-free survival when GISTs are treated with high-dose imatinib compared to standard dosages; such benefits are not observed in GISTs with other mutations.78 Mutational status may also predict response to the second-line agent sunitinib as those harboring KIT exon 9 mutations appear to be more sensitive to this agent than those harboring exon 11 mutations. The clinical benefit of sunitinib in wild-type cases has also been reported.71 The type of mutation may also have prognostic implications. For example, gastric GISTs with missense mutations in exon 11 seem to have a better prognosis than those with exon 11 deletions (no such association is found for small intestinal GISTs). Earlier reports of an unfavorable outcome for exon 9 mutations seem to be related to the poorer prognosis in small intestinal GISTs where these mutations are markedly overrepresented. At present, there are insufficient data for incorporation of mutational status into risk stratification schemes.15, 71 Mutational analysis facilitates the diagnosis of familial GIST syndromes associated with germline mutations in KIT or PDGFRA. Here, both tumor and surrounding nontumorous tissue exhibit the same mutation. The demonstration of wild-type KIT and PDGFRA may provide indirect support for the diagnosis of neurofibromatosis 1 (NF1) or Carney’s-associated GISTs in the right clinical setting (see below). Mutational studies can demonstrate secondary KIT mutations associated with the onset of secondary resistance to imatinib. Finally, mutational analysis can distinguish multiple sporadic GISTs (showing distinct KIT mutations in different tumors) from metastases or multiple familial GIST, which share a common mutation (see Sporadic multiple GIST below).
There are differing views as to whether mutational analysis should be a part of the routine diagnostic workup of all GISTs.41 Most would argue that this is not justified at present and should be reserved for specific situations such as those outlined above, or when contemplating neoadjuvant therapy in unresectable tumors. However, mutational studies should continue to be performed in the research setting particularly in studies evaluating new therapies.
Succinyl dehydrogenase subunit B (SDHB) expression, NF1 mutations, and BRAF mutations. Of the 10% to 15% of GISTs that are wild type for KIT or PDGFRA, a proportion will show other molecular abnormalities, including loss of succinyl dehydrogenase subunit B (SDHB) expression, NF1 mutations, and occasionally BRAF mutations (the latter reported in 7%-11% of wild-type GIST).32, 79, 80, 81, 82 Mutations in any one of the SDH subunits A to D can result in loss of immunohistochemical expression of SDHB.83
SDHB-deficient GISTs. SDHB-deficient GISTs have distinctive clinicopathologic features. They are seen almost exclusively in children or young adults, have a female predominance, have a gastric location, are frequently multicentric and/or multinodular (often with a plexiform growth pattern), have epithelioid morphology, and frequently display lymphovascular invasion (Fig. 7-9F,G). Despite a high frequency of liver and lymph node metastasis, survival is disproportionately long compared to unselected metastatic GISTs, and clinical behavior is not predicted by size and mitotic rate.81, 83, 84 SDHB-deficient GISTs are resistant to imatinib but may show a greater sensitivity to the second-line agent sunitinib than do KIT– and PDGFRA-mutated GISTs.81
Four clinical subgroups are strongly associated with SDHB-deficient GISTs, namely Carney’s triad, Carney-Stratakis syndrome, pediatric GIST (see below), and a proportion of sporadic GIST in young adults. In an unselected series of 746 gastric GISTs, 7.5% were found to be SDHB deficient.81 SDHB-deficient GISTs have been shown to overexpress insulin-like growth factor receptor 1 (IGFR1).84 Thus, it could be argued that GISTs with classical clinicopathologic features and an SDHB−/IGFR1+ immunoprofile do not require the more costly KIT and PDGFRA mutational analysis.
