DOI: 10.1148/rg.235035013
(Radiographics. 2003;23:1117-1131.)
© RSNA, 2003
Peripancreatic Masses That Simulate Pancreatic Disease: Spectrum of Disease and Role of CT1
Leo P. Lawler, MD, FRCR,
Karen M. Horton, MD and
Elliot K. Fishman, MD
1 From the Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins Medical Institutions, 601 N Caroline St, Rm 3254, Baltimore, MD 21287-0801. Presented as an education exhibit at the 2002 RSNA scientific assembly. Received January 27, 2003; revision requested March 6 and received April 18; accepted May 14. Address correspondence to L.P.L. (e-mail: efishman@jhmi.edu).
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Abstract
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A number of entities can simulate pancreatic disease at computed tomography (CT), which may lead to misdiagnosis. Common pitfalls include peripancreatic lesions of the foregut, adrenal gland, and kidney as well as disease of the mesentery and neurovascular structures. Optimal design and application of multi–detector row CT protocols with multiplanar reformation and maximum-intensity-projection and volume-rendering postprocessing improves the specificity of image interpretation. In most cases, helical CT is highly accurate for distinguishing primary disease of the pancreas from adjacent disease, although there are cases in which the differential diagnosis is more challenging and the potential for misdiagnosis still exists. Familiarity with some of the entities that can simulate pancreatic disease, careful attention to scanning protocol and contrast material administration, use of the full potential of multi–detector row CT data sets, and judicious application of postprocessing tools may help avoid some of the pitfalls caused by peripancreatic lesions.
© RSNA, 2003
Index Terms: Computed tomography (CT), multi–detector row, 77.1211 • Pancreas, anatomy, 77.92 • Pancreas, CT, 77.1211 • Pancreas, diseases
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LEARNING OBJECTIVES FOR TEST 4
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After reading this article and taking the test, the reader will be able to:
- Discuss current protocols for pancreatic multi–detector row CT.
- Describe 3D postprocessing techniques for pancreatic and peripancreatic imaging.
- Recognize entities that can simulate pancreatic disease and lead to misdiagnosis.
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Introduction
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The pancreas, which lies within the anterior pararenal space of the retroperitoneum, is intimately related to the peritoneal and retroperitoneal viscera and has a rich investment of lymphovascular structures. In patients with a moderate amount of fat in proximity to a normal pancreas, the interfaces between these lymphovascular structures are usually well defined. However, these interfaces may be more difficult to define in patients with limited intraabdominal fat and indistinct tissue planes, or in patients in whom a pathologic process disrupts normal tissue planes. Helical computed tomography (CT) is highly accurate in the detection of pancreatic and peripancreatic disease. However, over the past few years, many patients have been referred to our institution for potential resection of a pancreatic mass that subsequently proved to be extrapancreatic in nature. In addition, we have occasionally encountered similar problems in our own CT practice. To our knowledge, there is little in the radiology or surgery literature on this subject, although there are multiple case reports of misdiagnosis (1–15). Multi–detector row CT with three-dimensional (3D) postprocessing allows accurate imaging of the pancreas and peripancreatic structures and displays the anatomy and disease entity in an orientation that simulates a direct surgical approach.
In this article, we review pitfalls that we have experienced when imaging the pancreatic region and suggest ways to minimize them. We also discuss and illustrate CT protocol and applications, normal pancreatic and peripancreatic anatomy, and potential imaging pitfalls involving the foregut, lymph nodes, adrenal and renal tissue, mesentery, and lymphovascular structures.
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CT Protocols: Design and Applications
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Optimal imaging of the pancreas and peripancreatic area requires spiral CT (a type of multi–detector row CT) combined with dual-phase arterial and venous imaging (16–20). Adequate distention of the stomach and proximal small bowel with orally administered contrast material is critical in reducing misdiagnosis related to the gastrointestinal tract. We now routinely administer 1,000 mL of neutral contrast material (water) 20 minutes prior to a study, followed by an additional 250 mL of water before the start of acquisition. Neutral contrast material such as water is now preferred for 3D postprocessing with volume rendering (VR) because overlying loops may be "removed" by manipulating the attenuation value threshold (trapezoid histogram) without laborious editing to remove overlying high-attenuation bowel. Traditional positive oral contrast agents such as barium and iodinated contrast material are now reserved for routine abdominopelvic imaging or special cases and problem solving.
