Imaging of uncommon tumors of the pancreas

Radiol Clin N Am 40 (2002) 1273 – 1287

Imaging of uncommon tumors of the pancreas Sheila Sheth, MD, Elliot K. Fishman, MD* The Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins Medical Institutions, 601 North Caroline Street, JHOC-3250, Baltimore, MD 21287, USA

Although ductal adenocarcinoma represents approximately 90% of all tumors arising in the pancreas, neoplasms originating outside the exocrine ductal epithelium are rare. These include tumors arising from the neuroendocrine cells of the islets of Langerhans (islet cell tumors [ICTs]), lymphomas (usually the non-Hodgkin’s B-cell type), and metastases to the gland. Mesenchymal neoplasms of the pancreas are exceedingly rare. This article reviews the radiological manifestations of these uncommon tumors, with emphasis on their CT appearance.

Islet cell tumors Islet cell tumors (ICTs) are rare neoplasms arising from neuroendocrine cells in the pancreas or the periampullary region. Despite their rarity— an incidence of five cases per million persons per year is reported in the literature [1]—they present a special challenge for the radiologist. The diagnosis of functioning ICT is almost always established biochemically, when the lesion is of small size. Successful curative surgical resection is greatly facilitated by preoperative imaging depicting the precise location and number of lesions [1]. Nonfunctioning ICTs usually manifest themselves when they have reached a large size. There has been a continuing debate in the literature about the ideal

* Corresponding author. E-mail address: [email protected] (E. Fishman).

imaging modality for patients with a suspected ICT. Although invasive imaging techniques—such as selective celiac and mesenteric arteriography, venography, and venous sampling—are progressively being abandoned, gadolinium-enhanced MRI, somatostatin receptor imaging, and endoscopic ultrasound have emerged as potentially competing or complementary techniques to CT [2 – 4]. The difficulty in assessing the accuracy of these different modalities is compounded by the rarity of ICT and the small size of individual series. Because of its widespread availability and continuing technical improvements that allow for constant improvement in image quality, dual-phase helical CT remains the dominant imaging modality for the diagnosis of all pancreatic neoplasms, including ICT, in many centers. Clinical presentation Islet cell tumors are classified as functioning if they produce symptoms related to excessive hormone production, or nonfunctioning. In one large series of 125 patients who underwent surgical resection in a tertiary referral center, approximately half of the ICTs were functioning and 52% of the tumors were malignant [1]. These neoplasms tend to affect a younger age group than the more common adenocarcinoma of the exocrine pancreas.

Functioning ICTs Functioning ICTs are subdivided according to the hormone they produce (Table 1). Insulinomas are the most common, usually benign, and often quite small at

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Table 1 Clinical presentation in patients with functioning islet cell tumors

nomas are uncommon, representing 5% or less of all ICTs [1].

Functioning islet Clinical cell tumor presentation

Laboratory abnormalities

Insulinoma

Low fasting plasma glucose Hyperinsulinemia

Nonfunctioning ICTs Nonfunctioning ICTs usually reach a large size before the patient experiences symptoms of abdominal pain, weight loss, or jaundice. Signs related to extensive hepatic or nodal metastases may dominate the clinical picture when the tumors are malignant. With the proliferation of high-quality cross-sectional imaging studies, however, an increasing number of small, asymptomatic ICTs are being discovered serendipitously [5]. Although the majority of ICTs are sporadic, an increased prevalence of these tumors is seen in patients with von Hippel Lindau disease and those affected by Multiple Endocrine Neoplasia type I [6].