Chromosomal alterations. While KIT and PDGFA mutations are the earliest genetic changes in most GISTs, chromosomal losses (and gains) accumulate with progression to more aggressive phenotypes. Most low/very low malignant potential GISTs have a normal karyotype or loss of 14q (the latter occurs in 2/3 of GISTs overall). Intermediate malignant potential GISTs may show further loss of 22q (present in 1/2 of GISTs overall) and/or loss of lp. Overtly malignant GISTs usually show further chromosomal loss (such as 9p, 11p, 13, 15, or 18) or gains (such as 5p, 8q, 17q, or 20q). Of interest, chromosome 9p21 contains the tumor suppressor gene CDKN2A, which encodes the cell cycle inhibitors p16INK4A and p14ARF. Immunohistochemical and comparative genomic hybridization studies have shown p16 loss in GIST to be associated with a poor prognosis.70, 71, 85
Predicting Behavior
The presence of histologically confirmed metastases clearly indicates a malignant tumor. It is good practice to histologically confirm the nature of suspected metastases found at surgery by biopsy; rarely these prove to be from a different tumor that demands different therapy. Such “metastases” may occasionally prove to be unrelated benign lesions (e.g., focal nodular hyperplasia in the liver, granulomata, or Von-Meyenburg complexes). Biopsy eliminates such uncertainties. Stromal tumors are also lethal by virtue of their local recurrence and invasion of other organs. Incomplete resection clearly indicates an increased likelihood of local recurrence even in the absence of metastases.23 Tumor rupture at presentation or at surgery is independently associated with a poor outcome; fortunately, this complication occurs in <5% of patients.86, 87
A variety of prognostic factors have been evaluated in localized, surgically resectable GIST. These including tumor size, mitotic rate, site, tumor rupture, serosal involvement, mucosal ulceration, histologic type or pattern, lamina propria infiltration, coagulative necrosis, cellularity, degree of atypia, Ki67/MIB-1 index, p53, p16, ploidy, and others. Of these, tumor size and mitotic rate have consistently emerged as important independent prognostic factors. The significance of other factors has been difficult to ascertain due to small cohort size, selection bias, and association with other high-risk factors.87 The NIH consensus classification system—based on a GIST workshop in 2001—stratified risk of aggressive behavior on the basis of size and mitotic rate (Table 7-3).88 This system was based on consensus opinion rather than distinct data sets but accumulating evidence has confirmed its prognostic utility. Two very large studies from the
AFIP including 1,765 gastric GISTs (1,074 with full long-term follow-up data; 48% alive and disease free at a median of 14.1 years) and 906 small intestinal GISTs (629 with full long-term follow-up data; 34% alive and disease free at median of 15.5 years) have shown tumor site to be an important independent predictor of prognosis,2, 3 confirming the findings of previous smaller studies.89, 90, 91
AFIP including 1,765 gastric GISTs (1,074 with full long-term follow-up data; 48% alive and disease free at a median of 14.1 years) and 906 small intestinal GISTs (629 with full long-term follow-up data; 34% alive and disease free at median of 15.5 years) have shown tumor site to be an important independent predictor of prognosis,2, 3 confirming the findings of previous smaller studies.89, 90, 91
Table 7-3 The NIH 2001 Consensus Classification Scheme for Risk Stratification in GIST | |||||||||||||||||||||||||||
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Failure to account for site may result in significant overestimation of the risk of aggressive behavior in gastric GIST using the NIH 2001 risk stratification scheme. For example, while NIH scheme would place gastric GISTs >10 cm in size and with <5 mitoses/HPF into the high risk category, the AFIP study found that only 11% of these patients died of their GISTs during long-term follow-up. Other adverse prognostic features in the AFIP series included coagulative necrosis, mucosal invasion, and ulceration (gastric and small intestinal GISTs); proximal location in stomach versus antrum and exon 11 deletions versus point mutations (gastric GISTs); and diffuse cytologic atypia and epithelioid cytology (small intestinal GISTs). The authors have proposed a new set of guidelines with separate criteria for gastric and small intestinal GISTs (modified in Table 7-4). The currently available data on GISTs at uncommon sites such as esophagus, colon, and rectum, although limited, suggest that these should be stratified similar to small intestinal GISTs.15 Proliferation markers might be useful in assessing the proliferative rate but have not proved superior to mitotic rate in predicting aggressive behavior. As noted earlier, the behavior of SDHB-deficient GIST (which are exclusively gastric) is not predicted by size and mitotic rate.