Whether variable or fixed array multi–detector row CT is used, the goal is to minimize section width, bearing in mind that smaller section collimation also means more image noise and slower table translation. For most current systems, this translates into a target section width of 1.25 mm with 1-mm reconstruction intervals from 1-mm detectors (19,20). Use of these parameters generates a data set that is best suited to nearly isotropic multidimensional reformation. Section widths must not be smaller than the smallest detector chosen, although larger, less noisy images may be constructed. Although section widths on the order of 1 mm are preferred, if a multisection system is not available, scanning protocols that minimize both section thickness and interscan spacing should be used, which usually means 3-mm-thick sections at 2–3-mm reconstruction intervals.
Once images are generated, they may be viewed as planar two-dimensional axial images on film or on a soft-copy workstation. Typically, the use of film is impractical for dual-phase pancreatic imaging with data sets containing 400–500 images. More recently, the concept of volume visualization (as opposed to a series of single sections) has been introduced as a method for data set display and interpretation. Although the retroperitoneal structures such as the pancreas and duodenum are relatively fixed, we have observed a high degree of individual variation in their normal anatomic appearance and configuration. Such variants can simulate disease, which may be difficult to appreciate on axial images alone. We currently use a combination of display techniques including multiplanar reformation (MPR) and 3D imaging (VR and maximum intensity projection) (Siemens Leonardo; Siemens Medical Solutions, Malvern, Pa) for both analysis and interpretation (19,21). MPR is simply a reordering of voxels into an alternate plane of display. This additional perspective is valuable but is limited in its capacity to demonstrate the pancreas and peripancreatic tissues on a single image. Maximum intensity projection projects the highest in-ray attenuation onto a single plane. VR is a segmentation technique with high fidelity to the high-quality multi–detector row CT data, preserving all contributing attenuation values in the final display. The trapezoid histogram representation of the attenuation values inherent in VR allows infinite manipulation of the contributing tissue properties of opacity, brightness, and window width and level while preserving the valuable differential enhancement seen at dual-phase contrast material–enhanced imaging (19,21). We have found that, together with the real-time editing planes and projections, VR plays a critical role in mapping the normal and collateral peripancreatic vasculature, which may be directly involved in a disease process or secondarily distorted in a manner that aids in tumor localization. Maximum intensity projection segments a portion of the data and is often used to supplement VR images to create entire vascular maps of both the arterial and venous systems and of collateral vessels or shunt formation. Finally, we have found that the anatomy of the biliary and pancreatic ducts may be clearly defined with an MPR or VR approach, which has value for localizing a dilatation transition point representing a secondary sign of obstruction. Approximately 15 minutes is required to perform the 3D processing in an individual case using the latest software, with processing speeds and graphics cards to permit rapid image transfer, loading with high frame rates, and true real-time editing.
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Normal Anatomy
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The anatomy of the pancreas and the surrounding area has been well described, but we will review several essential points. The lobulated pancreas lies draped across the prevertebral aorta and inferior vena cava in the anterior pararenal space and is obliquely oriented superolaterally toward the splenic hilum. Significant individual variation in the relatively fixed retroperitoneal peripancreatic tissues is observed in both healthy and diseased tissue. Although the pancreas lacks a serosa, it is invested with retroperitoneal parietal peritoneum, has the transverse mesocolon attached along its anterior surface, and lies just superior to the origin of the small bowel mesentery. The pancreas forms part of the floor of the lesser sac, and its tail lies within the splenorenal ligament, so that its position and shape are dictated in part by the splenic orientation.
The pancreas sits deep in the retroperitoneum and is richly invested with lymphovascular supply and drainage. It has a strategic relationship with the celiac axis and superior mesenteric artery and their branches, from which it receives most of its arterial supply. It lies in close relation to the confluence of the superior mesenteric, inferior mesenteric, splenic, and portal veins. Peripancreatic nodes of the celiac axis, porta hepatis, and portocaval region as well as the aortocaval and paraaortic chains are frequently involved by diseases of the gastrointestinal tract, liver, and pancreas.