Gastrinoma

Glucagonoma Vipoma

Hypoglycemic attacks Atypical seizures Peptic ulcers Diarrhea, malabsorption Necrolytic migratory erythema Watery diarrhea, flushing

Elevated serum gastrin Hyperglycemia Hypokalemia, achlorhydria Elevated plasma peptide

diagnosis. Patients experience symptomatic intractable hypoglycemia, low blood levels of glucose, and high-level circulating plasma insulin. Gastrinomas are the second most common functioning ICTs; patients with this type of ICT present with symptoms of peptic ulcer disease, abdominal pain, or diarrhea. About 60% of gastrinomas are malignant. The demonstration of gastric hypersecretions and an elevated serum gastrin level confirm the diagnosis of Zollinger-Ellison syndrome. Other functioning tumors such as vipoma, glucagonomas, stomatostatinomas, and corticotropi-

Treatment and outcome Complete surgical removal of the tumor offers the only hope for definitive cure. Depending on the location and size of the lesion, surgical options available include enucleation of small ICT or distal pancreatectomy or pancreaticoduodenectomy for large or malignant tumors involving the pancreatic head or periampullary region [1]. Even when malignant, ICTs are slow growing and relatively indolent, with reported 5-year survival of 49% to 56%, justifying aggressive management even when the tumor has spread beyond the pancreas [1,7]. In patients with liver metastases, surgical removal of

Fig. 1. 65-year-old man with history of recurrent pancreatitis, elevated serum amylase, and lipase. Distal pancreatectomy confirmed the diagnosis of small, nonfunctioning islet cell tumor. (A) Axial CT image in the arterial phase of enhancement shows an 8-mm hypervascular mass in the distal body of the pancreas (arrow). The mass is clearly hyperattenuating to the normal pancreatic parenchyma. (B) Axial CT image at the same level in the venous phase of enhancement shows the lesion (arrow) to be slightly less conspicuous.

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the primary tumor combined with resection or transcatheter embolization of hepatic metastases prolongs survival [8].

CT appearance CT protocol for suspected ICTs There is convincing evidence in the literature that there is a small window of opportunity to achieve maximal tumor to pancreas attenuation differences and that dual-phase scanning is necessary to adequately eval-

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uate pancreatic lesions and achieve optimal enhancement of the surrounding vascular structures [9,10]. In our protocol, the entire pancreas is imaged using a 4  1.0 mm collimator setting to obtain 1.25-mm slices reconstructed at 1-mm intervals. Using a power injector, 120 mL of iohexol (Omnipaque 350; Nycomed Amersham, Princeton, NJ) is injected intravenously at a rate of 3mL/second. For the arterial phase, scanning is initiated following a 25-second delay from the time of initiation of contrast material injection. The liver and pancreas are imaged from the diaphragm to the inferior edge of the liver. This

Fig. 2. 34-year-old woman with a history of peptic ulcer disease, diarrhea, and an elevated serum gastrin level. The patient underwent a pylorus preserving pancreaticoduodenectomy and a gastrinoma was confirmed at histology. (A) Axial CT image in the arterial phase of enhancement shows a 2-cm homogeneous mass (arrow) in the uncinate process of the pancreas. The lesion was hyperattenuating to the normal pancreas. (B) Axial CT image at the same level in the venous phase of enhancement: the degree of enhancement within the mass has decreased. Note the thickened duodenal folds (arrowhead). (C) Axial CT image at the level of the gastric fundus shows thickened gastric folds (arrows).

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Fig. 2 (continued ).

technique is used to maximize the detection of potential hypervascular hepatic metastases from ICT. At the same time, the pancreas is imaged in the late arterial/ early parenchymal phase to optimize the detection of the primary pancreatic mass. Subsequently, venous phase imaging of the entire liver and pancreas is

initiated after a scan delay of 50 seconds from the time of initiation of contrast material injection. We use water as an oral contrast agent to optimize visualization of potential small periampullary masses and perform CT angiography and three-dimensional reconstruction for surgical planning.

Fig. 3. 16-year-old girl presenting with hypoglycemic seizures. The diagnosis of insulinoma was confirmed at endoscopic biopsy and surgery. The mass was found to be adherent to the splenic hilum, and the patient underwent a distal pancreatectomy and a splenectomy. (A) Axial CT image in the arterial phase of enhancement shows a 2-cm mass in the tail of the pancreas (arrow). The mass is not enhancing prominently. (B) Axial CT image at the same level in the venous phase of enhancement: the lesion is hypoattenuating to the normal pancreatic parenchyma. This appearance is not typical for an islet cell tumor. (C) Coronal oblique reconstruction image shows the mass to be intimately apposed to the spleen.

