DOI: 10.1148/rg.254045167
RadioGraphics 2005;25:949-965
© RSNA, 2005
Pancreatic and Peripancreatic Diseases Mimicking Primary Pancreatic Neoplasia1
Katherine J. To’o, BS,
Steven S. Raman, MD,
Nam C. Yu, MD,
Young Jun Kim, MD2,
Tyler Crawford, MD,
Barbara M. Kadell, MD and
David S. K. Lu, MD
1 From the Department of Radiology, David Geffen School of Medicine at UCLA, 10833 Le Conte Ave, BL-428 CHS, Box 951721, UCLA Medical Center, Los Angeles, CA 90095-1721. Presented as an education exhibit at the 2003 RSNA Annual Meeting. Received August 26, 2004; revision requested September 23 and received November 19; accepted November 23. All authors have no financial relationships to disclose.
Address correspondence to S.S.R. (e-mail: sraman{at}mednet.ucla.edu).
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Abstract
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A variety of anatomic variants and pathologic conditions in and around the pancreas may simulate primary pancreatic neoplasia at routine abdominal cross-sectional imaging. An ambiguous lesion whose appearance suggests a pancreatic origin requires a broad differential diagnosis that can subsequently be narrowed on the basis of both clinical history and features at optimal computed tomography (CT) and magnetic resonance (MR) imaging. Pancreas-specific multidetector CT and MR imaging techniques with thin collimation, multiplanar and multiphasic scans, and newly introduced curved planar reformation may help avoid potential diagnostic pitfalls. These techniques can help identify and characterize a mass in multiple viewing planes, thereby helping distinguish a true pancreatic neoplasm from peripancreatic adenopathy or from a tumor of the adjacent duodenum or small bowel. They can also help determine the cause of a tumor. It is important that the radiologist be familiar with the wide spectrum of anatomic variants and disease entities that can mimic primary pancreatic neoplasia in order to initiate the appropriate lesion-specific work-up and treatment and avoid unnecessary tests or procedures, including surgery.
© RSNA, 2005
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LEARNING OBJECTIVES
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After reading this article and taking the test, the reader will be able to:
- Describe the differential diagnoses of various lesions that may mimic primary pancreatic neoplasia.
- Identify associated clinical and imaging features that are helpful in identifying and characterizing these lesions.
- Discuss the imaging techniques that can help distinguish between these lesions and primary pancreatic neoplasia.
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Introduction
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Computed tomography (CT), magnetic resonance (MR) imaging, and ultrasonography (US) (transabdominal, endoscopic, intraoperative) have revolutionized the diagnosis of pancreatic and peripancreatic disorders. However, the complex anatomic relationships of a variety of structures in the upper abdomen (both peritoneal and retroperitoneal) have given rise to diagnostic challenges in that a variety of benign and neoplastic processes often mimic primary pancreatic neoplasia. Knowledge of the many processes that mimic primary pancreatic cancer (Table) and of the imaging techniques that aid in their differentiation, such as pancreas-specific CT and MR imaging with thin-section, multiplanar, and multiphasic scans helps avoid diagnostic mishaps.
In this article, we review the CT, MR imaging, US, and positron emission tomographic (PET) techniques for imaging of the pancreas. We also discuss and illustrate the broad array of entities that may mimic primary pancreatic neoplasia, including normal anatomic variants, inflammatory and infectious diseases of the pancreas, peripancreatic nodal enlargement and lymphoma, vascular lesions, disease in surrounding structures, metastases to the pancreas, and miscellaneous pancreatic diseases.
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Pancreatic Imaging Techniques
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Computed Tomography
Although pancreatic tumors may be evaluated with a variety of imaging modalities, the most significant advances in pancreatic imaging have come as a result of improvements in CT and then in MR imaging. Pancreatic imaging with multidetector helical CT allows very thin collimation and multiphase acquisition (arterial, pancreatic, and portal venous phases), with the subsequent ability to view the data with a variety of postprocessing techniques, such as multiplanar reformation or volume rendering (1). One particular advantage of multidetector CT is the ability to obtain high-quality off-axis scans, which can be especially helpful in evaluating the surrounding vessels. Curved planar reformatted images allow single two-dimensional (2D) image display of structures that run through multiple oblique planes, as well as visualization of the pancreatic and common bile ducts in their entirety (2).