The C loop of the duodenum abuts the head and uncinate process of the pancreas. The inferior pancreatic border is directly adjacent to the third portion of the duodenum and is closely related to the fourth portion, the ligament of Treitz, and the proximal jejunum. In some patients, the immediacy of the pancreas to the splenic or hepatic flexure may be noted as well. The stomach sits in close apposition to the anterior pancreas, being separated from it by the potential space of the lesser sac, and a redundant stomach fundus may lie adjacent to the pancreatic tail.
Finally, the extrahepatic biliary duct serves as a border to the foramen of Winslow, is closely apposed to the posterior pancreatic head, and meets the main pancreatic duct at the ampulla of Vater. A small accessory duct of Santorini may be seen superiorly and inferiorly, and the uncinate process duct is not infrequently visualized.
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Potential Pitfalls
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Stomach, Duodenum, and Proximal Jejunum
Two of the entities that more commonly mimic a pancreatic mass are a healthy foregut and foregut disease. A normal redundant gastric fundus may lie posteroinferiorly and abut the pancreatic tail. The normal fourth portion of the duodenum or proximal jejunum abuts the pancreatic tail and may simulate a mass (Fig 1). Because small bowel will enhance to 110–120 HU after fast intravenous administration of a bolus of contrast material, it is not surprising that, in some cases, this phenomenon can result in a false-positive diagnosis of a pancreatic tail tumor. Although we recognized this pitfall in the past and gave affected patients additional positive oral contrast material for clarification, misdiagnosis can be more of an issue with the use of neutral oral contrast material.
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Figure 1a. Normal jejunum that simulates a mass in a 77-year-old man. (a) On a CT scan, the jejunum (M) simulates a mass in the region of the pancreatic tail (arrowheads). (b) CT scan demonstrates jejunal loops (white arrowheads) containing some neutral contrast material in the region of the pancreatic tail (black arrowhead).
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Figure 1b. Normal jejunum that simulates a mass in a 77-year-old man. (a) On a CT scan, the jejunum (M) simulates a mass in the region of the pancreatic tail (arrowheads). (b) CT scan demonstrates jejunal loops (white arrowheads) containing some neutral contrast material in the region of the pancreatic tail (black arrowhead).
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The normal second portion of the duodenum and the pancreatic head tend to be displaced to the right posterolaterally after right nephrectomy and can be markedly distorted as a result. It is also important to recognize the surgically reconstructed areas after a Whipple procedure and gastric bypass surgery because the frequently unopacified afferent loops may simulate tumor recurrence. Doubts regarding bowel loops are best resolved with administration of additional positive oral contrast material, a change in patient posture, or off-axial MPR and VR images (22,23).
Large tumors of the gastric fundus may displace it inferiorly so that it abuts the pancreatic tail, and tumors of the posterior gastric wall may simulate a mass of the anterior pancreatic body (Fig 2). Most of the duodenum is directly apposed to the pancreas, so that there is no identifiable plane of delineation. Infiltrating processes and large masses of the pancreas or duodenum may obscure the known tissue boundaries (11, 13). Many duodenal lesions, both neoplastic (11,13,15) and nonneoplastic (2,7,24,25), have been confused with primary pancreatic disease. Localization may be improved with use of coronal or off-axial reformation to identify the center of the lesion and vascular landmarks.
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Figure 2a. Gastric tumor in a 50-year-old man with abdominal pain. (a) CT scan demonstrates a GIST (M) in the region of the fundus of the stomach (G). (b) On a CT scan obtained inferior to a, the gastric tumor (M) simulates a mass of the pancreas (arrowheads). G = stomach.
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Figure 2b. Gastric tumor in a 50-year-old man with abdominal pain. (a) CT scan demonstrates a GIST (M) in the region of the fundus of the stomach (G). (b) On a CT scan obtained inferior to a, the gastric tumor (M) simulates a mass of the pancreas (arrowheads). G = stomach.
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Gastrointestinal stromal tumors (GISTs) (Figs 2–4) are commonly found in the foregut (26,27) and may simulate a primary pancreatic malignancy (11). Larger lesions tend to develop necrotic centers, and multidimensional image reformation may display their origin and characteristic exophytic nature (Figs 3–5). Awareness of the fact that GISTs enhance up to 1.5 times baseline at CT can help prevent any confusion. Care must be taken not to confuse a large, nonfunctioning islet cell tumor of the pancreas with a GIST. Unlike GISTs, islet cell tumors have an epicenter within the pancreas. In addition, they typically demonstrate greater enhancement and more commonly contain calcification.