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Fig. 3 (continued).

Enhancement pattern of ICTs at dual-phase helical CT Because of their rich vascular supply, ICTs classically are hyperattenuating compared with the surrounding pancreatic parenchyma on contrast-enhanced CT. This characteristic tumor blush of ICTs allows for their differentiation from other pancreatic neoplasms—particularly adenocarcinomas, which are hypovascular

lesions and almost invariably of lower attenuation then the normal gland, regardless of the phase of enhancement used for image acquisition. Capturing the vascular blush is essential for the diagnosis of small tumors, which often do not distort the contour of the pancreas (Fig. 1) [11,12]. This is particularly true in the investigation of functioning insulinomas, because these are often very small, with

Fig. 4. 67-year-old-woman with history of abdominal pain. The patient underwent a distal pancreatectomy and splenectomy and histologic analysis revealed a malignant islet cell tumor. (A) Axial CT image in the arterial phase of enhancement shows a 7-cm heterogeneous hyperattenuating mass in the tail of the pancreas (arrow). Note the central low attenuation area (arrowhead). (B) Axial CT image at the same level in the venous phase. The enhancement within the tumor is less pronounced and the central necrosis is nearly imperceptible.

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Fig. 4 (continued ).

50% measuring less then 1.3 cm2. Nonfunctioning ICTs often reach a large size before they become symptomatic and thus easily are detected by the mass effect they produce [13]. Some nonfunctioning ICTs are also small at diagnosis, however, either because they are strategically located, obstructing the biliary tree or the pancreatic duct, or if they are found incidentally [5]. Several studies have emphasized the value of dual-phase helical CT for the detection of small ICTs. Appropriate timing of image acquisition is critical, but it may be difficult to predict which phase of enhancement will best depict a particular lesion. Because ICTs are hypervascular, tumor conspicuity is expected to be greater in the arterial phase. The mean attenuation difference between the lesion and the normal parenchyma is often greater during this phase than in the venous phase [14]. The attenuation of ICTs relative to the surrounding pancreas changes on different phase acquisitions, however. Several patterns of enhancement have been described in the literature and confirmed by our own observations. The classic and most common enhancement pattern of ICTs is that of a hyperattenuating mass in the arterial and venous phase. Many small lesions enhance more prominently and thus are easier to detect in the arterial phase (Fig. 2). In a series of 11 cases of functioning ICTs reported by Van Hoe and colleagues [15], most lesions were hyperattenuating and two were more conspicuous on arterial phase imaging. King and co-authors [16] reported similar results. Five of seven small ICTs reported by Chung et al [17] were only depicted in the arterial phase. In

their protocol, however, the delayed phase was initiated particularly late, after 180 seconds. Small, nonfunctioning ICTs have similar enhancement characteristics, with tumor conspicuity also reported to be better on the arterial phase [14]. Careful analysis of the venous phase is essential because some tumors are better seen in this phase. In their review of 26 ICTs, Ichikawa and colleagues [18]

Fig. 5. 56-year-old woman with history of abdominal pain and calcifications in the left upper quadrant seen on plain radiograph. Axial CT image shows a 6-cm mass in the tail of the pancreas (arrow). The tumor is mildly hyperattenuating to the normal pancreas and coarse peripheral calcifications are present. A benign islet cell tumor with extensive hyalinization and ossification was found at surgery.

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found no statistically significantly higher tumor-topancreas contrast in the arterial phase compared with the venous phase. In their series, venous phase imaging was in fact superior for tumor detection, perhaps because they encountered several lesions exhibiting delayed enhancement and best seen or only apparent in the portal venous phase [18]. In addition, some tumors have an atypical enhancement pattern and appear hypo- or isoattenuating to the normal parenchyma on one or both imaging sequences (Fig. 3) [14,15]. Small ICTs are usually solid and homogeneous. Larger lesions, which are usually non-functioning, often demonstrate a heterogeneous enhancement pattern [12,19]. Many contain central areas of necrosis and cystic degeneration (Fig. 4), or calcifications (Fig. 5) [13].