Our protocol for pancreas-specific CT is as follows: Patients generally fast for 3 hours before the examination and drink 1 L of low-density oral contrast material such as water prior to scanning, with another smaller oral bolus of water (300 mL) immediately before image acquisition. Precontrast images (5-mm reconstruction interval) are obtained through the abdomen to localize the pancreas. Nonionic iohexol (Omnipaque 350; Amersham Health, Princeton, NJ) is power injected at a rate of 3–4 mL/sec using the bolus tracking feature. The aorta is monitored starting 10 seconds after injection until a trigger threshold (150 HU above baseline) is reached. Thin-collimation pancreatic phase volumetric scans are obtained 70 seconds after injection, followed by thin-collimation portal venous phase scans. True arterial phase scans are obtained for evaluation of vascular lesions, such as islet cell tumors. Three-dimensional (3D) images are created and reviewed on a workstation with multiplanar and volumetric postprocessing capability.
MR Imaging
Development of faster MR imaging systems has significantly expanded the role of MR imaging in the evaluation of pancreatic lesions (3). MR imaging provides unparalleled tissue contrast with the advent of high-performance 1.5T and 3T scanners and breath-hold imaging protocols, as well as the potential for high-resolution isotropic voxels if necessary. MR imaging is particularly useful as an adjunct to CT in the imaging of pancreatic neoplasms that are difficult to identify, such as small, non-organ-deforming pancreatic ductal adenocarcinomas and islet cell tumors. Other indications for MR imaging include the detection of choledocholithiasis, pancreatic duct calculi, or ductal cholangiocarcinomas and further evaluation of cases of pancreatic enlargement (eg, early autoimmune pancreatitis) (4). MR imaging has the unique capacity to allow noninvasive evaluation of the pancreatic ducts and their relationship to the common bile duct (5). MR cholangiopancreatography can be performed rapidly with fast 2D and 3D sequences to provide high-resolution visualization of the extrahepatic biliary tract and pancreatic duct. Some authors have stressed the value of secretin administration in providing more detailed depiction of the pancreatic duct at MR cholangiopancreatography.
At our institution, MR imaging is performed on a 1.5T or 3T scanner with phased array coils. General axial and coronal 2D and 3D spoiled gradient-echo T1-weighted imaging is performed. In-phase and out-of-phase T1-weighted images are acquired along with fat-saturated T1-weighted images through the abdomen and with thin-collimation (2–3-mm) images through the pancreas. T2-weighted imaging includes axial and coronal half-Fourier rapid acquisition with relaxation enhancement (RARE) and single-shot fast spin-echo T2-weighted sequences, along with fat-saturated breath-hold fast spin-echo T2-weighted sequences. With either a test injection or a bolus tracking algorithm, dynamic 2D and 3D gradient-echo T1-weighted acquisitions are performed through the pancreas with thin collimation. Three-dimensional acquisition, although yielding somewhat "noisy" (grainy) images, allows beautiful postprocessing images that display precise relationships to major vessels. Thin- and thick-slab 2d and 3D or half-Fourier low-angle shot MR cholangiopancreatography may be easily performed to evaluate ductal anatomy.
Ultrasonography
The diagnostic role of conventional transabdominal US in the evaluation of pancreatic lesions is limited. It is mainly used for screening patients with abdominal pain, assessing the gallbladder and bile ducts, and assessing flow in cystic lesions. However, new advances in US equipment, such as color power Doppler sequences, harmonics, and compound imaging have improved visualization of the pancreatic head and body in many patients. Endoscopic US has significantly improved visualization of small tumors and provides a means of sampling small lesions.
Endoscopic US can often detect small pancreatic lesions that are undetectable at routine CT and MR imaging. Endoscopic US allows high-resolution imaging of cystic tumors of the pancreas and offers two means of diagnosis: morphologic imaging and US-guided fine-needle aspiration. The success of conventional and endoscopic US is greatly dependent on operator skill, and its use is based on local expertise.
PET and CT-PET
PET is complementary to cross-sectional imaging techniques such as CT and MR imaging in patients with suspected pancreatic carcinoma at initial presentation and allows detection of unsuspected distant metastases, assessment of tumor viability, and monitoring of tumor response to treatment, especially in nonmucinous tumors. One study has demonstrated encouraging results with fusion of multidetector CT and PET in pancreatic lesions, a combined modality that demonstrates improved sensitivity in differentiating between benign and malignant pancreatic lesions without significant change in specificity (6).