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Figure 3. Duodenal mass in a 70-year-old woman with abdominal pain. Contrast-enhanced CT scan demonstrates a GIST of the duodenum (arrowheads). The tumor causes circumferential thickening and aneurysmal enlargement and could be mistaken for a pancreatic mass.
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Figure 4. Duodenal mass in a 70-year-old woman with abdominal pain. CT scan shows a solid GIST of the duodenum (M) that simulates a mass of the pancreatic head. Arrowheads indicate the neck and body of the pancreas.
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Figure 5a. Duodenal carcinoma in a 78-year-old woman with abdominal pain. (a) CT scan shows a mass (black arrowheads) with a central cavity containing air and blood (white arrowhead). The mass is difficult to differentiate from the pancreatic head (arrow). (b) MPR image reveals that the cavitating mass (black arrowheads) arises from the second portion of the duodenum (white arrowheads). Arrow indicates the pancreas. (c) VR image shows that the cavitating mass (arrowheads) arises from the second portion of the duodenum (long arrow) and is located inferior to the pancreas (short arrow).
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Figure 5b. Duodenal carcinoma in a 78-year-old woman with abdominal pain. (a) CT scan shows a mass (black arrowheads) with a central cavity containing air and blood (white arrowhead). The mass is difficult to differentiate from the pancreatic head (arrow). (b) MPR image reveals that the cavitating mass (black arrowheads) arises from the second portion of the duodenum (white arrowheads). Arrow indicates the pancreas. (c) VR image shows that the cavitating mass (arrowheads) arises from the second portion of the duodenum (long arrow) and is located inferior to the pancreas (short arrow).
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Figure 5c. Duodenal carcinoma in a 78-year-old woman with abdominal pain. (a) CT scan shows a mass (black arrowheads) with a central cavity containing air and blood (white arrowhead). The mass is difficult to differentiate from the pancreatic head (arrow). (b) MPR image reveals that the cavitating mass (black arrowheads) arises from the second portion of the duodenum (white arrowheads). Arrow indicates the pancreas. (c) VR image shows that the cavitating mass (arrowheads) arises from the second portion of the duodenum (long arrow) and is located inferior to the pancreas (short arrow).
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Adenocarcinoma of the small bowel most commonly occurs in the proximal portion (28). The typically infiltrating pattern of adenocarcinoma obscures the margins of the duodenum and pancreas, making it difficult to determine whether a primary disease process of the duodenum is invading the pancreas or vice versa (Figs 6, 7). The use of additional neutral oral contrast material may improve duodenal distention and increase the conspicuity of the enhancing primary mass. Positive oral contrast material allows better differentiation between a collapsed duodenum and the pancreas. Three-dimensional imaging may depict the typical "apple core" appearance of a circumferential adenocarcinoma. A large lymphoma that either primarily or secondarily involves the duodenum can be extremely difficult to isolate to the duodenum due to its typically infiltrative nature (Fig 8). Metastasis to the duodenum is uncommon but can present interpretive difficulties (Fig 9).
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Figure 6. Duodenal carcinoma in a 61-year-old woman with abdominal pain. CT scan demonstrates circumferential thickening of the third portion of the duodenum (black arrowheads). The second portion of the duodenum is well enhanced (white arrowhead).
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Figure 7. Duodenal carcinoma in a 60-year-old woman with abdominal pain. CT scan shows an infiltrating mass of the third portion of the duodenum (arrow). It is difficult to determine whether the mass is duodenal or pancreatic in origin.
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Figure 8a. Duodenal lymphoma in a 63-year-old woman. (a) Follow-up CT scan demonstrates a large, lobulated mass of the duodenum (arrows) that obscures normal tissue planes. The second portion of the duodenum contains a biliary stent (arrowhead). (b) VR image demonstrates a large, lobulated lymphoma (arrowheads) that infiltrates the pancreas and obscures known tissue boundaries. The biliary stent (long arrow) is seen in the second portion of the duodenum (short arrow).