Table 7-4 Risk Stratification Scheme Based on Long-term Follow-up Data from >1,600 Patients from the Armed Forces Institute of Pathology Prior to the Era of Imatinib Therapy | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Mitotic rate counting may be subject to number of influences including interobserver variability, specimen fixation, section thickness, size of fields examined, variation in tumor cellularity and tumor cell size, and fastidiousness of the pathologist, all of which may affect reproducibility. If modern wide-field eyepieces are used, the mitotic rate should be adjusted to correspond to the same total area examined in the largest (AFIP) studies on which prognostic data are based (i.e., 5 mm2). Counts should be performed in the most mitotically active area and should proceed until either 50 fields are viewed or 100 mitoses have been counted.2, 3 Tissue should be adequately fixed and sections should not exceed 4 to 5 µm in thickness. Care should be taken not to count pyknotic nuclei or nuclear debris as mitotic figures. Two studies have stratified GISTs into high- and low-risk groups based on gene expression profiles and chromosome complexity, both outperforming the AFIP grading system.92, 93 It is therefore possible that morphology-based risk stratification schemes might ultimately be replaced by risk stratification based on a molecular profiling.
GIST Syndromes
Four GIST syndromes have been described: (1) familial GIST syndrome, (2) neurofibromatosis type 1 (von Recklinghausen’s disease), (3) Carney’s triad, and (4) GIST paraganglioma syndrome (Table 7-5).
Familial GIST syndrome. Familial GIST syndrome, associated with germline mutations in KIT94, 95, 96, 97, 98, 99, 100 or PDGFRA101 genes, is extremely rare with at least 25 kindreds reported.102, 103 Those affected have several family members who develop multiple primary
GISTs at early age (on average two decades earlier than sporadic GISTs) (Fig. 7-9A-E). GISTs typically develop in a background of diffuse ICC hyperplasia. Depending on the mutation involved, patients may develop a variety of cutaneous lesions (including hyperpigmentation of per oral, axillary, perineal regions and hands, lentigines, café au lait macules, benign nevi, urticaria pigmentosa, and melanoma), hematologic malignancies, or dysphagia (presumably due to dysmotility associated with ICC hyperplasia) (Table 7-5). There is an autosomal dominant pattern of inheritance.42, 94, 95, 96, 97, 98, 99, 100, 101 In this setting, multiple primary GISTs are distinguished from metastases on the basis of their small size, lack of mitotic activity, and uneventful follow-up (unless associated with a concurrent malignant tumor).2
GISTs at early age (on average two decades earlier than sporadic GISTs) (Fig. 7-9A-E). GISTs typically develop in a background of diffuse ICC hyperplasia. Depending on the mutation involved, patients may develop a variety of cutaneous lesions (including hyperpigmentation of per oral, axillary, perineal regions and hands, lentigines, café au lait macules, benign nevi, urticaria pigmentosa, and melanoma), hematologic malignancies, or dysphagia (presumably due to dysmotility associated with ICC hyperplasia) (Table 7-5). There is an autosomal dominant pattern of inheritance.42, 94, 95, 96, 97, 98, 99, 100, 101 In this setting, multiple primary GISTs are distinguished from metastases on the basis of their small size, lack of mitotic activity, and uneventful follow-up (unless associated with a concurrent malignant tumor).2
Table 7-5 Molecular Classification of Gastrointestinal Stromal Tumors | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Neurofibromatosis type 1 (von Recklinghausen’s disease). Patients with NF1 are at increased risk of the development of GISTs. A Swedish study found the incidence of clinically apparent GISTs to be approximately 7% in NF1 patients and 33% in an autopsy series.104 NF1 is overrepresented by 50- to 150-fold among patients with GISTs.10, 105, 106 GISTs associated with NF1 are frequently multicentric (60%) and often accompanied by ICC hyperplasia. The vast majority occurs in the small intestine (usually jejunum) with the stomach infrequently involved.105, 106, 107, 108 On average, NF1-associated GISTs present approximately 10 years earlier than sporadic GISTs.106 Most GISTs occurring in the setting of NF1 have low-grade morphology, low mitotic rates, abundant skenoid fibers, and a good clinical outcome,4, 105, 106, 109 but a minority may show more aggressive behavior.106, 108 Despite CD117 overexpression, almost all lack KIT or PDGFRA mutations.105, 106, 107 NF1-associated GISTs show diffuse S100 immunoreactivity in up to 40% to 60% of cases.106, 109 Occasionally, GISTs may be the first presentation in clinically unrecognized NF1.110 The pathologist should be alert to this possibility when confronted with multiple GIST, ICC hyperplasia, strong S100 immunoreactivity, and absence of KIT or PDGFRA mutations.