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Anatomic Variants
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Normal anatomic structures and anomalies of the foregut are common entities that may simulate primary pancreatic cancer. The normal fourth portion of the duodenum or proximal jejunum borders the pancreatic tail. Unopacified adjacent bowel (Fig 1 ) or a redundant or underdistended gastric fundus may simulate a mass. Such cases can be easily clarified with the use of positive oral contrast material at CT or of water at MR imaging. Variations in the lateral contour of the pancreatic head and neck are common and may also mimic pancreatic masses (7). Typically, the lobules are isoattenuating relative to the adjacent parenchyma at contrast material–enhanced CT and show normal pancreatic attenuation on both arterial and portal venous phase images. Anatomic distortions from prior surgery, including nephrectomy, Whipple procedure, and Puestow procedure (Fig 2), may also appear as pancreatic masses due to unopacified afferent loops or pancreatic displacement (8,9).
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Figure 1a. Normal unenhanced duodenum mimicking a pancreatic mass. (a) CT scan shows findings that suggest enlargement of the head of the pancreas (*). (b) Repeat CT scan obtained with additional oral contrast material shows a normal pancreatic head, with contrast material in the duodenum (arrow) and gallbladder (*).
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Figure 1b. Normal unenhanced duodenum mimicking a pancreatic mass. (a) CT scan shows findings that suggest enlargement of the head of the pancreas (*). (b) Repeat CT scan obtained with additional oral contrast material shows a normal pancreatic head, with contrast material in the duodenum (arrow) and gallbladder (*).
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Pancreas divisum, which arises from aberrant fusion of the embryologic dorsal and ventral pancreatic ducts, is the most common congenital anatomic variant of the pancreas, with a prevalence approaching 10% in the general population. Its clinical relevance is most often discussed in the context of recurrent pancreatitis; rarely, however, it may cause enlargement of the pancreatic head and be mistaken for a mass (10). Annular pancreas is the second most common congenital pancreatic anomaly and results in pancreatic tissue partially or completely encircling the second part of the duodenum. A diagnosis of annular pancreas may be suspected if barium studies show narrowing at the level of the major papilla. Endoscopic retrograde cholangiopancreatography (ERCP) or noninvasive MR cholangiopancreatography can be performed to delineate the pancreatic duct encircling the duodenum (Fig 3) (11).
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Figure 3a. Annular pancreas. (a) CT scan shows pancreatic tissue encircling the duodenum (*). (b) ERCP image shows the main pancreatic duct wrapped around the endoscope (arrows). In addition, the annular pancreas causes stenosis of the common bile duct, which is distended proximally (arrowhead).
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Figure 3b. Annular pancreas. (a) CT scan shows pancreatic tissue encircling the duodenum (*). (b) ERCP image shows the main pancreatic duct wrapped around the endoscope (arrows). In addition, the annular pancreas causes stenosis of the common bile duct, which is distended proximally (arrowhead).
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Duodenal duplications (Fig 4) (12) and diverticula (13,14) may also be misinterpreted as pancreatic masses at CT or MR imaging, especially if their content is purely fluid. Although duplications in the gastrointestinal tract are rare, approximately 12% occur in the gastroduodenal region. They are usually noncommunicating and are most often located on the mesenteric side of the second and third portions of the duodenum. Affected patients typically present with symptoms of obstruction but may also develop biliary obstruction and pancreatitis. At barium examination, the duodenum usually appears compressed by a mass in the C loop, and the duplication appears as a smoothly rounded, fluid-filled cyst located in or adjacent to the wall (15). Duodenal diverticula, which may be of either the congenital intraluminal or, more commonly, the acquired extrinsic pulsion type, are incidental findings at up to 14.5% of barium examinations of the upper gastrointestinal tract. Most intraluminal diverticula are found within the second portion of the duodenum and may show the classic "wind sock" deformity at barium examination, with the contrast material–filled diverticulum projecting into the true lumen (16). However, if the diverticulum is entirely filled with fluid or other material and if it does not fill with barium, it may mimic a cystic pancreatic neoplasm. At T2-weighted MR imaging, duodenal diverticula may contain both high-and low-signal-intensity areas, which are related to the presence of fluid and gas, respectively. Careful scrutiny of images for evidence of small amounts of gas or air-fluid levels may help establish the correct diagnosis. In addition, multidetector CT and coronal volume rendering may better delineate the thin, intraluminal diverticulum sac wall and the plane of separation between the duodenum and pancreas (17).
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Figure 4a. Duodenal duplication. (a) CT scan shows a fluid-filled, circumferential duodenal duplication, which may be mistaken for a pancreatic mass on axial sections. (b) Coronal multiplanar reformatted image demonstrates the full extent of the duodenal duplication (*) adjacent to the pancreas (P).
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Figure 4b. Duodenal duplication. (a) CT scan shows a fluid-filled, circumferential duodenal duplication, which may be mistaken for a pancreatic mass on axial sections. (b) Coronal multiplanar reformatted image demonstrates the full extent of the duodenal duplication (*) adjacent to the pancreas (P).
