Gross findings

FNH is usually a subcapsular, well-circumscribed, bulging, tan, nodular mass. Most measure less than 5 cm in diameter, but larger lesions occur. The surrounding liver is normal. The most characteristic finding is the presence of a central stellate scar with radiating fibrous septa that subdivide the mass into multiple smaller nodules ( Fig. 20-1 ). The central scar may be inconspicuous in smaller lesions. Hemorrhage, necrosis, and bile staining are rare.

Focal nodular hyperplasia (FNH) is a sharply circumscribed nodular mass often appearing lighter than the surrounding liver. The central stellate scar is characteristic.

Microscopic findings

FNH is composed of nodules of hepatocytes surrounded by fibrous septa that contain artery branches, bile ductules (a variable but key feature), a variable chronic and/or acute inflammatory infiltrate, and decreased or absent interlobular bile ducts and portal vein branches. The central scar is composed of dense collagen ( Fig. 20-2 A) and contains numerous thick-walled arteries and bile ductules (see Fig. 20-2 B). In the telangiectatic variant, numerous small, dilated vessels (resembling hemangioma) are seen centrally, and the adjacent sinusoids are markedly dilated. The histologic features overall resemble a biliary type of cirrhosis with ductopenia.

Focal nodular hyperplasia. A, The central stellate scar is composed of dense collagen. B, A variable degree of bile ductular proliferation is present at the interface of the scar with the parenchyma.

The hepatocytes in FNH lesions are similar to those in the surrounding liver, although they may be somewhat larger and paler and may incorporate variable quantities of fat or glycogen. Nuclear pleomorphism, prominent nucleoli, and mitotic figures are not found in classic FNH, but cytologic atypia may be seen in variant forms. The variant formerly referred to as “telangiectatic FNH” has been shown to be a form of hepatic adenoma by genetic and molecular studies.

Fine-needle aspiration biopsy

Fine-needle aspiration (FNA) biopsies are generally not useful for definitive diagnosis because the central stellate scar is usually not represented. The hepatocytes of FNH are cytologically indistinguishable from normal.

Gross findings

HA is a well-circumscribed, often subcapsular mass in the right lobe of an otherwise normal liver. Most are larger than 10 cm. Ruptured HAs may be obscured by blood clot and difficult to recognize, but the tumor is usually paler or more yellow than the adjacent liver ( Fig. 20-3 ). The cut surface has a variegated appearance, generally due to the presence of blood clot. Fibrous septa are usually a result of previous infarct.

Hepatic adenomas are sharply circumscribed lesions. Intrahepatic hemorrhage is common in resected specimens.

Microscopic findings

On first glance, most HAs resemble normal liver microscopically, being composed of virtually normal hepatocytes in cords that are one to two cells thick and separated by sinusoids lined by inconspicuous Kupffer cells. However, no normal portal tracts are present, and there is a notable lack of any biliary epithelium. The third key feature of HA is the presence of haphazardly distributed arteries and thin-walled veins ( Fig. 20-4 ). Hepatocellular rosettes (pseudoglands) should not be confused with bile ducts.

Hepatic adenoma. Normal portal structures are lacking, and there is no biliary epithelium. The hepatocytes are bland. Haphazardly arranged parenchymal vessels are characteristic.

The hepatocytes of HA frequently display clear or vacuolated cytoplasm due to accumulation of glycogen or fat. Cytoplasmic inclusions representing α ₁ -antitrypsin, megamitochondria, and Mallory’s hyalin is occasionally found. Focal cellular pleomorphism and prominent nucleoli may occasionally be seen, especially in tumors associated with androgenic steroid use. Mitoses, vascular invasion, and stromal invasion are not seen in HA and are suggestive of malignancy. Various degenerative changes in HA include dilated sinusoids and larger blood-filled (pelioid) spaces, a myxoid stroma, and areas of necrosis, infarct, and hematoma.

A new classification of HAs has been proposed based on genotype-phenotype correlation. Based on two molecular criteria (presence of HNF1α or β-catenin mutation) and one histologic criterion (presence or absence of an inflammatory infiltrate), HAs can be subclassified into four categories: (1) HAs with mutations of the HNF1α gene, (2) HAs with β-catenin gene mutation, (3) tumors without β-catenin or HNF1α mutations with inflammatory features, and (4) lesions showing no mutation and no inflammatory component. Tumors in group 1 are the most common (30% to 50% of adenomas) and histologically show prominent steatosis without cellular atypia or inflammation. Type 2 tumors are more common in males, tend to show cytologic abnormalities and acinar growth pattern, and may show areas of either borderline lesion between HA and HCC or unequivocal HCC. Type 3 tumors are characterized by presence of inflammatory infiltrates (especially around portal tractlike structures) and telangiectatic areas ( Fig. 20-5 ). Most lesions previously classified as “telangectatic FNHs” are now classified as inflammatory/telangectatic adenoma, in that their molecular features and clinical behavior (propensity to bleed if not resected) mimic those of adenomas rather than FNHs. Finally, type 4 lesions comprise a minority of lesions and lack distinctive features.