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Staging of malignant ICTs at dual-phase CT Large tumors with diameters over 5 cm are frequently malignant [3,5]. As portions of the tumor may become isoattenuating on the venous phase, lesion size is often larger on the arterial phase [14]. In addition to local extension and encasement of the major peripancreatic arteries and veins, the liver and regional lymph nodes are the most common sites for metastases. Like the primary tumor, liver metastases are hypervascular. Arterial phase imaging demonstrates the number and size of the hepatic lesions better than images acquired in the venous phase, particularly for small metastases. In the series reported by StaffordJohnson et al [14], 17 hepatic metastases were identified in the arterial phase, but only 9 were seen in the venous phase (Fig. 6). Spread to regional

Fig. 6. 62-year-old man with history of abdominal pain. Percutaneous biopsy of one of the liver lesions confirmed the diagnosis of metastatic well-differentiated islet cell tumor. (A) Axial CT image in the arterial phase of enhancement shows an ill-defined, lobulated, heterogeneous, mildly enhancing, 3.5-cm mass in the head of the pancreas (arrow). The mass is abutting the portal confluence and the superior mesenteric vein is not seen. Mildly enhancing peripancreatic nodes are present (open arrows). Several enhancing liver lesions are also seen (arrowheads). (B) Axial CT image in the venous phase of enhancement shows the pancreatic head mass (arrow) and the peri-pancreatic adenopathy (curved arrows). The liver lesions, however, have become imperceptible. (C) Coronal reconstruction in the venous phase confirmed occlusion of the superior mesenteric vein (arrow).

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Fig. 7. 40-year-old man with a pancreatic mass discovered on MRI performed for vague abdominal pain. (A) Axial CT image in the arterial phase of enhancement: no pancreatic mass was identified prospectively. In retrospect, a subtle area of higher attenuation (arrow i) than the surrounding pancreas may represent the tumor. (B) Axial CT image in the venous phase of enhancement. (C) Endoscopic ultrasound of the pancreas followed by biopsy confirmed the presence of a small hypoechoic islet cell tumor (ICT) (arrow). (Courtesy of M. Canto, Baltimore, MD.) (D) Intraoperative ultrasound shows the small hypoechoic lesion (arrow), which was proven to represent a 1-cm ICT at distal pancreatectomy.

lymph nodes also appears more conspicuous in the arterial phase [14]. As expected, arterial encasement is best depicted on the arterial phase and venous involvement is more readily appreciated on the venous phase. Both are exquisitely demonstrated on three- dimensional CT angiographic mapping, which is essential for preoperative planning (see Fig. 3). Sensitivity of dual-phase CT in the diagnosis of ICTs The reported sensitivity of CT in localizing functioning islet cell tumors (FICT) varies from 71% to 82% [12,15,18]. The difficulty in interpreting sometimes-conflicting data is compounded by

the small size of individual series; the variety of CT equipment used, including older, conventional CT scanners; and variation in scanning protocols. As expected, small tumors are more frequently missed (Fig. 7) [19]. Endoscopic ultrasound is an excellent alternative imaging technique with reported sensitivity and specificity of 93% and 95%, respectively [20].

Lymphoma Lymphoma, predominantly the non-Hodgkin’s Bcell subtype, involves the pancreas secondarily in

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approximately 30% of patients with widespread disease. It usually spreads to the pancreas by direct extension from peripancreatic lymphadenopathy. Primary involvement of the pancreas is uncommon, representing 2% to 5% of cases of extranodal lymphomas. Many of these cases occur in patients with immunocompromised hosts, particularly patients infected with HIV [21,22].

jaundice and hyperbilirubinemia are uncommon, even with large lesions involving the pancreatic head [24]. With current chemotherapeutic regimen, the prognosis for pancreatic lymphoma is more favorable then for adenocarcinoma, with cure or remission rates of up to 30% reported.