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Splenic variants, such as accessory spleen or splenosis, may also mimic a pancreatic mass (Fig 5) (18). Accessory spleens occur in approximately 10% of the population, and intrapancreatic accessory spleens are found in the pancreatic tail in roughly 16% of these patients (19). Technetium 99m (99mTc) sulfur colloid, radiolabeled heat-damaged red blood cells, and indium 111–labeled autologous platelets may be used to differentiate splenic from pancreatic tissue (20,21). In addition, MR imaging with ferumoxides (Feridex; Berlex, Wayne, NJ) may be used to demonstrate preferential uptake in hepatic and splenic tissue, with their rich reticuloendothelial composition.
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Figure 5a. Intrapancreatic splenic rest. (a) Axial contrast-enhanced spoiled gradient-echo T1-weighted MR image shows a lesion (*) that is isointense relative to the spleen (S). P = pancreatic tail. (b) Coronal T1-weighted MR image demonstrates the pancreas wrapped around the lesion (*). P = pancreatic tail, S = spleen.
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Figure 5b. Intrapancreatic splenic rest. (a) Axial contrast-enhanced spoiled gradient-echo T1-weighted MR image shows a lesion (*) that is isointense relative to the spleen (S). P = pancreatic tail. (b) Coronal T1-weighted MR image demonstrates the pancreas wrapped around the lesion (*). P = pancreatic tail, S = spleen.
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Choledochal cysts are rare congenital malformations of the bile duct that usually manifest in infancy and childhood. They appear as cystic or fusiform dilatation of the extrahepatic biliary tree and may simulate a cystic mass in the head of the pancreas (Fig 6) (22). Hepatobiliary scintigraphy with 99mTc-isofenin can help confirm excretion into the choledochal cyst, and other studies such as ERCP, percutaneous transhepatic cholangiography, and intraoperative cholangiography can definitively demonstrate the lesion (23). MR cholangiopancreatography also allows confirmation of the diagnosis and noninvasive delineation of the anatomy (24). Biliary contrast-enhanced CT or MR imaging may also be used for this purpose.
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Figure 6a. Choledochal cyst. (a) CT scan demonstrates a dilated, redundant common bile duct simulating a cystic lesion within the pancreatic head (*). (b) ERCP image shows contrast material filling a choledochal cyst (*).
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Figure 6b. Choledochal cyst. (a) CT scan demonstrates a dilated, redundant common bile duct simulating a cystic lesion within the pancreatic head (*). (b) ERCP image shows contrast material filling a choledochal cyst (*).
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Inflammatory and Infectious Diseases of the Pancreas
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As an exocrine and endocrine gland lying at the crossroads of the enteric and biliary tracts, the pancreas is susceptible to unique inflammatory and infectious diseases. Because these pathologic conditions may mimic pancreatic malignancy at cross-sectional imaging as well as at clinical presentation, surgical resection is often required (25,26).
Chronic pancreatitis can be associated with a range of anatomic abnormalities of the pancreas, including atrophy or enlargement of the organ and ductal dilatation. When enlargement is focal with parenchymal changes due to chronic inflammation, it may be virtually indistinguishable from adenocarcinoma on the basis of morphologic features or enhancement pattern at MR imaging and CT (27). Furthermore, inflammatory changes in chronic pancreatitis may result in local lymphadenopathy and vessel involvement, raising further concern for malignancy. This problem is compounded by the fact that adenocarcinoma often develops in the setting of chronic pancreatitis, so that the two conditions may coexist. Ichikawa et al (28) have suggested that MR cholangiopancreatography may be superior to both MR imaging and CT in differentiating inflammatory pancreatic masses from carcinomas. Studies have also suggested that MR imaging with mangafodipir may improve the detection rate and characterization of focal pancreatic lesions due to improved lesion contrast (29). Complications of acute pancreatitis such as hemorrhage (30), pseudocyst formation, and peripancreatic fat necrosis (31) may also mimic a pancreatic tumor (Fig 7).
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Figure 7a. Complications of pancreatitis mimicking a pancreatic tumor. (a) CT scan demonstrates hemorrhagic pancreatitis as a heterogeneous mass in the area of the pancreatic bed (*). Arrow indicates active extravasation (hemorrhage). (b) Pancreatic phase helical CT scan obtained in a 70-year-old woman with a history of pancreatitis who presented with abdominal pain shows numerous thick-walled cystic lesions throughout the pancreas (*). The patient’s condition was initially diagnosed as an intraductal papillary mucinous neoplasm but later proved to be a complex pseudocyst at surgery.