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Figure 8b. Duodenal lymphoma in a 63-year-old woman. (a) Follow-up CT scan demonstrates a large, lobulated mass of the duodenum (arrows) that obscures normal tissue planes. The second portion of the duodenum contains a biliary stent (arrowhead). (b) VR image demonstrates a large, lobulated lymphoma (arrowheads) that infiltrates the pancreas and obscures known tissue boundaries. The biliary stent (long arrow) is seen in the second portion of the duodenum (short arrow).
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Figure 9a. Duodenal renal cell metastasis in a 70-year-old woman. (a) Follow-up early phase contrast-enhanced CT scan shows a hyperattenuating mass (long arrow) of the duodenum (arrowheads). The mass could be mistaken for an islet cell tumor of the pancreas (short arrow). (b) On a delayed phase contrast-enhanced CT scan, the duodenal mass is washed out (arrow). Arrowheads indicate the duodenum.
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Figure 9b. Duodenal renal cell metastasis in a 70-year-old woman. (a) Follow-up early phase contrast-enhanced CT scan shows a hyperattenuating mass (long arrow) of the duodenum (arrowheads). The mass could be mistaken for an islet cell tumor of the pancreas (short arrow). (b) On a delayed phase contrast-enhanced CT scan, the duodenal mass is washed out (arrow). Arrowheads indicate the duodenum.
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Occasionally, an extrinsic duodenal diverticulum (Fig 10) (7) or duplication cyst (Fig 11) will simulate a necrotic mass or be confused with a pancreatic or peripancreatic abscess (24). This is especially true if there is significant foreign matter mixed with air within the diverticulum. We have also seen a case of an intrinsic duodenal diverticulum leading to distortion of the pancreas and bowel that simulated a pancreatic mass at initial axial examination (Fig 12). Perforated duodenal ulcers (25,29) and reactive duodenal changes in pancreatitis (2) may also have a complex, nonspecific appearance.
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Figure 10. Extrinsic duodenal diverticulum in a 70-year-old woman with abdominal pain. CT scan shows an air-filled duodenal diverticulum (long arrow) that extends into the region of the pancreatic head (arrowhead) and could be mistaken for a pancreatic abscess. Short arrow indicates the duodenum. P = pancreatic body and tail.
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Figure 11. Duodenal duplication cyst in a 42-year-old man with abdominal pain. CT scan shows a fluid-filled duplication cyst (arrowheads) of the medial second portion of the duodenum (arrow). The cyst simulates a cystic mass of the pancreas (P).
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Figure 12a. Intraluminal duodenal diverticulum in a 61-year-old woman with abdominal pain. (a) CT scan shows a debris-filled intrinsic "windsock" diverticulum (large arrowhead) that distorts the pancreas (short arrow) and the second portion of the duodenum (small arrowhead), part of which is well enhanced (long arrow). The diverticulum simulates a pancreatic mass. (b) MPR image demonstrates the diverticulum (T), which distorts the pancreas (short arrows) and the second and third portions of the duodenum (arrowhead) and simulates a pancreatic mass. Long arrow indicates the ligament of Treitz. (c) VR image demonstrates the diverticulum (T) within the proximal jejunum (arrow). The diverticulum distorts the pancreas (P) and duodenum (arrowheads), thereby simulating a pancreatic mass.
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Figure 12b. Intraluminal duodenal diverticulum in a 61-year-old woman with abdominal pain. (a) CT scan shows a debris-filled intrinsic "windsock" diverticulum (large arrowhead) that distorts the pancreas (short arrow) and the second portion of the duodenum (small arrowhead), part of which is well enhanced (long arrow). The diverticulum simulates a pancreatic mass. (b) MPR image demonstrates the diverticulum (T), which distorts the pancreas (short arrows) and the second and third portions of the duodenum (arrowhead) and simulates a pancreatic mass. Long arrow indicates the ligament of Treitz. (c) VR image demonstrates the diverticulum (T) within the proximal jejunum (arrow). The diverticulum distorts the pancreas (P) and duodenum (arrowheads), thereby simulating a pancreatic mass.
