Fig. 5.1
Lipid-rich adenoma in a 69-year-old man. Unenhanced CT image shows a left adrenal mass (arrow) which is measured 1 HU, consistent with a lipid-rich adenoma
Subsequent studies have shown that percentage loss of iodine contrast material is useful characterizing a lipid-poor adenoma at early and delayed contrast-enhanced CT examinations (Fig. 5.2). This concept is based on that adenoma shows early wash-in and washout of contrast material. This is why adenoma is histologically hypervascular and why adenoma does not retain contrast material for delayed time periods, as opposed to most of non-adenoma. Washout values can be calculated using a few different methods, with most institutes adopting non-contrast images, early contrast-enhanced images (usually 1 min after contrast material injection), and delayed contrast-enhanced images (at 15 min). Absolute percentage washout (APW) and relative percentage washout (RPW) are calculated using the attenuation values that are measured to place a region of interest (ROI) on the lesion at the different phases of contrast enhancement.
Fig. 5.2
Lipid-poor adenoma in a 57-year-old woman. (a) The signal intensity of a right adrenal mass (arrow) does not show any difference between in-phase (left-side) and opposed-phase (right-side) MR images. Adrenal-to-spleen ratio is 1.2 and signal intensity index is 4.2 %. Accordingly, MR diagnosis of the lesion is non-adenoma. (b) The right adrenal mass (arrow) is measured 35 HU, 129 HU, and 62 HU on unenhanced (right-side image), early enhanced (middle), and delayed (left-side image) enhanced CT images, respectively. Absolute and relative washout values are calculated 71 % and 52 %, respectively. Accordingly, CT diagnosis of the lesion is adenoma
Generally, formulas for calculating APW or RPW are as follows: APW = [early contrast-enhanced CT (HU) − delayed contrast-enhanced CT (HU)] x 100/[early contrast-enhanced CT (HU) − unenhanced CT (HU)]. RPW = [early contrast-enhanced CT (HU) − delayed contrast-enhanced CT (HU)] x 100/[early contrast-enhanced CT (HU)]. APW of 60 % or more or RPW of 40 % or more offers confident diagnosis of adenoma, with sensitivity ranging from 88 to 96 % and specificity ranging from 96 to 100 %.
Chemical shift MR imaging uses the different precession frequencies of protons in both water fat within the same voxel and creates in-phase and opposed-phase images in which signal from the protons is either additive or subtractive from one another (Fig. 5.3). This MR technique is useful in detecting abundant intracytoplasmic lipid found. Regarding detection of adenoma, chemical shift MR imaging is better than unenhanced CT because this MR technique detects more adenomas that are measured more than 10 HU at unenhanced CT. Visual MR assessment provides a confident diagnosis of adenoma that is measured 10 HU or less at unenhanced CT (Fig. 5.3). However, quantitative MR assessment should be performed for characterizing adenoma that is measured between 10 and 20 HU at unenhanced CT (Fig. 5.4) [12–14]. For quantitative analysis, adrenal-to-spleen ratio (ASR) and signal intensity index (SII) are frequently used. SII is more sensitive for adenoma than ASR (Fig. 5.4) [12, 15]. ASR and SII are calculated as follows:
SIOP and SIIP indicate signal intensity on opposed-phase image and in-phase image, respectively.
Fig. 5.3
Lipid-rich adenoma in a 62-year-old man. A right adrenal mass (arrow) is hyperintense on in-phase MR image (left-side image), while the lesion shows signal void on opposed-phase MR image (right-side image)
Fig. 5.4
Lipid-rich adenoma in a 43-year-old man. Unenhanced CT image (left-side image) shows a right adrenal mass (arrow) which is measured 12 HU. CT diagnosis is not consistent with a lipid-rich adenoma. The lesion is slightly hyperintense on in-phase MR image (middle image) and slightly hypointense on opposed-phase MR image (right-side image). The adrenal-to-spleen ratio is 0.92, but the signal intensity index is 24 %. MR diagnosis is lipid-rich adenoma
Adenoma can be diagnosed if ASR is less than 0.71 or if SII is more than 16.5 %. However, if adenoma is measured more than 20 HU on unenhanced CT, the sensitivity begins to be lower (Fig. 5.2). Adenoma that is measured more than 40 HU at unenhanced CT hardly is diagnosed on chemical shift MR imaging [12, 15]. Therefore, adrenal masses that is measured more than 20 HU on unenhanced CT should be evaluated using early and delayed contrast-enhanced CT imaging rather than chemical shift MR imaging [12].