Carney’s triad. Carney’s triad is a tumor syndrome characterized by multicentric, functioning extraintestinal paraganglioma, pulmonary chondroma, and
gastric epitheliod GIST (Fig. 7-9F-I). Gastric GIST may precede the other tumors by several years. About 80 cases have been reported in the literature to date.111, 112, 113 Most are sporadic. Distinctive clinicopathologic features include a striking female predominance (>85% of cases), young age at presentation (>80% of patients <30 years), frequent multifocality, consistent epithelioid morphology, and a relatively indolent course despite a high rate of local recurrence and metastases (41% and 55% respectively).111, 112 These GISTs also lack KIT/PDGFRA mutations as well as the nonrandom loss of 14q and 22q, characteristic of sporadic GIST. The response to imatinib in this setting is generally considered to be poor.114
gastric epitheliod GIST (Fig. 7-9F-I). Gastric GIST may precede the other tumors by several years. About 80 cases have been reported in the literature to date.111, 112, 113 Most are sporadic. Distinctive clinicopathologic features include a striking female predominance (>85% of cases), young age at presentation (>80% of patients <30 years), frequent multifocality, consistent epithelioid morphology, and a relatively indolent course despite a high rate of local recurrence and metastases (41% and 55% respectively).111, 112 These GISTs also lack KIT/PDGFRA mutations as well as the nonrandom loss of 14q and 22q, characteristic of sporadic GIST. The response to imatinib in this setting is generally considered to be poor.114
 Figure 7-9. GIST syndromes. A-E: A 33-year-old female with multiple GISTs associated with a germline, exon 13 KIT mutation. A: The gross specimen received in consultation was fragmented due to prior sampling but several GISTs can still be appreciated. B: Scanning view of two of the GISTs centered in the muscularis propria. C: CD117 immunohistochemistry highlights massive ICC hyperplasia adjacent to a GIST. ICC hyperplasia is characteristic of germline KIT/PDGFRA mutations but can also be seen in NF1. ICC hyperplasia is readily appreciated on H&E (D) shows an identical morphologic appearance to the adjacent GIST, complete with skenoid fibers (E, arrow). F,G: Gastric epithelioid GIST associated with wild type KIT/PDGFRA in a 22-year-old female patient. The young age, female gender, gastric location, multinodular and plexiform growth pattern, and epithelioid morphology are highly suggestive of a succinyl dehydrogenase subunit B (SDHB)-deficient GIST. Such GISTs may be associated with Carney’s triad or GIST-paraganglioma syndrome. No history of prior paraganglioma (H) or |
 Figure 7-9. (Continued) pulmonary chondroma (I) was forthcoming, but GISTs may be the initial presentation in such patients. |
GIST-paraganglioma syndrome (Carney-Stratakis syndrome). This novel familial GIST syndrome (involving 12 patients from 7 families) overlaps with Carney’s triad in that patients develop both paragangliomas and GISTs at a young age.115, 116 However, it differs from Carney’s triad in that there is an equal sex predilection, an absence of pulmonary chondromas, and an autosomal dominant pattern of inheritance with incomplete penetrance.115 The majority of patients with this syndrome have germline loss-of-function mutations in the succinyl dehydrogenase subunit genes SDHB, SDHC, and SDHD,116 leading to loss of SDHB expression and overexpression of IGF1R.