A, Portal tractlike structures with chronic inflammatory infiltrate and dilated (telangectatic) sinusoids are characteristic of telangectatic adenomas. B, Portal tract like structures may contain poorly formed ductules ( arrow ) but lack normal, well-formed bile ducts.

The term adenomatosis is sometimes used for those cases with multiple (usually more than 10) adenomas. Because the etiologic associations appear to be similar for multiple and solitary adenomas, distinction between these two groups is probably not warranted on clinicopathologic grounds.

Fine-needle aspiration biopsy

FNA biopsy shows a single population of benign hepatocytes and absence of other cellular elements such as bile duct epithelium and fibrous tissue.

Gross findings

Macroregenerative and dysplastic nodules are usually similar to other cirrhotic nodules, although they may be paler or more bile stained. A thick fibrous capsule or variegated appearance is more suggestive of small HCC. MRNs are larger than other cirrhotic nodules and may measure up to 5 cm or more in diameter ( Fig. 20-6 ). High-grade DNs resemble MRNs grossly but may appear less well circumscribed.

The macroregenerative nodule in this cirrhotic liver ( arrow ) is distinguishable from the background cirrhosis only by its larger size. The variegated nodule of similar size is a small hepatocellular carcinoma.

Microscopic findings

Most of the MRNs in cirrhosis are multiacinar, containing more than one portal tract scattered throughout the nodule. The hepatocytes within these nodules are identical to those in the surrounding liver. The hepatocellular plates are one or two cells thick. Prominent bile ductular reaction may be seen in the adjacent fibrous scar. The reticulin framework is intact, as in a typical cirrhotic nodule.

In DNs, the hepatocytes typically display nuclear density more than twice normal. Occasional foci of small cell change (small cell dysplasia; Fig. 20-7 A), characterized by smaller size and a greater N:C ratio than the surrounding hepatocytes, are seen. Other focal architectural and cytologic abnormalities include pseudoglands, large cell change (see Fig. 20-7 B), rare mitoses, and isolated liver plates that are three cells thick. Portal tracts may be structurally abnormal. Increased iron deposition is sometimes noted.

A, Dysplastic nodules often exhibit small cell change ( arrow ), a dysplastic change that is recognized by the presence of densely packed hepatocytes smaller than those of the surrounding liver. B, In large cell change, the hepatocytes are enlarged, with atypical, irregular nuclei and large nucleoli. Aberrant architecture of hepatocellular carcinoma is absent.

Gross findings

Several macroscopic patterns of HCC are reported. HCC arising in a noncirrhotic liver usually grows as a single large mass, with or without satellite nodules, and may also exhibit this pattern in the setting of cirrhosis ( Fig. 20-8 A). However, tumors arising in cirrhosis often grow as numerous smaller nodules (diffuse type) that may be difficult to distinguish from the background liver (see Fig. 20-8 B). Tumor nodules are soft, bile stained, or yellow to tan; they often appear variegated due to foci of hemorrhage and necrosis (see Fig. 20-6 ). Separate tumor nodules may represent multicentric growth or may represent tumor spread via intrahepatic vascular routes. Macroscopic portal vein, hepatic vein, and bile duct invasion are present in some cases (see Fig. 20-8 C). Involvement of the inferior vena cava, sometimes with extension into the right atrium, may be found.

A, Variegated appearance and greenish color (due to the presence of bile) are characteristic of hepatocellular carcinoma (HCC). Satellite nodules are often present. B, In the cirrhotic liver, nodules of HCC may be difficult to distinguish from cirrhosis. C, Invasion of large veins is common.

Microscopic findings

Hepatocellular carcinoma typically displays obvious hepatocellular differentiation, with arrangement of cells resembling hepatocytes in a trabecular pattern outlined by sinusoids. Most HCCs have scant stroma. Bile production by neoplastic cells is pathognomonic of HCC but should not be confused with bile production by trapped non-neoplastic hepatocytes. The presence of bile canaliculi is also diagnostic. Cytoplasmic inclusions such as Mallory’s hyalin, α ₁ -antitrypsin inclusions, and fat are frequently present, often in focal areas of the tumor or in individual tumor nodules. Vascular lakes may simulate peliosis hepatis. The World Health Organization classification divides tumors into well-, moderately, poorly, and undifferentiated grades. Although most tumors are moderately differentiated, more than one histologic grade is often present within a given tumor.