Clinical presentation

Pancreatic lymphoma generally appear as homogeneous soft tissue masses, showing little enhancement after intravenous contrast administration. Intrinsic involvement of the pancreas may be difficult

The most common clinical manifestation of pancreatic lymphoma is abdominal pain [23]. Obstructive

Appearance at CT

Fig. 8. 46-year-old man with history of abdominal pain and history of pancreatitis. The patient underwent a pancreaticoduodenectomy and the pathology specimen revealed primary pancreatic B-cell lymphoma. (A) Axial CT image in the arterial phase of enhancement shows a 5-cm heterogeneous hypoattenuating mass in the head of the pancreas. (B) Axial CT image in the venous phase of enhancement. There is mild compression but no invasion of the portal vein. No significant adenopathy is seen. The absence of any significant dilatation of the pancreatic duct or the biliary tree would be atypical for an adenocarcinoma and suggests the possibility of lymphoma.

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Fig. 9. 19-year-old man presenting with abdominal and back pain. Contrast-enhanced axial CT image shows a diffusely enlarged and heterogeneous pancreas (arrows). Percutaneous biopsy of large retroperitoneal nodal mass (not shown) yielded Burkitt’s-like B-cell lymphoma.

to differentiate from lymphoma affecting the peripancreatic lymph nodes. Two distinct CT patterns have been described, including focal and circumscribed single or multiple masses (Fig. 8) and diffuse enlargement of the gland by an infiltrating tumor (Fig. 9) [25]. This latter appearance can be associated with involvement of the peripancreatic fat and can mimic acute

pancreatitis at CT. Encasement of the peripancreatic vessels may occur, but dilatation of the pancreatic duct is uncommon, despite the presence of bulky tumor, a helpful distinguishing feature from adenocarcinoma (see Fig. 8). The presence of associated lymphadenopathy below the level of the renal veins also favors the diagnosis of lymphoma [24]. If the imaging findings

Fig. 10. 69-year-old man presenting to the emergency room with leg pain due to metastatic squamous cell lung cancer to the femur. Contrast-enhanced axial CT image shows a solitary well-defined hypoattenuating mass in the head of the pancreas (arrow). Multiple liver metastases are seen (arrowheads).

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are suggestive of lymphoma, a definitive diagnosis can be achieved by percutaneous or endoscopic biopsy, averting unnecessary surgery [26].

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creas. In these latter cases, surgical resection may prolong survival for these patients [29]. Appearance at CT

Pancreatic metastases Metastases to the pancreas from a variety of primary malignancies are being diagnosed more frequently with the increasing use of CT in oncology. These mestastases usually develop late in the course of the disease and are often associated with widespread metastatic disease. Clinical presentation The majority of patients with pancreatic metastases do not have organ specific symptoms, although abdominal pain, jaundice, weight loss, or symptoms of acute pancreatitis may be present in some cases [27]. Among primary malignancies with a propensity to spread to the pancreas, renal cell carcinoma was the most common in one large series, followed by bronchogenic carcinoma, breast and colon cancer, melanoma, and soft tissue sarcomas [28]. The interval between initial diagnosis of the primary tumor and the detection of pancreatic metastases varies from a few months to several years. Specifically, patients with a history of renal cell carcinoma can present 5 to more than 10 years later, sometimes with isolated involvement of the pan-

Metastases do not show any predilection for a specific area of the pancreas. The majority of metastases present as large solitary masses with welldefined margins (Fig. 10) [28,30]. Other patterns include multifocal lesions (Fig. 11) and diffuse infiltration of the gland. Dilatation of the pancreatic duct by an obstructing metastasis can occur in up to one third of cases, obstruction of the biliary tree is less common, and encasement of the major peripancreatic vascular structure is rare. The enhancement pattern is variable and often mimics the enhancement characteristic of the primary tumor [30,31]. Many lesions, as they grow to a significant size tend to exhibit heterogeneous enhancement. Pancreatic metastases from hypervascular malignancies, particularly renal cell carcinoma display enhancement characteristics similar to the primary tumor (Fig. 12) and may mimic an ICT [32].

Rare malignant tumors of the pancreas Case reports or small series of rare malignant tumors of the pancreas are scattered through the imaging literature.

Fig. 11. 59-year-old woman presenting with brain metastases due to small cell lung cancer. Contrast-enhanced axial CT image shows two well-defined hypoattenuating masses in the pancreas (arrows).