Imaging features of adenoma may overlap with those of other benign and malignant lesions, including pheochromocytomas, metastases, and cortical carcinomas [16–19]. While small size (<3 cm) of adrenal mass can suggest a benign lesion, the size criterion is not a reliable finding. Adrenal lesion’s lipid or fat which is detected on CT or MR imaging is not totally specific for adenoma because adrenal hyperplasia, adenoma with a carcinoma or metastasis focus or metastases from renal cell carcinoma, hepatocellular carcinoma, and liposarcoma have potential to contain intracytoplasmic lipid or adipose tissue [16–18, 20]. Furthermore, hypervascular pheochromocytomas, adrenal hyperplasia, and hypervascular metastasis from renal cell carcinoma or hepatocellular carcinoma may meet washout criteria of adenoma.
Adenoma may include cystic degeneration, hemorrhage, and calcification, which are frequent when adenoma grows 3 cm or more (Fig. 5.5) [16, 19]. These unusual features make it difficult to differentiate adenoma from cortical carcinoma. Right adrenal adenoma may arise from the adrenohepatic tissue and subsequently can manifest as a primary or metastatic hepatic tumor (Fig. 5.6) [21].
Fig. 5.5
Degenerated adenoma in a 47-year-old woman. Contrast-enhanced CT image shows a left adrenal mass (arrow) which is measured 3.3 cm. The central area (asterisk) of the lesion is not enhanced, but the medial aspect (arrowhead) shows crescent-like marginal enhancement. She underwent left adrenalectomy, and histologic diagnosis was adenoma with cystic degeneration and hemorrhage
Fig. 5.6
Adrenohepatic fusion adenoma in a 59-year-old woman. Unenhanced CT shows a hypoattenuating hepatic mass (arrow) which is measured 0 HU. Right adrenal limb (arrowhead) is attached to the medial aspect of the lesion. The lesion shows adenoma-like enhancement (early wash-in and washout) on early and delayed enhanced CT images (not shown). Clinically, the diagnosis is considered adenoma arising adrenohepatic fusion
5.4.1.2 Hyperplasia
The prevalence of adrenal hyperplasia is estimated to be 0.51 % in the autopsy series of approximately 3500 subjects [22]. Hyperplasia manifests as either a diffuse or focal enlargement of adrenal gland. Hyperplasia involves adrenal gland multifocally and bilaterally. A series of 113 consecutive adult necropsies reported that 35 % appear normal, 50 % mildly nodular, and 15 % distinctly nodular [23]. The mean age of patients with distinctly nodular hyperplasia was 65 years while that of normal adrenal group was 50 years. With age increasing, adrenal gland becomes nodular [23].
Hyperplasia is histologically composed of mainly lipid-rich cortical cells which form variable-sized nodules due to overgrowth. Hyperplastic nodules appear micronodular or macronodular in appearance, depending on whether or not the nodules are visible with at gross examination.
Clinically, hyperplasia often is similar to hyperfunctioning adenoma. Actively hormone-producing hyperplasia can cause pituitary- or ACTH-independent Cushing syndrome or hyperaldosteronism [5].
Hyperplasia is detected at CT or MR imaging if the hyperplastic nodules are apparently large. At unenhanced CT, hyperplasia appears frequently hypodense nodules which are measured 10 HU or less because of lipid-rich cells as it were adenoma [16] (Fig. 5.7). Less frequently, hyperplasia appears isodense compared to adjacent normal adrenal tissue, which tends to be atrophic due to decreased ACTH. Accordingly, it is not unusual that hyperplasia shows signal drops on opposed-phase MR imaging (Fig. 5.7). In addition, hyperplastic nodules may show high washout which meets criteria for adenoma (Fig. 5.7) [16]. For these reasons, hyperplasia is not easy to differentiate from multiple adenomas clinically, radiologically, and histologically.