Four major histologic patterns are described and are frequently found together in the same tumor. Only the fibrolamellar type (FL-HCC) appears to have prognostic significance. The trabecular pattern consists of trabeculae that vary in thickness and are surrounded by sinusoids lined by flattened endothelial cells and Kupffer cells ( Fig. 20-9 A). In the compact or solid pattern, compression of broad trabeculae forms sheets with inconspicuous sinusoids (see Fig. 20-9 B). The pseudoglandular (acinar) variant, with dilated rounded spaces rounded by cytologically malignant hepatocytes, may be mistaken for adenocarcinoma (see Fig. 20-9 C). The lumen may contain bile. The rare scirrhous variant displays prominent desmoplastic stroma (see Fig. 20-9 D). Pleomorphic (giant cell), clear cell, and sarcomatoid variants are rare subtypes (see Figs. 20-9 E and F).

A, Trabecular architecture, with markedly thickened cell plates lined by endothelial cells, is the most common microscopic pattern in HCC. B, Pseudoglandular pattern may be prominent in HCC and may lead to an erroneous diagnosis of adenocarcinoma. Note the absence of a desmoplastic background. C, Solid (compact) pattern. D, Cytoplasmic clearing of neoplastic hepatocytes due to glycogen accumulation (clear cell pattern). E, Prominent bands of collagen are seen in the rare scirrhous pattern. Note that the cytologic features of fibrolamellar-hepatocellular carcinoma (FL-HCC) are not present. F, Pleomorphic pattern of HCC showing markedly enlarged, hyperchromatic nuclei, which were present throughout this lesion.

Ultrastructural findings

Electron microscopy is rarely indicated, but demonstration of canaliculi or bile may be useful.

Fine-needle aspiration biopsy

Aspirates of HCC are generally highly cellular, and the tumor cells are polygonal with centrally placed hyperchromatic nuclei and variably prominent nucleoli. The N:C ratio is typically increased, varying with the degree of differentiation. Naked tumor cell nuclei are common and should not be confused with lymphoma. Intranuclear cytoplasmic invaginations and the various cytoplasmic deposits (bile, hyaline globules, Mallory’s hyalin), described in the section on histologic features, can be identified in cytologic preparations. Peripheral rimming of the tumor cell clusters by endothelial cells ( Fig. 20-10 ) is a helpful feature.

Hepatocellular carcinoma. Aspirates are hypercellular; large clusters of neoplastic hepatocytes rimmed by endothelial cells are common.

Immunohistochemistry

The most useful immunohistochemical markers for HCC are canalicular staining pattern with antibodies to CD-10 and certain antibodies to carcinoembryonic antigen (CEA), as well as positivity for hepatocyte (HepPar-1). Immunostaining with polyclonal anti-CEA antiserum or certain monoclonal CEA (m-CEA) antibodies that cross-react with canalicular biliary glycoprotein 1 demonstrates a canalicular pattern in up to 80% of HCCs ( Fig. 20-11 A). Positive staining with antisera to AFP is very suggestive of HCC (see Fig. 20-11 B). HepPar-1, a relatively hepatocyte-specific monoclonal antibody that reacts with a hepatocyte epitope that is resistant to formalin fixation and tissue processing, produces a granular cytoplasmic staining pattern in HCC and normal hepatocytes (see Fig. 20-11 C). Positive staining for α ₁ -antitrypsin and alpha-1-antichymotrypsin is nonspecific.

A, A canalicular staining pattern with cross-reactive (polyclonal) antibodies to carcinoembryonic antigen (shown) and CD10 is characteristic of HCC. Note that both normal liver ( upper right ) and HCC ( lower left ) show the typical canalicular pattern. B, Immunohistochemistry for α-fetoprotein is patchy; only approximately 50% of HCC will stain for this marker. C, Hepatocyte antibody (HepPar-1) has a granular cytoplasmic staining pattern in HCC. Normal liver ( upper left ) shows similar pattern. D, Cytoplasmic and membranous pattern of glipican-3 reactivity in HCC. No staining is seen in normal liver ( right ).

Glypican-3 (an oncofetal protein) immunohistochemistry has also been shown to be a useful marker. Glypican-3 has a high sensitivity for poorly differentiated HCCs and can be used in this situation to differentiate the latter from cholangiocarcinomas and metastatic lesions (see Fig. 20-11 D). This marker is also expressed by most well-differentiated HCCs (although sensitivity may be lower than in poorly differentiated lesions). Positive staining is not seen in benign hepatocytic neoplasms.

Cytokeratin (CK) profiles are not particularly useful in diagnosis of HCC. In situ hybridization for albumin messenger RNA appears to be highly sensitive but is not widely used. In practice, many investigators currently use a panel of p-CEA or CD10 (canalicular pattern), m-CEA, HepPar-1, and AFP antibodies for diagnosis of HCC.