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Figure 12c. Intraluminal duodenal diverticulum in a 61-year-old woman with abdominal pain. (a) CT scan shows a debris-filled intrinsic "windsock" diverticulum (large arrowhead) that distorts the pancreas (short arrow) and the second portion of the duodenum (small arrowhead), part of which is well enhanced (long arrow). The diverticulum simulates a pancreatic mass. (b) MPR image demonstrates the diverticulum (T), which distorts the pancreas (short arrows) and the second and third portions of the duodenum (arrowhead) and simulates a pancreatic mass. Long arrow indicates the ligament of Treitz. (c) VR image demonstrates the diverticulum (T) within the proximal jejunum (arrow). The diverticulum distorts the pancreas (P) and duodenum (arrowheads), thereby simulating a pancreatic mass.
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Duodenal hematomas may simulate a cystic pancreatic mass or a complex pseudocyst (Fig 13). Acute duodenal hematomas may be more clearly depicted on unenhanced images, but after contrast material administration their high attenuation can be similar to pancreatic enhancement. As duodenal hematomas age, their heterogeneous attenuation may make them more difficult to differentiate from the adjacent pancreas, although they are usually clearly contained within a circumscribed, distended duodenum.
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Figure 13. Duodenal hematoma in a 41-year-old coagulopathic man who presented with acute duodenal obstruction. CT scan demonstrates a hematoma with heterogeneous attenuation (H) in the third portion of the duodenum (arrowheads). Such a hematoma may simulate a pancreatic mass or cyst.
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Adenopathy
Peripancreatic nodal enlargement can be due to a wide variety of pathologic processes ranging from lymphoma to gastrointestinal adenocarcinoma to granulomatous disease. In addition, enlarged reactive nodes are commonly seen in chronic hepatitis. Although at times these nodes may abut the pancreas, they are usually easy to distinguish because of their sharp borders, different enhancement patterns and textures, and known anatomic location. In other cases, especially in patients who are thin or have indistinct tissue planes or in the setting of bulky adenopathy, it may be difficult to distinguish nodes from the pancreas (3,30). The use of thin-section CT coupled with a properly timed contrast material injection usually makes this distinction clear by allowing optimal differentiation between the enhancing gland and the nonenhancing nodes. In other cases, review of the scanning data in the sagittal projection can be valuable, especially for assessing nodes located inferoposterior to the neck and superolateral to the head of the pancreas. Examples of some more commonly observed adenopathies are discussed in the text that follows.
Portocaval Nodes.—
Enlarged portocaval nodes are classically a site of recurrence in patients with right-sided colon cancer. Other tumors that do not infrequently involve this nodal chain are lymphoma and metastatic esophageal and gastric cancer. Inflammatory processes such as tuberculosis, Mycobacterium avium-intracellulare, and sarcoidosis can involve this chain as well. Nonpathologic nodes at this site can exceed 1 cm in diameter (Figs 14, 15).
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Figure 14. Colorectal carcinoma in a 75-year-old woman who underwent follow-up CT for colon cancer. CT scan shows an enlarged portocaval node (short arrow) just anterior to the inferior vena cava (arrowhead). Such a lesion may simulate a mass of the pancreas (long arrow).
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Figure 15. Merkel cell tumor in a 62-year-old woman with known tumor. CT scan shows infiltrating Merkel cell tumor metastases (arrowheads) that simulate a mass originating from the pancreas (long arrow). Short arrow indicates the second portion of the duodenum.
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Peripancreatic Nodes.—
Peripancreatic nodes are involved in a wide range of inflammatory and neoplastic diseases and can come into intimate contact with the pancreas. Many of these nodes lie near or around the second portion of the duodenum and between the duodenum and the pancreatic head. In most cases, these nodes are small (<1 cm) even in disease states, with lymphoma as a notable exception (Figs 16, 17).
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Figure 17. Metastatic breast carcinoma in a 62-year-old woman with abdominal pain. CT scan shows peripancreatic adenopathy (arrowhead) that simulates a primary pancreatic mass. Arrow indicates a normal pancreas.
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Perihepatic or Portal Chains.—
A wide range of disease processes can result in nodal enlargement in the perihepatic and portal chains. In our experience, however, the most common cause is the presence of parenchymal liver disease. Up to 70% of patients with parenchymal liver disease will have nodes near the porta hepatitis as a reactive phenomenon. In most cases, these nodes are small (<1 cm), but they are often multiple. Although the nodes are usually not confused with primary pancreatic disease, they may be confused with contiguous spread of pancreatic disease. Imaging in the plane of the portal vein and bile duct can best help distinguish lymph nodes from the pancreatic head.