Fig. 5.7
Macronodular hyperplasia in a 53-year-old man. (a) Unenhanced CT image (right-side image) shows bilateral adrenal hypodense masses (arrows) which are measured less than 10 HU. Early (middle image) and delayed (left-side image) enhanced images show adenoma-like enhancement with APW ≥ 60 % and RPW ≥ 60 %. (b) Bilateral adrenal masses are hyperintense on in-phase MR image and hypointense on opposed-phase MR image with ASR < 0.71 and SII > 16.5 %. Histologic diagnosis was macronodular hyperplasia after percutaneous biopsy
5.4.1.3 Hemorrhage
Adrenal hemorrhage may result from various conditions including trauma, burn, sepsis, surgery, hypotension, and anticoagulant therapy [5, 24]. Trauma is the most common cause to adrenal hemorrhage [24, 25]. Adrenal hemorrhage involves unilateral or bilateral adrenal glands.
Adrenal hemorrhage may occur in adenoma, myelolipoma, pheochromocytoma, metastasis, and cortical carcinoma, which may be spontaneously ruptured due to excessive bleeding [26].
Usually, adrenal hemorrhage is not symptomatic and is mostly an incidental finding at CT or MR imaging. Symptom is flank or back pain that ranges from mild to severe pain. It depends on the degree of hemorrhage. However, adrenal insufficiency is rare even if adrenal hemorrhage is bilateral [25, 27].
On CT images, adrenal hemorrhage appears a round or oval mass in which the attenuation value depends on the stage of hemorrhage (Fig. 5.8). Less commonly, adrenal hemorrhage may manifest as extensive suprarenal hematoma that obliterates the gland entirely [5, 28]. More recent hemorrhage appears relatively hyperdense. Over time, the size and attenuation value of adrenal hemorrhage decreases. In most cases, adrenal hemorrhage spontaneously disappears. Calcification may develop in the late stage of adrenal hemorrhage (Fig. 5.9).
Fig. 5.8
Adrenal hematoma in a 28-year-old man. Unenhanced CT image shows a hyperdense right adrenal mass which is measured 62 HU. He underwent right nephrectomy due to autosomal dominant polycystic kidney disease (asterisk)
Fig. 5.9
Adrenal hematoma in a 29-year-old man. Unenhanced CT image (right-side image) shows subacute hematoma (arrow) in the right adrenal gland. The lesion (22 HU) is more hyperdense than stomach fluid (asterisk). The adrenal hematoma (arrow) is decreased and calcified (arrowhead) on 2 years later CT image (left-side image)
On MR imaging, the signal intensity of adrenal hemorrhage also depends on the stage of hemorrhage. Early hemorrhage tends to be isointense on T1-weighted images and hypointense on T2-weighted images. Intermediate hemorrhage tends to be hyperintense on both T1- and T2-weighted images due to the paramagnetic effects of methemoglobin. Finally, adrenal hemorrhage shows a T1- and T2-hypointense rim due to formation of fibrous capsule and deposition of hemosiderin, resulting in a variable degree of susceptibility artifact on gradient-echo images [24, 25, 29].
5.4.1.4 Cyst
Most of adrenal cysts are incidentally detected at CT or MR imaging that is performed for unrelated reason. Usually, these lesions are asymptomatic but infrequently cause discomfort or mild pain because of mass effect to adjacent organs. Pathologically, adrenal cysts consist of endothelial cyst, epithelial cyst, pseudocyst, and parasitic cyst [30]. Almost all adrenal cysts appear a unilocular cyst with fine wall and simple fluid [31]. However, pseudocysts may have a minimally thick cyst wall and complicated fluid resulting from internal hemorrhage or fluid-fluid level [31, 32].
At CT, adrenal cyst usually shows a well-demarcated, non-enhancing, hypoattenuating lesion with simple water attenuation (<20 HU) and a thin wall (Fig. 5.10) [31]. Sometimes, internal hemorrhage or calcification can be detected. Wall calcification is seen more often in the pseudocysts and parasitic cysts, while septum calcification is present in endothelial cysts [31].
Fig. 5.10
Adrenal cyst in a 35-year-old woman. Contrast-enhanced CT image shows a left adrenal cyst (arrow) with thin wall and simple fluid. An arrowhead indicates the other left adrenal tissue
At MR imaging, adrenal cysts show a well-demarcated, non-enhancing, thin-walled lesion which is uniformly hypointense on T1-weighted images and hyperintense on T2-weighted images similar to those of simple cyst [29]. Hemorrhagic fluid within an adrenal cyst may show T1 hyperintense signal [29]. The imaging appearances of an echinococcal adrenal cyst may include a simple cyst, a multilocular cyst with septa, daughter vesicles, the “water lily” sign, and calcifications depending on the stage of infection [31, 33]. Detecting extra-adrenal Echinococcus is useful in making a confident diagnosis of echinococcal cysts, since isolated adrenal involvement is rare [33].