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Fig. 12. 63-year-old man with history of left nephrectomy for renal cell carcinoma 6 years prior to this CT. (A) Contrastenhanced axial CT image in the arterial phase shows a 2-cm hypervascular mass in the body of the pancreas (arrow). The patient underwent distal pancreatectomy and the pathology specimen confirmed the presence of a single metastasis from renal cell carcinoma. (B) Venous phase: the lesion is not as well seen.

Pancreatoblastoma This is a very unusual tumor of acinar cell origin that affects infants and young children. Most commonly, it involves the body and tail of the gland and presents as a large heterogeneous multicystic mass at CT [33,34].

metastatic fat necrosis. These tumors tend to reach a large size at presentation and frequently undergo cystic degeneration or central necrosis (Fig. 13). An association with elevated serum alpha-fetoprotein levels has been reported [35].

Mesenchymal tumors of the pancreas Acinar cell carcinoma Acinar cell carcinoma tends to affect older patients and may present with bone and skin lesions caused by

Mesenchymal tumors represent 1% to 2% of pancreatic neoplasms. These unusual tumors arise from the connective, vascular, and neuronal tissue

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Fig. 13. 55-year-old woman with history of abdominal pain. Contrast-enhanced axial CT image shows a 4-cm heterogeneous mass (arrow) with a central low attenuation area. The tumor appears adjacent but extrinsic to the pancreatic head. Note the mild dilatation of the pancreatic duct (arrowhead). The patient underwent a pancreaticoduodenectomy. At surgery, the mass appeared to originate from the posterior portion of the pancreatic head. The pathology specimen revealed an acinar tumor of the pancreas.

in the pancreas and are classified according to their main histologic component.

usually asymptomatic, detected incidentally at abdominal CT, and appear as well-circumscribed fat attenuation masses within the pancreatic parenchyma.

Lymphangioma Lymphangiomas—benign tumors of the lymphatic system—most commonly manifest as a neck or axillary mass in a pediatric patient. In the abdomen, these lesions are usually found in the retroperitoneum or the mesentery. Pancreatic lymphangiomas are exceedingly rare and can grow quite large before they become symptomatic [34]. These tumors are composed of multiloculated cystic masses surrounded by a thin fibrous capsule and containing serous, serosanguineous or chylous fluid. At histology, dilated lymphatic channels of various sizes are separated by thin septa [36]. At CT, lymphangiomas present as a multilocular cystic mass containing fine enhancing septations in or adjacent to the pancreas [36]. Calcifications are rare. These lesions cannot be distinguished from pseudocysts, hydatid cysts, and cystadenomas and cystadenocarcinomas preoperatively. Complete surgical excision is curative. Fat-containing tumors Lipoma Few cases of pancreatic lipomas have been reported in the imaging literature [37,38]. They are

Teratoma Mature cystic teratomas arise from pluripotential cells and contain elements of all three germ layers in

Fig. 14. 57-year-old woman with history of abdominal pain. Contrast-enhanced axial CT image shows a 6-cm low-attenuation mass in the body of the pancreas (arrow). Fine enhancing septations are present. The patient underwent a pancreaticoduodenectomy and the pathology specimen revealed an intrapancreatic schwannoma with cystic degeneration.

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varying amount, including cystic and solid components, sebaceous material, hair, teeth, calcium, and skin appendages. Extragonadal teratomas are rare, and location in the pancreas is exceedingly unusual. CT feature of a heterogeneous mass containing a fat/ fluid level with or without calcifications is pathognomonic [39].

[10]

[11]

Tumors of neural origin Rare cases of pancreatic schwannomas and plexiform neurofibromas associated with von Recklinghausen neurofibromatosis have been reported in the literature [34,40,41]. The CT appearance of schwannomas may be related to the proportion of organized cellular spindle cell component (Antoni type A area) and loose hypocellular component (Antoni type B area) within the mass [34]. The CT appearance varies from a homogeneous enhancing mass to a heterogeneous lesion with central low-attenuation necrosis (Fig. 14).

[12]

[13]

[14]

[15]

[16]

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