Nodes in the Root of the Mesentery.—
Nodal disease in the mesentery may be a result of inflammatory or neoplastic disease. When bulky, these nodes may extend up to or directly involve the lower aspects of the pancreas and encase the vasculature. This potential pitfall can usually be avoided by looking at the epicenter of the lesion, especially on sagittal 3D reformatted images (Figs 18, 19).
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Figure 18. Lymphoma in a 68-year-old woman with an abdominal mass. CT scan demonstrates bulky adenopathy from lymphoma (arrows) that encases the vasculature and obscures the planes with the pancreas (arrowheads).
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Adrenal Masses
Masses arising from the adrenal gland can typically be recognized as adrenal in origin, especially when they are 4 cm or less in diameter. As tumors get larger (especially when 8–10 cm or more in diameter), the normal gland is obscured and anatomic localization is difficult. We are all familiar with this potential pitfall in cases of lesions that arise in the right adrenal gland but may be difficult to classify as adrenal, renal, or hepatic in origin on axial images alone. Lesions of the right adrenal gland can occasionally be confused with a pancreatic lesion. On the left side, misdiagnosis may be more of an issue with larger tumors. This pitfall may also occur with a potentially benign adrenal lesion such as an adrenal cyst (Fig 20), which can be confused with pancreatic pseudocyst, intraductal papillary mucinous tumor, cystadenoma, carcinoma, or cystic islet cell tumor. We have seen a case in which an adrenal cyst was aspirated because it was believed to be a pancreatic pseudocyst. Coronal reformatted images may better show the relationship of adrenal tissue to the upper pole of the kidney and help localize adrenal veins.
Renal Masses
Larger renal tumors or masses that arise from the medial upper pole of the kidney may also be a source of confusion. Such tumors can displace or abut the pancreas and be confused with primary pancreatic disease. This problem is more common with involvement of the upper pole of the left kidney, where larger masses may interface with the pancreatic tail through the lienorenal ligament or directly traverse the retroperitoneal layers. We have also seen several cases of large left upper pole renal cysts that were confused with cystic tumors of the pancreas, especially when the cystic lesions had thick or atypical calcifications (Fig 21). Three-dimensional VR may be used to demonstrate the 3D equivalent of the "beak sign" (ie, a mass that arises from the kidney and manifests as a crescentic defect in the renal parenchyma). Lymphomatous infiltration of the kidney and of the peri- and pararenal spaces may extend to involve the pancreas. However, in these cases, diagnosis is usually fairly straightforward and is facilitated by observing the mantle of tumor around the kidney.
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Figure 21a. Renal cyst in a 40-year-old woman with abdominal pain. MPR images demonstrate a partially calcified cyst (C) that arises from the upper pole of the left kidney (arrow) and is separate from the pancreatic tail (arrowhead). G = stomach.
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Figure 21b. Renal cyst in a 40-year-old woman with abdominal pain. MPR images demonstrate a partially calcified cyst (C) that arises from the upper pole of the left kidney (arrow) and is separate from the pancreatic tail (arrowhead). G = stomach.
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Mesenteric Masses
Mesenteric masses (eg, nodes) and tumors or tumor-like diseases (eg, desmoid tumors, carcinoid tumors) may extend up to the base of the pancreas along the superior mesenteric vascular pedicle (Fig 22) (9,31). Likewise, large cancers of the colon can spread to the pancreas (and vice versa) along the transverse mesocolon. Mesenteric lesions tend to remain intimately related to the vasculature as they infiltrate inferiorly or anteriorly, often preserving a fat plane, whereas primary pancreatic lesions tend to spread within the anterior pararenal space. It is unusual for a primary pancreatic tumor to spread inferiorly to any great extent. Documentation of the epicenter of the disease process usually prevents misdiagnosis, especially if a good arterial phase study allows detection of the normally enhancing pancreas. Sagittal images of the mesentery are optimal for distinguishing lesions at the root of the small bowel from pancreatic tissue (31).
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Figure 22. Sclerosing mesenteritis in a 49-year-old woman with abdominal pain. CT scan demonstrates infiltrative sclerosing mesenteritis (arrowheads) that encases the celiac vasculature in a fashion similar to that of adenocarcinoma of the pancreas (P). The diagnosis was made at core biopsy.