Sporadic multiple GIST. Multiple primary GISTs were thought to occur only in the clinical syndromes described above. However, a total of 21 patients with multiple primary sporadic GISTs (with differing KIT or PDGFRA mutations in each lesion) have been described in four studies.117, 118, 119, 120 In 19 patients, the tumors arose in the same organ (16 stomach, 3 small intestine), while in 2 cases they involved different organs (stomach and small intestine, small intestine and peritoneum). Most were situated within 3 cm of one another. The mean age at presentation was 72.4 years. In about half of the cases, at least one of the lesions was under 1 cm in size. However, in one study five patients had two separate lesions >1.5 cm size (mean of 4.3 cm).118 Sporadic multiple GISTs differ from those associated with GIST syndromes in that patients are older, have fewer GISTs (usually 2-3), lack ICC hyperplasia, often have smaller tumors, show a predilection for the proximal stomach (versus small bowel for familial and NF1-associated GISTs), are usually closely approximated (<3 cm), and show different KIT or PDGFRA mutations in each tumor. The existence of multiple sporadic GISTs has important clinical implications. Patients presenting with multiple GISTs in the absence of clinicopathologic features of GIST syndromes may be misdiagnosed as having metastatic disease (as occurred in 2/6 patients in one study) and treated with imatinib.118 Molecular testing of each tumor is therefore recommended in this setting.
Pediatric GIST. Very occasionally, GISTs may present in children or young adults. Pediatric GISTs display a number of characteristic clinicopathologic features including a striking female predominance, a predominantly gastric location, a strong tendency for multifocality, a frequent epithelioid morphology, and high rate of local recurrence and lymph node metastasis.81, 121, 122, 123 Almost all reported pediatric GISTs so far are negative for KIT or PDGFRA mutations,122, 123, 124 although an isolated case with a novel exon 9 KIT mutation was reported.124
Small GIST. Small, clinically silent GISTs are being detected with increasing frequency,125 raising questions as to their clinical significance and management. Several studies suggest a high prevalence in the general population. Agaimy et al. detected small GIST in 22.5% of 98 consecutive autopsy cases by gross examination (this included only grossly recognizable lesions).126 Kawanowa et al. detected microscopic GIST in 35 out of 100 gastrectomies (performed for unrelated tumors) when sectioned at 5 mm intervals.127 Two studies of esophagogastrectomy specimens found microscopic GIST or minute lesions (≤1 mm) termed “ICC hyperplasia” in approximately 10% of patients128, 129; good criteria for separating focal ICC hyperplasia and microscopic GIST do not exist. Small GISTs show a predilection for the proximal stomach and gastroesophageal junction. Most are solitary but up to 30% may be multiple. Reported cases have ranged from GISTs range from 0.2 mm in size to up
to 10 mm. The lesions frequently show hyalinization and calcification leading to the suggestion that many may regress/involute.126 The reported rates of KIT and PDGFRA mutations vary but they occur in up to 85% of GISTs ≤10 mm in size.130 Variable terminology has been applied to these lesions (microscopic GIST, minimal GIST, minute GISTs, GIST tumorlets, ICC hyperplasia, etc.), and there is no consensus on terminology. The simplest approach might be to regard all GIST-like lesions ≤10 mm in size as “small, incidental GIST” given their uniformly indolent behavior. The vast majority either involute or remain asymptomatic. A small minority may accrue additional genetic alterations and evolve into clinically significant GIST.
to 10 mm. The lesions frequently show hyalinization and calcification leading to the suggestion that many may regress/involute.126 The reported rates of KIT and PDGFRA mutations vary but they occur in up to 85% of GISTs ≤10 mm in size.130 Variable terminology has been applied to these lesions (microscopic GIST, minimal GIST, minute GISTs, GIST tumorlets, ICC hyperplasia, etc.), and there is no consensus on terminology. The simplest approach might be to regard all GIST-like lesions ≤10 mm in size as “small, incidental GIST” given their uniformly indolent behavior. The vast majority either involute or remain asymptomatic. A small minority may accrue additional genetic alterations and evolve into clinically significant GIST.
Table 7-6 Differential Diagnosis Based on Morphologic Subgroup | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Differential Diagnosis
In our experience, overdiagnosis of GIST is more frequent an occurrence than underdiagnosis. This is likely due to the increased awareness of GIST that has followed the advent of targeted therapy. The differential diagnosis is best considered in the context of six broad morphologic patterns of GIST (Table 7-6). The majority of lesions entering the differential diagnosis fall into the “spindled, cytologically bland” category with desmoid tumor, leiomyomas, schwannoma, and IFP, the most frequent mimics. Salient anatomical, morphologic, and immunohistochemical features separating these lesions are summarized in Table 7-7.