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Figure 7b. Complications of pancreatitis mimicking a pancreatic tumor. (a) CT scan demonstrates hemorrhagic pancreatitis as a heterogeneous mass in the area of the pancreatic bed (*). Arrow indicates active extravasation (hemorrhage). (b) Pancreatic phase helical CT scan obtained in a 70-year-old woman with a history of pancreatitis who presented with abdominal pain shows numerous thick-walled cystic lesions throughout the pancreas (*). The patient’s condition was initially diagnosed as an intraductal papillary mucinous neoplasm but later proved to be a complex pseudocyst at surgery.
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Groove pancreatitis, a rare segmental chronic pancreatitis, may also mimic a pancreatic head carcinoma. This type is localized to the groove between the head of the pancreas, the duodenum, and the common bile duct. T1-weighted MR imaging may demonstrate a hypointense sheetlike mass between the pancreatic head and the thickened duodenal wall. Dynamic CT may demonstrate a poorly enhancing lesion extending between the pancreatic head and the duodenum, reflecting a fibrous mass (32).
Although autoimmune pancreatitis typically manifests with diffuse pancreatic enlargement, the focal type may manifest as a distinct mass in the pancreatic head (Fig 8) (33,34). CT features suggestive of autoimmune pancreatitis include minimal peripancreatic inflammation and absence of calcification or vascular encasement. Multiple biliary strictures and diffuse irregular narrowing of the main pancreatic duct at ERCP or MR cholangiopancreatography are other typical features (35). In the context of a coexisting autoimmune disease such as primary sclerosing cholangitis or ulcerative colitis, suspecting this diagnosis will help avoid unnecessary surgical intervention. Although exceptionally rare in the pancreas, parasitic cysts, such as Echinococcus granulosis, of the pancreas have been described (36) and may be unilocular, multilocular, or complex cystic. Because differentiation of these from other cystic masses is difficult on the basis of imaging studies alone, this diagnosis should be entertained in endemic regions.
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Figure 8a. Autoimmune pancreatitis. (a) CT scan demonstrates an enlarged, heterogeneous pancreatic head (*) with perivascular inflammation simulating neoplastic vascular involvement (arrow). (b) Coronal half-Fourier RARE T2-weighted MR image obtained in a different patient with abdominal pain demonstrates narrowing of the intrapancreatic common bile duct (arrow) by a diffusely enlarged pancreatic head without evidence of a specific mass. (c) On an axial contrast-enhanced fat-saturated spoiled gradient-echo T1-weighted MR image obtained in a third patient, the pancreatic tail is enlarged and demonstrates a hypointense rim (arrowheads).
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Figure 8b. Autoimmune pancreatitis. (a) CT scan demonstrates an enlarged, heterogeneous pancreatic head (*) with perivascular inflammation simulating neoplastic vascular involvement (arrow). (b) Coronal half-Fourier RARE T2-weighted MR image obtained in a different patient with abdominal pain demonstrates narrowing of the intrapancreatic common bile duct (arrow) by a diffusely enlarged pancreatic head without evidence of a specific mass. (c) On an axial contrast-enhanced fat-saturated spoiled gradient-echo T1-weighted MR image obtained in a third patient, the pancreatic tail is enlarged and demonstrates a hypointense rim (arrowheads).
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Figure 8c. Autoimmune pancreatitis. (a) CT scan demonstrates an enlarged, heterogeneous pancreatic head (*) with perivascular inflammation simulating neoplastic vascular involvement (arrow). (b) Coronal half-Fourier RARE T2-weighted MR image obtained in a different patient with abdominal pain demonstrates narrowing of the intrapancreatic common bile duct (arrow) by a diffusely enlarged pancreatic head without evidence of a specific mass. (c) On an axial contrast-enhanced fat-saturated spoiled gradient-echo T1-weighted MR image obtained in a third patient, the pancreatic tail is enlarged and demonstrates a hypointense rim (arrowheads).