5.4.1.5 Myelolipoma
Adrenal myelolipoma is a benign tumor histologically composed of adipose and myeloid tissues. The prevalence of myelolipoma is estimated 0.08–0.2 % in autopsy series [35]. Usually, these tumors are hormonally inactive and are asymptomatic. In rare incidence, discomfort or pain may occur due to mass effect or internal hemorrhage when myelolipoma grows to a large size.
At CT, myelolipoma is easily diagnosed because gross fat (less than −30 HU) is almost always visible (Fig. 5.11). Calcification can be detected in approximately 24 % [36]. At MR imaging, myelolipomas show hyperintense foci on T1-weighted images due to gross fat. These T1 hyperintense signals are suppressed on fat-saturated MR images [1, 5, 6, 29]. However, internal hemorrhage may obliterate adipose tissue so that hemorrhagic myelolipoma is not easy to diagnose.
Fig. 5.11
Myelolipoma in a 67-year-old man. Contrast-enhanced CT image shows a left adrenal myelolipoma (arrow) in which attenuation value is measured as low as −54 HU. An arrowhead indicates the other left adrenal tissue
5.4.1.6 Pheochromocytoma
Pheochromocytoma is a catecholamine-producing tumor arising from chromaffin cells in the adrenal medulla. This tumor is closely related to extra-adrenal sympathetic and parasympathetic paraganglia, so-called as extra-adrenal pheochromocytoma. Pheochromocytomas typically produce and secrete both norepinephrine and epinephrine. Norepinephrine is usually the predominant catecholamine. The prevalence of pheochromocytoma is still unknown.
Widespread use of CT or MRI increases detection of asymptomatic pheochromocytoma. Therefore, the number of small or asymptomatic pheochromocytoma appears greater than that of large or symptomatic pheochromocytoma [5]. The number of pheochromocytoma with classical triad symptoms becomes fewer. Biochemical test should be performed to detect subclinical or asymptomatic pheochromocytoma or other hormone-producing adrenal tumors.
Pheochromocytomas are a typically firm and gray-white tumor that may have areas of central degenerative changes including fibrosis, hemorrhage, and cystic change.
CT is the first-line modality for detecting, localizing, and characterizing pheochromocytoma. These tumors are round or oval masses of similar attenuation to the back muscles at unenhanced CT [37]. Scattered or punctate calcifications can be detected in approximately 10 % [17, 38]. The size of pheochromocytoma becomes smaller because of early detection. These tumors usually are measured 3–5 cm in diameter when they are detected but can grow to 10 cm or more mostly in symptomatic cases [5, 17, 26]. Degenerated pheochromocytoma may show hyperdense texture on unenhanced CT due to hemorrhage or poor enhancement due to cystic or necrotic change on contrast-enhanced CT. When the lesion is small, it can show adenoma-like enhancement due to strong enhancement on early enhanced CT (Fig. 5.12).
Fig. 5.12
Pheochromocytoma in a 49-year-old woman. (a) Contrast-enhanced CT image shows a left pheochromocytoma (arrow) with non-enhancing degeneration area (asterisk). (b) I123 MIBG scan shows a hypermetabolic mass (arrow) which is well correlated with CT findings
At MR imaging, the classic sign “light bulb” on T2-weighted image is not so frequently encountered because it can be seen in case of a pheochromocytoma that contains at least more than one half of cystic or necrotic change [17, 39]. Majority of pheochromocytomas show slightly high signal intensity compared to adjacent adrenal tissue on T2-weighted images [5]. Also, hyperintense foci due to cystic or necrotic changes can be encountered within the tumor on T2-weighted images. These masses are homogeneous or heterogeneous depending on internal components of calcification, fibrosis, necrosis, hemorrhage, and cyst degeneration [39].
CT or MR imaging features of pheochromocytoma are overlapped with adenoma and cortical carcinoma. Pheochromocytoma is usually so hypervascular that this tumor cannot be easily differentiated with adenoma regarding washout of contrast material [16, 17, 40]. When the size of pheochromocytoma is less than 3 cm, this tumor is a homogeneous solid mass because of rare cystic or necrotic changes. From this point of view, small pheochromocytoma may meet the criteria for washout of adenoma. When pheochromocytoma becomes larger, this tumor contains frequently hemorrhage, calcification, cystic or necrotic change, and fibrosis. These imaging appearances often overlap with those of large adenoma (≥3 cm) and cortical carcinoma [17, 19, 41, 42].