In the spindled, cytologically malignant category, the most important considerations include spindle cell carcinoma, melanoma, and sarcomas. Bland epithelioid tumors should be distinguished from glomus tumor, carcinoid, and paraganglioma, while melanoma and carcinoma should be excluded when confronted with malignant epithelioid tumors. Both may also show mixed spindled and epithelioid morphology. Pleomorphic morphology is highly unusual for GIST and should prompt consideration of alternative diagnoses including carcinoma, melanoma, or sarcoma. A variety of less common lesions also enter the differential diagnosis from time to time (Table 7-6). Demographic, anatomic, gross, morphologic, and immunohistochemical features can all provide useful clues to the diagnosis and are outlined in the text that follows. Practical approaches to the differential diagnosis of GIST have been the subject of numerous reviews.131, 132 It should be emphasized that small mesenchymal polyps confined to the mucosa and/or submucosa cannot be GISTs, as the latter originate in the muscularis propria and only involve the submucosa or mucosa by extension from a larger mural mass.
In the spindled, cytologically malignant category, the most important considerations include spindle cell carcinoma, melanoma, and sarcomas. Bland epithelioid tumors should be distinguished from glomus tumor, carcinoid, and paraganglioma, while melanoma and carcinoma should be excluded when confronted with malignant epithelioid tumors. Both may also show mixed spindled and epithelioid morphology. Pleomorphic morphology is highly unusual for GIST and should prompt consideration of alternative diagnoses including carcinoma, melanoma, or sarcoma. A variety of less common lesions also enter the differential diagnosis from time to time (Table 7-6). Demographic, anatomic, gross, morphologic, and immunohistochemical features can all provide useful clues to the diagnosis and are outlined in the text that follows. Practical approaches to the differential diagnosis of GIST have been the subject of numerous reviews.131, 132 It should be emphasized that small mesenchymal polyps confined to the mucosa and/or submucosa cannot be GISTs, as the latter originate in the muscularis propria and only involve the submucosa or mucosa by extension from a larger mural mass.
Table 7-7 Key Anatomic, Morphologic, and Immunohistochemical Features of Lesions to Be Considered in the Differential Diagnosis of Spindled, Cytologically Bland GIST | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Table 7-8 Example of GIST Synoptic Report | ||||||
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Reporting GIST
The pathologist should ensure that at a minimum, all information required to accurately prognosticate is included in the pathology report. Synoptic reports are gaining in popularity and we prefer this form of reporting for GISTs; an example of our GIST synoptic report is illustrated in Table 7-8. The minimum information necessary includes site of tumor, extent of disease (i.e., localized or invasion surrounding tissues), tumor size, resection margins, perforation or serosal involvement, and mitotic rate. Along with the diagnosis, the pathologist should convey the risk of aggressive behavior based on the above observations. Metastatic, unresectable, or locally advanced disease will by definition be associated with a high risk of aggressive behavior. In localized, surgically resected GISTs, we take into account size, mitotic rate, site, and serosal involvement (perforation) and favor risk stratification schemes that include these factors (see Table 7-4). Since more than one risk stratification scheme now exists, it is important to document which scheme has been used for risk stratification.
SMOOTH MUSCLE TUMORS
Leiomyomas
In the era of CD117 immunohistochemistry, true smooth muscle tumors of the GI tract are rare and substantially outnumbered by GISTs. Two notable exceptions include leiomyomas of the esophagus and the colorectal muscularis mucosae, which remain the most common mesenchymal tumors at their respective sites. In the esophagus, leiomyomas outnumber GISTs by two- to threefold, occur in younger patients (mean 35 years), show a mild male predominance, and have a predilection for the distal esophagus and esophagogastric junction. They are often discovered incidentally but can present with dysphagia, retrosternal or epigastric pain or bleeding. In one large series, esophageal leiomyomas ranged from 1 to 18 cm in size (mean 5 cm).26 Clinically occult “seedling” tumors are common. In a study of 150 esophagogastrectomy specimens for gastric or esophageal carcinoma, careful examination revealed incidental esophageal leiomyomas in 47% of patients. These had a mean size of 1.7 mm (range 1-13 mm) with a mean of 3 leiomyomas per patient (range 1-13).128 The main differential diagnosis includes a small
GIST (see above). Grossly, esophageal leiomyomas generally arise from the muscularis propria and are usually well demarcated and spherical to oval shaped. Larger tumors may be elongated with sausage, dumb-ell or horseshoe shapes. They may form polypoid lesions, which project into the lumen. The cut surface is firm, white to gray-tan, whorled, and often lobulated resembling leiomyomas of the myometrium.