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Peripancreatic Nodal Enlargement and Lymphoma
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Certain nodal chains, when involved in neoplastic, inflammatory, or infectious disorders resulting in lymphadenopathy, may mimic lesions of the pancreas. These nodes include portocaval, portal, peripancreatic, and celiac nodes. Bulky lymphadenopathy due to lymphoma, usually non-Hodgkin B-cell type, can occasionally mimic a pancreatic carcinoma (Fig 9) (37,38). The presence of associated lymphadenopathy below the level of the renal veins is not seen in pancreatic adenocarcinoma and favors the diagnosis of lymphoma (39). Metastatic disease from a variety of primary malignancies such as those involving the esophagus, stomach, colon, breast, adrenal gland, and kidney may also manifest with lymph node involvement in this region. Inflammatory disorders associated with lymphadenopathy include granulomatous disorders such as sarcoidosis (40) and angioproliferative disorders such as Castleman disease (41). Although nonspecific, sarcoidosis may be diagnosed if concomitant lesions are present in the liver or spleen and the typical perihilar and paratracheal adenopathy or parenchymal lung changes are present. Castleman disease may sometimes manifest with variably but intensely enhancing adenopathy. Tuberculosis of the pancreas and peripancreatic lymph nodes (42,43), although rare, may also mimic a tumor of the pancreatic head (44). Lymphadenopathy is the most common manifestation of abdominal tuberculosis and often occurs without radiologic evidence of pulmonary disease. The mesenteric, omental, and peripancreatic lymph nodes are most commonly involved. Contrast-enhanced CT classically shows enlarged, multilocular lymph nodes, sometimes having central areas of low attenuation and immediate postcontrast peripheral rim enhancement (45). The presence of low-attenuation nodes without evidence of a granulomatous infection may also suggest Whipple disease. Finally, lymphoproliferative disorders must be considered in posttransplantation patients who present with bulky lymphadenopathy (46). The presence of intact fat planes between the nodes and the pancreas and anterior displacement of the pancreas help in distinguishing peripancreatic lymphadenopathy from a primary pancreatic neoplasm.
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Figure 9. Intraabdominal lymphoma simulating a primary pancreatic mass. CT scan demonstrates an enlarged lymph node (*) adjacent to the pancreatic body and mimicking a pancreatic mass.
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Vascular Lesions
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The pancreas, although derived from the foregut, receives its rich collateral vascular supply from both the foregut and the midgut via the superior mesenteric and celiac arteries. Abnormalities such as aberrant vessels (47) or pseudoaneurysms (48,49) secondary to complicated pancreatitis or vascular surgical anastomoses may mimic a mass at routine imaging, especially if the lumen is completely thrombosed and fails to enhance at CT (Fig 10) (50). Pseudoaneurysms complicate 10% of cases of acute pancreatitis, most commonly affecting the splenic artery, although the hepatic, gastric, gastroduodenal, and pancreaticoduodenal arteries may also be involved. Spectral Doppler US may show turbulent combined venous and arterial flow within a pseudoaneurysm, whereas color flow Doppler US may show bidirectional flow ("to-and-fro sign") and a "swirling" flow pattern within the anechoic mass. Diagnostic specificity may be improved with 3D multidetector CT angiography (51). A superselective conventional angiogram may then be obtained for embolization.
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Figure 10. Superior mesenteric artery aneurysm mimicking a primary pancreatic lesion. Contrast-enhanced CT scan shows an enlarged, enhancing lumen (*) surrounded by mural thrombus in the pancreatic head. Without arterial phase imaging, these lesions may be mistaken for pancreatic masses.
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Venous structures may also be mistaken for pancreatic masses. Examples include an unenhanced portal vein at true arterial phase imaging and poorly enhanced collateral vessels in thromboses of the portal vein or inferior vena cava (Fig 11) (52). Three-dimensional gadolinium-enhanced MR angiography has been advocated for the noninvasive differentiation of the portal venous system from parenchymal lesions of the pancreas (53).
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Figure 11. Venous vascular lesion mimicking a primary pancreatic lesion. CT scan shows an occlusive portal vein thrombus (arrow). When extensive, this finding may be mistaken for a mass.
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Disease in Surrounding Structures
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A variety of lesions from surrounding structures, especially large or infiltrative lesions, can be mistaken for primary pancreatic lesions. They may originate from the retroperitoneum, mesentery, gastrointestinal tract (stomach, duodenum, small bowel, colon), kidney, adrenal gland, or nervous system. Primary retroperitoneal processes such as hematoma, fibrosis, or neoplasms may mimic a pancreatic mass (Fig 12). Some mesenteric processes (eg, retractile mesenteritis) or masses (eg, carcinoid tumors [Fig 13a], desmoid tumors [Fig 13b], sclerosing mesenteritis) may pose diagnostic difficulty unless a clear plane of separation is identified (54).
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Figure 12. Fibrohistiocytoma mimicking a pancreatic mass. CT scan shows a retroperitoneal fibrohistiocytoma (*), which was initially diagnosed as a mucinous adenocarcinoma of the pancreas.