Gross findings

Most FL-HCCs arise in the left lobe, but large tumors may affect both lobes. The tumors are usually solitary, firm, and well circumscribed and may be bile stained, necrotic, or hemorrhagic. They are typically large, measuring up to 25 cm. A central stellate scar similar to that seen in FNH may be present ( Fig. 20-12 ). The adjacent non-neoplastic liver is unremarkable.

Fibrolamellar carcinoma (FL-HCC) is usually a single mass; note the noncirrhotic adjacent liver and the central stellate scar.

Microscopic findings

The tumor cells of FL-HCC are very large, display abundant granular eosinophilic cytoplasm (due to large numbers of mitochondria) and distinct cell borders, and have a distinctive oncocytic appearance ( Fig. 20-13 A). A defining feature of FL-HCC is the presence of parallel bands of hyalinized fibrous tissue separating the tumor cells into nests (see Fig. 20-13 B). Cytoplasmic eosinophilic hyaline globules, which may be periodic acid-Schiff (PAS) positive diastase resistant, may represent α ₁ -antitrypsin. Cytoplasmic pale bodies contain fibrinogen and are present in about 50% of cases.

A, The cells of FL-HCC are very large and polygonal, with abundant granular pink cytoplasm (oncocytes). Central nucleus with prominent nucleolus is also characteristic. B, Dense collagen bundles are arranged in parallel layers.

Ultrastructural findings

The cytoplasm contains numerous mitochondria, as expected from the oncocytic appearance of the tumor cells. Dense core neuroendocrine-like granules may also be found.

Gross findings

HB is typically a solitary mass most often located in the right lobe. Tumors may be quite large, measuring up to 20 cm. Purely epithelial HB is a soft, fleshy, tan-white mass ( Fig. 20-14 ); those with a prominent mesenchymal component are often firm and may be calcified. Cystic degeneration, necrosis, and hemorrhage may be seen.

Hepatoblastoma (HB) is a single large mass; epithelial tumors are soft and fleshy.

Microscopic findings

HBs may be classified as either epithelial or mixed epithelial-mesenchymal and are subclassified into six patterns.

• 1.
  

  Fetal pattern (30%). In this pattern the hepatocytes are uniform and similar to normal hepatocytes, although they have a slightly higher N:C ratio. An alternating light and dark pattern is characteristic on low power ( Fig. 20-15 A). Mitoses are typically rare (less than 2 per 10 high-power fields). The tumor cells form cords two to three cells thick, separated by sinusoids, in a pattern reminiscent of normal liver. However, normal structures such as portal tracts, bile ducts, and bile ductules are absent.

  
  

  
  

  
  

  

  
  

  
  

  A, The fetal pattern of HB exhibits alternating areas of light- and dark-staining tumor cells showing hepatocellular differentiation. B, Osteoid is the most common mesenchymal element and may be abundant following chemotherapy.

  
  
  
  
• 2.
  

  Embryonal pattern (20%). The tumor cells in the embryonal pattern are less differentiated and cohesive compared with those of the fetal pattern and have a higher N:C ratio and coarser chromatin. The trabecular pattern is not well developed, and the tumor cells may form sheets. Mitoses and necrosis are more common than in the fetal pattern.

  
  
• 3.
  

  Macrotrabecular pattern (3%). This pattern is characterized by broad trabeculae 10 or more cells thick. Trabeculae may be composed of embryonal or fetal cells.

  
  
• 4.
  

  Small cell undifferentiated pattern (3%). This pattern is the least differentiated pattern and resembles other small blue cell tumors of childhood. The tumor is composed of loosely cohesive sheets of uniform small cells with scanty cytoplasm. The tumor cells are positive for CK.

  
  
• 5.
  

  Mixed epithelial and mesenchymal pattern (44%). This pattern is characterized by areas of both epithelial and mesenchymal differentiation. The primitive mesenchymal component may be primitive, with spindle or stellate cells with little cytoplasm, or display differentiation along chondroid and rhabdomyoblastic lines. Osteoid is the most frequent heterologous element and may be more prominent after chemotherapy (see Fig. 20-15 B). The mesenchymal elements may be CK positive, suggesting sarcomatoid metaplasia of the epithelial component.

  
  
• 6.
  

  A subtype of mixed pattern tumors displays teratoid features and may contain elements of mature teratoma such as keratinized squamous epithelium, intestinal epithelium, and neuroectodermal structures.

Fine-needle aspiration biopsy

FNA biopsy yields highly cellular smears with cohesive nests and sheets of small atypical hepatocytes. The aspirate may contain a spindle cell or mesenchymal component.