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Neurovascular Structures
CT provides excellent vascular mapping of both the arterial and venous systems. Although normal anatomic variants are not uncommon, they are usually easily recognized. However, in cases of portal vein thrombosis with cavernous transformation, a suboptimal injection of contrast material may result in vessels being confused with nodes or even with a tumor of the pancreatic head (Fig 23). Even with a properly timed injection, the capacious collateral veins may enhance poorly. This problem is most common in thin patients and in patients with minimal intraabdominal fat. In patients with extensive collateral vessels, arterial phase imaging may yield a false-positive diagnosis of a pancreatic mass, which can have serious consequences if biopsy is performed. Usually, the serpiginous collateral vessels are well depicted after the appropriate venous delay or at multidimensional reformation.
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Figure 23a. Peripancreatic varices in a 71-year-old man with elevated liver enzyme levels. (a) Arterial phase CT scan shows how low-attenuation peripancreatic varices (long arrow) in the region of the pancreatic head (short arrow) can simulate a pancreatic mass. Arrowhead indicates the second portion of the duodenum. (b) On a venous phase contrast-enhanced CT scan, the varices (long arrow) are more clearly distinguished from the pancreatic head (short arrow). Arrowhead indicates the duodenum.
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Figure 23b. Peripancreatic varices in a 71-year-old man with elevated liver enzyme levels. (a) Arterial phase CT scan shows how low-attenuation peripancreatic varices (long arrow) in the region of the pancreatic head (short arrow) can simulate a pancreatic mass. Arrowhead indicates the second portion of the duodenum. (b) On a venous phase contrast-enhanced CT scan, the varices (long arrow) are more clearly distinguished from the pancreatic head (short arrow). Arrowhead indicates the duodenum.
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Pseudoaneurysms of the pancreaticoduodenal, hepatic, or mesenteric artery may also be confused with a pancreatic mass, especially on unenhanced or late-phase images or in lesions with large amounts of thrombus (Figs 24–26) (1,4,5, 8,14). During arterial phase studies, a hyperenhancing islet cell tumor may also be simulated. Splenic artery aneurysms lie close to the pancreatic tail and require careful attention to be identified. We have seen several cases of patients who were referred for either biopsy or resection of a pancreatic mass that was vascular in nature. Proper attention to scanning protocols and use of 3D multi–detector row CT angiography will increase diagnostic confidence (32).
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Figure 24. Pseudoaneurysm of the gastroduodenal artery in a 60-year-old man. CT scan shows a pseudoaneurysm (arrowheads) with a central high-attenuation lumen (short arrow). The pseudoaneurysm is intimately associated with the pancreatic head (long arrow). P = pancreas.
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Figure 25. Pseudoaneurysm in a 40-year-old man with abdominal pain. CT scan shows a partially thrombosed peripancreatic arterial pseudoaneurysm (arrowheads) with a high-attenuation lumen (a). This lesion may simulate a mass on unenhanced images. Arrow indicates the pancreatic tail.
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Figure 26. Pseudoaneurysm in a 53-year-old man with abdominal pain. CT scan demonstrates a large, partially thrombosed peripancreatic pseudoaneurysm (arrowheads) with a patent lumen (a). Arrow indicates the pancreas.
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Tumors of nerve cell origin within or adjacent to the duodenum are uncommon but frequently present a diagnostic dilemma (Fig 27) (33,34).
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Figure 27. Neurofibroma in a 56-year-old woman who presented for evaluation of a pancreatic mass. CT scan shows a neurofibroma (arrowhead) in the region of the pancreas (arrow). The neurofibroma simulates a primary pancreatic lesion.
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Conclusions
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In most cases, helical CT is highly accurate in distinguishing primary disease of the pancreas from adjacent disease, although there are cases in which the differential diagnosis is more challenging and the potential for misdiagnosis exists. Familiarity with some of the pitfalls discussed in this article is helpful during interpretation. Careful attention to scanning protocol and contrast material administration, use of the full potential of multi–detector row CT data sets, and judicious application of postprocessing tools may help avoid some of these pitfalls.
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Footnotes
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Abbreviations: GIST = gastrointestinal stromal tumor,
MPR = multiplanar reformation,
VR = volume rendering,
3D = three-dimensional
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References
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Abstract
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