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Figure 13a. Mesenteric tumorlike diseases mimicking pancreatic masses. (a) CT scan shows a carcinoid tumor manifesting as a heterogeneously attenuating mass with calcifications (*) adjacent to the head of the pancreas in the root of the mesentery. (b) CT scan obtained in a different patient shows a desmoid tumor as a large, ovoid mass compressing the pancreas dorsally (*). The tumors in both a and b simulate lesions of pancreatic origin.
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Figure 13b. Mesenteric tumorlike diseases mimicking pancreatic masses. (a) CT scan shows a carcinoid tumor manifesting as a heterogeneously attenuating mass with calcifications (*) adjacent to the head of the pancreas in the root of the mesentery. (b) CT scan obtained in a different patient shows a desmoid tumor as a large, ovoid mass compressing the pancreas dorsally (*). The tumors in both a and b simulate lesions of pancreatic origin.
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Relevant gastric masses include tumors of the gastric fundus or posterior gastric wall, large gastric lymphomas, and large exophytic GISTs (gastric leiomyomas, leiomyosarcomas [Fig 14]), which may develop a necrotic center and be mistaken for a cystic pancreatic neoplasm. Gastric pseudomasses may include gastric diverticula. Three-dimensional CT or MR imaging can help in better characterizing these masses and determining their origin.
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Figure 14a. Gastric GIST. (a) Initial CT scan shows findings that suggest a large mass (*) abutting the pancreatic tail. (b) Focused pancreatic CT scan shows a sulcus between the mass (*) and the pancreatic tail, a finding that suggests an extrapancreatic origin for the mass.
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Figure 14b. Gastric GIST. (a) Initial CT scan shows findings that suggest a large mass (*) abutting the pancreatic tail. (b) Focused pancreatic CT scan shows a sulcus between the mass (*) and the pancreatic tail, a finding that suggests an extrapancreatic origin for the mass.
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Many duodenal processes may mimic pancreatic disease. Neoplastic lesions such as lymphoma (Fig 15), adenocarcinoma (Fig 16), leiomyosarcoma, duodenal spindle cell sarcoma (Fig 17), Brunner gland hamartoma (55), and metastasis to the duodenum (eg, bronchogenic carcinoma) (Fig 18) may simulate primary pancreatic masses. Benign entities may include fluid- or debris-filled diverticula, hematomas, contained perforation (56), duplication cysts, and leiomyomas. Proximal jejunal loops, especially when unopacified, may simulate a lobulated or ovoid mass of the pancreatic tail. Adenocarcinoma of the small bowel and a variety of small bowel tumors may mimic pancreatic disease but can be identified with the use of an adequate amount of oral contrast material and multiplanar views. Thinner-collimation multidetector CT with coronal or off-axial 3D reformation can help detect even subtle duodenal adenocarcinoma. The tumor frequently appears as eccentric or circumferential wall thickening involving a short segment of bowel and may result in an "apple core" appearance, similar to that seen at barium examination (57). CT enteroscopy may also be useful, although, to our knowledge, its role in this setting has not yet been established.
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Figure 15a. Duodenal lymphoma. (a) CT scan demonstrates an ill-defined mass (arrowhead) involving the inferior pancreatic head and duodenum. (b) Image from an upper gastrointestinal study shows nodular mural thickening of the duodenum (arrowheads).
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Figure 15b. Duodenal lymphoma. (a) CT scan demonstrates an ill-defined mass (arrowhead) involving the inferior pancreatic head and duodenum. (b) Image from an upper gastrointestinal study shows nodular mural thickening of the duodenum (arrowheads).
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Figure 16. Duodenal adenocarcinoma. Contrast-enhanced CT scan shows an annular mass adjacent to the pancreatic head (arrow) and a filling defect in the distal duodenum (arrowhead). The lesion could be mistaken for a pancreatic mass owing to its location.
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Figure 17. Spindle cell sarcoma of the duodenum. CT scan demonstrates a lobular exophytic mass (*) simulating a pancreatic lesion. The mass later proved to be a metastatic duodenal lesion.
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Splenic tissue may sometimes be closely apposed to the pancreatic tail and simulate a primary hypervascular pancreatic mass. Other splenic masses such as lymphoma may, if diffuse, involve the pancreatic tail. Splenic diseases, including cysts, metastases, and angioma (Fig 19), may also appear to arise from the pancreas.
Large renal masses, especially those involving the upper pole of the kidney (particularly the left kidney), may simulate primary pancreatic disease (8). These masses may be solid tumors, cysts, or hematomas. Renal lymphoma may also extend to involve the pancreas.