GIST (see above). Grossly, esophageal leiomyomas generally arise from the muscularis propria and are usually well demarcated and spherical to oval shaped. Larger tumors may be elongated with sausage, dumb-ell or horseshoe shapes. They may form polypoid lesions, which project into the lumen. The cut surface is firm, white to gray-tan, whorled, and often lobulated resembling leiomyomas of the myometrium.
Focal hemorrhage may be present. Histologically, the tumors are of low to moderate cellularity and composed of intersecting fascicles of bland spindled cells with abundant eosinophilic cytoplasm, which is often fibrillary or clumped. The nuclei are elongated, usually with blunt ended, and may be indented by a paranuclear vacuole. The arrangement of tumor cells is more haphazard than in the surrounding muscle layers with which it may merge at the periphery. Occasionally, the tumors may show epithelioid morphology. Rarely, leiomyomas may have a plexiform appearance similar to that seen in neural tumors.26, 133 Areas of hyalinization may be seen. Mitotic figures are rare or absent.26, 131, 134 In the colon, leiomyomas are usually found incidentally as small polyps at screening colonoscopy or in resection specimens (Fig. 7-10A-C).135 They arise almost exclusively from the muscularis mucosae and, because of their superficial location, are usually amenable to endoscopic removal by snare resection. Grossly, they are white, firm, circumscribed nodules. Most are <5 mm in size but examples of up to 2.2 cm are described. Histologically, they often exhibit a storiform rather than herringbone pattern. Older tumors may be heavily collagenized (Fig. 7-10C). They typically merge with the surrounding muscularis mucosae. Some display prominent eosinophilic hyaline globules with immunohistochemical and ultrastructural features of actin and desmin filaments (Fig. 7-10B, inset). Skenoid fibers are absent.136 Rarely, colorectal leiomyomas display marked atypia resembling that seen in symplastic uterine leiomyomas but mitotic figures are rare or absent.135 Leiomyomas occur uncommonly in the small intestine, stomach, appendix, or intramural within the anorectum. At all sites, leiomyomas behave in a clinically benign manner with little tendency for recurrence; morbidity, if any, is related to local effects of the lesions.
The major issue with leiomyomas is to avoid misdiagnosis as other tumors, especially GIST. Leiomyomas are generally less cellular than GISTs, have plumper cells with more abundant and more eosinophilic cytoplasm, and in the case of colorectal leiomyomas, their origin from the muscularis mucosae is an important clue to their diagnosis. Leiomyomas are immunoreactive for smooth muscle markers and, more importantly, negative for CD117 and DOG1. Some apparently typical GI leiomyomas contain abundant admixed CD117+/DOG1+ ICC (or occasionally S100+ fibers) throughout the tumor (Fig. 7-10G,H). Such tumors may represent a form of nodular hyperplasia or hamartoma rather than a true leiomyoma.32 They behave identically to leiomyomas, and the key issue here is not to misinterpret the admixed CD117+/DOG1+ cells as evidence of a GIST. In addition, care should also be taken not to interpret intralesional mast cells as focal CD117 immunoreactivity. The storiform arrangement often seen in colorectal leiomyomas may lead to confusion with a small, fibrous histiocytoma, but the merging of the tumor with the muscularis mucosae should suggest the correct diagnosis. The distinction between leiomyomas (low to moderate cellularity, mitotically inactive, and clinically benign) and leiomyosarcomas (high grade, highly cellular, mitotically active, and clinically aggressive—see below) is usually easily made. Thus, unlike GISTs, the problem of predicting the behavior of paucicellular, well-differentiated tumors is not commonly encountered.26, 27 Finally, smooth muscle pseudotumors may be created artificially if part of the muscularis propria (or the muscularis mucosae in the esophagus) is sampled in deep biopsies. These are more likely to be obtained when punch or basket rather than flat surgical biopsy forceps are used.23
Epstein-Barr Virus-associated Smooth Muscle Tumors