Pheochromocytoma shows a wide range of imaging features, which is so-called an imaging chameleon [43]. Radiologists are kept in mind that pheochromocytoma should be included for the differential diagnosis of masses with high washout lesions or dominant cystic change, depending on the clinical and laboratory findings.
5.4.1.7 Hemangioma
Hemangioma is a rare adrenal tumor which is discovered incidentally on CT or MRI. The main histologic types of this tumor are capillary and cavernous hemangiomas. Cavernous hemangioma is more frequent than capillary hemangioma. These tumors tend to be highly vascular. Unenhanced CT shows these tumors are seen well-demarcated hypoattenuating solid masses [44]. Phleboliths or calcification is characteristic imaging features for diagnosis. Hemangioma is hypointense on T1-weighted images and hyperintense on T2-weighted images. Hemorrhage or necrosis increases signal intensity on T1-weighted image. Contrast-enhanced CT or MRI shows peripheral nodular enhancement with or without delayed central filling.
5.4.1.8 Lymphangioma
Lymphangioma is an exceedingly rare adrenal tumor which is discovered incidentally. CT shows a thin-walled non-enhancing cystic lesion with near-water attenuation. Mild enhancement can be identified in the cyst wall or thin septa. MRI shows the tumor is hypointense on T1-weighted images and hyperintense on T2-weighted images [29]. A multilocular cyst with thin septa and water-like fluid is most suggestive of a lymphangioma.
5.4.1.9 Schwannoma
Schwannoma is a benign nerve sheath tumor which is rarely discovered in the adrenal gland [45]. This tumor appears as a heterogeneously enhancing hypoattenuating mass at CT (Fig. 5.13). Adrenal schwannoma is isointense on T1-weighted images and hyperintense on T2-weighted images compared to muscle. However, these MR imaging features are nonspecific, as lesion signal intensity depends on the degree of degenerative change [45]. Accordingly, the differential diagnoses include pheochromocytoma and malignant tumors.
Fig. 5.13
Schwannoma in a 36-year-old man. Contrast-enhanced CT image shows a left adrenal schwannoma (arrow) which is heterogeneously enhanced. An arrowhead indicates the residual left adrenal tissue
5.4.1.10 Ganglioneuroma
Ganglioneuroma is a benign neurogenic tumor arising along the sympathetic nerve or adrenal gland. This lesion is more frequent in the mediastinum or retroperitoneum than in the adrenal medulla. Adrenal ganglioneuroma accounts for 20–30 % of all cases [46].
Unenhanced CT shows the lesion is so hypoattenuating that the attenuation value is measured <40 HU [47]. Punctate or discrete calcifications can be seen. Early enhanced CT shows a hypoattenuating mass while delayed enhanced CT shows that the lesion is hyperattenuating as compared to muscle due to persistent enhancement (Fig. 5.14a) [48]. Characteristically, the lesion grows by wrapping vessels without obstruction [49]. MRI shows the lesion is hypointense on T1-weighted images, hyperintense on T2-weighted images (Fig. 5.14b), and gradual or persistent enhancement on contrast-enhanced images.
Fig. 5.14
Ganglioneuroma in a 30-year-old man. (a) A left ganglioneuroma (arrow) is measured 12 HU, 21 HU, and 34 HU on unenhanced (right-side), early enhanced (middle), and delayed enhanced (left-side) CT images. The lesion shows gradual and persistent enhancement. (b) T2-weighted MR image shows that the lesion (arrow) is hyperintense
5.4.1.11 Adenomatoid Tumor
Adenomatoid tumor is a rare benign adrenal lesion of mesothelial origin. Contrast-enhanced CT shows the lesion is well-circumscribed and heterogeneously hypoattenuating. Calcification may be present. MRI shows lesion signal intensity is variable on T1-weighted, T2-weighted, and contrast-enhanced images. Differential diagnoses include pheochromocytoma and adrenocortical carcinoma [50].
5.4.1.12 Oncocytoma
Oncocytoma is a rare adrenal tumor which is histologically benign but clinically malignant in some cases [51, 52]. CT or MRI shows the lesion is a well-demarcated mass with heterogeneous enhancement [52]. Differential diagnoses include adrenocortical carcinoma, lymphoma, or lipid-poor myelolipoma. Generally, the lack of vascular invasion or lymphadenopathy argues against the former diagnoses.