Adrenal disease may also imitate pancreatic disease. Large lesions in the right adrenal gland most often create a diagnostic dilemma in this setting (8). In particular, large adrenal pheochromocytomas may be interpreted as pancreatic in origin. Finally, neoplasia of nerve tissue outside the adrenal gland, such as extraadrenal pheochromocytoma (paraganglioma) (Fig 20) and schwannoma, should also be considered.
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Figure 20a. Left extraadrenal paraganglioma. Axial fat-saturated fast spin-echo T2-weighted (a) and contrast-enhanced fat-saturated T1-weighted (b) MR images demonstrate the spatial relationship between a paraganglioma (*) and the pancreas. In this case, a clear plane of demarcation is noted between the lesion and the pancreas, which helps distinguish the mass from a primary pancreatic tumor.
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Figure 20b. Left extraadrenal paraganglioma. Axial fat-saturated fast spin-echo T2-weighted (a) and contrast-enhanced fat-saturated T1-weighted (b) MR images demonstrate the spatial relationship between a paraganglioma (*) and the pancreas. In this case, a clear plane of demarcation is noted between the lesion and the pancreas, which helps distinguish the mass from a primary pancreatic tumor.
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Metastases to the Pancreas
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Metastatic lesions to the pancreas are found at 3%–12% of autopsies performed in patients who died of advanced malignancies, and most patients with metastatic pancreatic lesions also had extrapancreatic metastatic disease (58). Primary tumors that commonly metastasize to the pancreas include those involving the lung, gastrointestinal tract, breast (Fig 21) (59), and kidney (Fig 22) (60), melanoma (61), lymphoma, and (osteo)sarcoma (Fig 23) (62). Metastases do not show any predilection for a specific area of the pancreas, and described patterns at CT include single localized masses, multifocal nodules, and diffuse infiltration of the gland (63). The majority of metastatic lesions appear as a large, solitary mass with well-defined margins, but cystic lesions may occur, particularly from cystadenocarcinoma of the ovary and melanoma. The enhancement pattern is variable and often mimics the enhancement characteristics of primary tumor. The use of US- or CT-guided fine-needle aspiration for the diagnosis of lesions that radiologically mimic primary cystic pancreatic neoplasms prompts appropriate clinical studies and treatment (64), thereby obviating open pancreatic biopsy. Of all tumors that metastasize to the pancreas, lymphomas, renal cell carcinomas, and melanomas cause the most difficulty in the differentiation from primary pancreatic cancer because of their tendency to form solitary pancreatic lesions, often without other identifiable metastatic sites (60). Such solitary pancreatic metastases are often virtually indistinguishable from a primary pancreatic tumor on CT scans. In general, imaging factors that may favor metastatic disease include (a) multiplicity of lesions, (b) hypervascularity (65), and (c) imaging features consistent with a specific primary tumor, such as hyperintensity of melanoma at T1-weighted MR imaging due to paramagnetic melanin content. Clinical history of a known primary malignancy would help guide a differential diagnosis.
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Figure 22. Metastatic renal cell carcinoma. Early arterial phase CT scan demonstrates a metastatic lesion in the pancreatic head (arrow), a finding that may be mistaken for a primary islet cell tumor. A renal mass arising from the left kidney with adjacent fat stranding is also seen (*).
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Figure 23a. Metastatic sarcoma. (a) CT scan shows a metastatic osteosarcoma (*) arising from the pancreatic tail. (b) CT scan obtained in a different patient demonstrates a large, metastatic myxoliposarcoma (*) in the pancreatic head. The primary lesion had previously been resected from the buttock. (c) CT scan obtained in a third patient shows a metastatic lobular soft-tissue sarcoma in the pancreatic head effacing the duodenal lumen (arrow).
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Figure 23b. Metastatic sarcoma. (a) CT scan shows a metastatic osteosarcoma (*) arising from the pancreatic tail. (b) CT scan obtained in a different patient demonstrates a large, metastatic myxoliposarcoma (*) in the pancreatic head. The primary lesion had previously been resected from the buttock. (c) CT scan obtained in a third patient shows a metastatic lobular soft-tissue sarcoma in the pancreatic head effacing the duodenal lumen (arrow).
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Figure 23c. Metastatic sarcoma. (a) CT scan shows a metastatic osteosarcoma (*) arising from the pancreatic tail. (b) CT scan obtained in a different patient demonstrates a large, metastatic myxoliposarcoma (*) in the pancreatic head. The primary lesion had previously been resected from the buttock. (c) CT scan obtained in a third patient shows a metastatic lobular soft-tissue sarcoma in the pancreatic head effacing the duodenal lumen (arrow).
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Miscellaneous Pancreatic Diseases
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Abstract
LEARNING OBJECTIVES