Distinctive smooth muscle tumors associated with Epstein-Barr virus (EBV) infection may arise in immunocompromised patients including those with HIV infection (predominantly children or young adults), organ transplant recipients, or children with congenital immunodeficient states.137, 138 The GI tract is one of the most common sites involved (after lung and liver). Endoscopically, these tumors often have a centrally ulcerated, nodular appearance. They are usually multiple. Histologically, they resemble typical smooth muscle tumors but show a spectrum of features, which range from paucicellular, uniform, mitotically inactive tumors resembling leiomyomas to densely cellular, pleomorphic, and mitotically active tumors resembling leiomyosarcomas. Small lesions often show a close relationship with the walls of blood vessels. Many contain foci of primitive round cells with irregular contours, which are actin positive. Such foci may be extensive and do not appear to have a prognostic significance. A minority contains a prominent T-cell infiltrate.137 The lesional cells are immunoreactive for SMA and variably reactive for desmin, CD21 (EBV receptor), and EBV nuclear antigen 2. EBV latent membrane protein is usually negative. In situ
hybridization for EBV early RNA (EBER) showing nuclear staining is the most helpful test to confirm the association with EBV.138 Although many tumors would meet the criteria for leiomyosarcoma as proposed by Billings, these patients have an excellent prognosis and very few die of their disease.137 Distinction from GISTs and true leiomyosarcomas is therefore important.
hybridization for EBV early RNA (EBER) showing nuclear staining is the most helpful test to confirm the association with EBV.138 Although many tumors would meet the criteria for leiomyosarcoma as proposed by Billings, these patients have an excellent prognosis and very few die of their disease.137 Distinction from GISTs and true leiomyosarcomas is therefore important.
 Figure 7-10. Leiomyomas. Submucosal leiomyomas of the colon (A-C). Note the intimate association with the muscularis mucosae, which is no longer readily identifiable. Inset B: Distinctive hyaline globules frequently described in these tumors. (C) In some tumors, a storiform pattern is readily apparent but much of the tumor consists of collagen. Leiomyoma arising in the muscularis propria of the small intestine (D-H) showing features typical of leiomyomas elsewhere, but with immunohistochemical evidence of admixed CD117- (and DOG1-) positive ICC (G), suggesting that this lesion might represent some form of nodular hyperplasia or hamartoma. Such staining should not be interpreted as evidence of a GIST. Rarely, a similar distribution of admixed S100-positive neural cells (H) may also be encountered. Leiomyomatosis disseminata peritonealis in a 43-year-old female (I). The patient had innumerable peritoneal nodules at laparotomy and was thought to have disseminated malignancy. These tumors are generally positive for both estrogen (J) and progesterone receptors. |
Leiomyomatosis
Esophageal leiomyomatosis is a rare condition characterized by multifocal, often confluent smooth muscle proliferations, which can form linear constricting lesions. Most patients are young (mean age 25.6 years) and present with dysphagia and achalasia-like manometric findings. Over 2/3 have Alport’s syndrome. Other associations include gastroesophageal-vulvar leiomyomatosis and multiple endocrine neoplasia type 1 (MEN 1). The stomach and tracheobronchial tree may be involved.139, 140, 141 The diagnosis is often difficult and patients may have symptoms for many years before the correct diagnosis is made. EUS is a useful diagnostic modality. Histologically, the lesions can arise from the muscularis propria or muscularis mucosae and are composed of well-differentiated smooth muscle cells arranged in fascicles, bundles, or whorls. Adjacent lesions may become confluent. Cellularity is low and cytologic atypia, and mitotic activity is absent or minimal. The vast majority of patients require esophagectomy with reconstruction due to intractable symptoms. About 5% of patients are asymptomatic and can be followed conservatively.139 Rarely, leiomyomatosis may involve the colon or small intestine.142 A leiomyomatosis-like lesion termed “leiomyomatosis-like lymphangiomatosis” has been described in the colon of a young patient with tuberous sclerosis and renal angiomyolipoma. Although overlapping morphologically with leiomyomatosis, it was distinguished by a component of epithelioid cells that were HMB-45 positive.143
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