5.4.1.13 Adrenal Infection
Adrenal infection is caused by fungus, bacteria, mycobacteria, parasite, and virus. Hydatid involvement shows the classic appearance of a primary cyst with daughter cysts internally. Histoplasmosis and paracoccidioidomycosis are the most common fungal infections [3]. Less common infections are blastomycosis, coccidioidomycosis, and cryptococcosis. Typically, bilateral adrenal enlargement is a common feature [53]. Lung infection can precede detection of adrenal involvement.
Tuberculosis also can affect both adrenal glands. CT shows decreased central attenuation due to necrosis and calcifications.
Adrenal abscess formation occurs rarely and is usually unilateral. Bacteria such as Escherichia coli, group B streptococcus, and Bacteroides spread hematogenously and may be associated with hemorrhage. So, this infection appears as adrenal tumor [7]. Immunocompromised patients can have cytomegalovirus (CMV) infection affecting the adrenals [54]. However, CMV infection is difficult to detect because of normal-appearing adrenal gland at cross-sectional imaging [55].
5.5 Malignant Adrenal Tumors
5.5.1 Cortical Carcinoma
Cortical carcinoma is an uncommon malignant adrenal tumor [56, 57]. This lesion shows a biphasic two peak incidences in early childhood and middle ages [57]. It is sporadic or associated with hereditary syndromes including Li-Fraumeni syndrome, Beckwith-Wiedemann syndrome, Carney complex, congenital adrenal hyperplasia, and multiple endocrine neoplasia (MEN) type 1 [58].
Clinically, cortical carcinoma may show abdominal pain, back pain, weight loss, or early satiety depending on cancer stage. This lesion may result in Cushing syndrome, hyperaldosteronism, or virilization or feminization due to overproduction of steroids [56, 57].
Usually, cortical carcinoma is detected as a large tumor which is superior to the kidney. Necrosis, hemorrhage, or calcification is frequent. This lesion shows typically unilateral involvement. Large cortical carcinoma tends to locally spread into adjacent organs, regional lymph nodes, and the renal vein or IVC [56, 57].
At US, a small cortical carcinoma shows homogeneous echotexture, while a large cortical carcinoma shows heterogeneous echotexture due to hemorrhage and necrosis. Both hyperechoic and hypoechoic regions may be present. US is good to determine whether or not adjacent organs are invaded because of real-time imaging.
Unenhanced CT shows cortical carcinoma is measured greater than 10 HU [19]. Contrast-enhanced CT show heterogeneous lesion enhancement due to necrosis, hemorrhage, or calcification (Fig. 5.15a) [19]. Generally, a large cortical carcinoma shows lower relative (<40 %) and absolute (<60 %) percentage washouts than adenoma [19]. However, small cortical carcinoma may show high relative percentage washout (≥40 %) [19]. Tumor thrombus within the renal vein or IVC is not uncommon. Local invasion, regional and para-aortic lymphadenopathy, and distant metastases to the lungs, liver, and bones are common initial presentations of large cortical carcinoma [59].
Fig. 5.15
Cortical carcinoma in a 47-year-old woman. (a) Contrast-enhanced CT shows a left cortical carcinoma (arrow) which is heterogeneously enhanced due to multifocal necrosis. An asterisk indicates a hepatic hemangioma. (b) T2-weighted MR image shows the lesion (arrow) contains multifocal hyperintense foci due to necrosis. An asterisk indicates a hepatic hemangioma
MRI shows cortical carcinoma is hypointense on T1-weighted images and hyperintense on T2-weighted images (Fig. 5.15b). Areas of hemorrhage appear hyperintense foci on T1-weighted images [59]. Occasionally, this lesion may contain small areas that lose signal on out-of-phase images [60]. Therefore, this feature alone cannot completely exclude the diagnosis of cortical carcinoma. Cortical carcinoma shows a heterogeneous enhancement and slow washout at contrast-enhanced MRI.
A heterogeneously enhancing large adrenal mass is one of important features for identifying cortical carcinoma. However, the high likelihood of cortical carcinoma is only 70 % although adrenal tumor is measured more than 6 cm [59, 61]. This is why there is some overlap between large adenoma (≥3 cm) and cortical carcinoma in terms of lesion size [19]. Therefore, CT or MRI features can provide more confident differentiation of adenoma from cortical carcinoma than lesion size.
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