Imaging Diagnosis of Cystic Pancreatic Lesions: Pseudocyst versus Nonpseudocyst
Although the clinical, radiologic, and pathologic features of cystic pancreatic lesions are well known, preoperative diagnosis is difficult. Differentiation between a pancreatic pseudocyst and a cystic pancreatic neoplasm is crucial in determining the proper treatment. Careful evaluation of the patient’s clinical history is important for accurate diagnosis of a pseudocyst. Clinical scenarios include a pseudocyst developing after acute pancreatitis and a pseudocyst superimposed on chronic pancreatitis. However, a pseudocyst in a patient with no clinical history of pancreatitis poses a diagnostic problem. The differential diagnosis of a neoplastic cystic lesion of the pancreas includes serous cystadenoma, mucinous cystic neoplasms, intraductal papillary mucinous tumor, and solid and papillary epithelial neoplasm. Definitive diagnosis is often possible when the lesion has a typical radiologic appearance, but in many cases characterization with imaging alone is impossible. Thin-section computed tomography with multiplanar reformation, magnetic resonance cholangiopancreatography, and endoscopic ultrasonography have emerged as modalities that can provide additional diagnostic information. Familiarity with the range of imaging appearances and awareness of the diagnostic strengths and limitations of each imaging modality are important for accurate diagnosis and management of cystic pancreatic lesions.
© RSNA, 2005
LEARNING OBJECTIVES FOR TEST 4
After reading this article and taking the test, the reader will be able to:
Identify the radiologic findings of pancreatic pseudocysts.
Describe the radiologic findings of cystic pancreatic neoplasms.
Discuss the findings that allow differential diagnosis between pancreatic pseudocysts and cystic pancreatic neoplasms.
Pseudocysts are the most common cystic lesions of the pancreas. All other cystic lesions including cystic neoplasms represent only 10%–15% of pancreatic cysts (,1). Recently, wide application of computed tomography (CT) and ultrasound (US) in asymptomatic and mildly symptomatic patients has increased detection of incidental cystic lesions of the pancreas so that the differential diagnosis of pancreatic cystic lesions has become more challenging. In addition, recent awareness of intraductal papillary mucinous tumor (IPMT) has resulted in increasing numbers of reported cases (,2). Differentiating pancreatic pseudocysts from nonpseudocysts is important for determining treatment.
It is well known that radiologic imaging alone has limited accuracy in differentiating between pseudocysts and nonpseudocysts due to similarities in the imaging findings of the lesions (,3,,4). Thin-section CT and magnetic resonance (MR) cholangiopancreatography have gained popularity for the potential advantages of visualizing communication between the main pancreatic duct and a cystic lesion noninvasively. Endoscopic US has emerged as a modality that can provide anatomic structure in greater detail and facilitate aspiration biopsy of smaller lesions (,5). When radiologic imaging findings and results of cyst fluid analysis with or without biopsy are interpreted in conjunction with a careful patient history, diagnostic accuracy may be increased substantially.
The purpose of this article is to illustrate the spectrum of imaging findings of cystic pancreatic lesions and highlight the roles, advantages, and limitations of various imaging modalities in the differential diagnosis of pancreatic cystic lesions, with particular emphasis on pseudocysts versus nonpseudocysts.
Most cystic masses of the pancreas encountered in clinical practice are postinflammatory pseudocysts. Pancreatic pseudocysts are defined as localized amylase-rich fluid collections located within the pancreatic tissue or adjacent to the pancreas and surrounded by a fibrous wall that does not possess an epithelial lining (,6,,7). The CT findings of a pseudocyst include a round or oval fluid collection with a thin, barely perceptible wall or thick wall that shows evidence of contrast enhancement (,7,,8). They develop most often as a complication of acute or chronic pancreatitis and may develop secondary to pancreatic trauma or surgery (,9). Although a prior history of pancreatitis cannot by itself justify the diagnosis of pancreatic pseudocyst, careful evaluation of the patient’s clinical history is important for the accurate diagnosis of pseudocyst. Clinical scenarios include a pseudocyst developing after identifiable acute pancreatitis (,Figs 1,,–,,,,,,,4), a pseudocyst resulting from an acute incident superimposed on chronic pancreatitis (,Fig 5,,), and a pseudocyst with an uncertain or no known previous clinical history of pancreatitis (,Fig 6,).
Classic Postinflammatory Pancreatic Pseudocyst
In the clinical setting of acute pancreatitis, observing peripancreatic inflammatory changes on initial CT scans and an evolving peripancreatic fluid collection that develops a wall or capsule provides a sufficient diagnostic clue to the diagnosis of pancreatic pseudocyst (,Fig 1,,). After an acute attack, the pseudocyst develops during a period of 4–6 weeks. Pseudocysts may be followed conservatively if they are smaller than 6 cm in diameter or the patient is asymptomatic because pseudocysts can resolve spontaneously (,Fig 2,,) (,10). Unilocular pseudocysts occur more frequently than multilocular pseudocysts. Pseudocysts usually have a smooth thin wall or a thick wall with uniform thickness. Complications related to pseudocysts include infection, hemorrhage, rupture, and obstruction of other abdominal organs. Secondary infection of the pseudocyst is a dreaded complication due to its high rate of morbidity and mortality, requiring drainage by radiologic, endoscopic, or surgical decompression (,11). Radiologic differential diagnosis of infected necrosis, pancreatic abscess, or hemorrhage within the cyst from a simple uncomplicated pseudocyst cannot be made without clinical information (,Figs 3,,, ,4).
Pancreatic Pseudocyst Superimposed on Chronic Pancreatitis
Chronic pancreatitis is most often caused by alcoholism. Conditions such as hyperlipidemia or hyperparathyroidism, trauma, and chronic obstruction of the pancreatic duct can also be associated with chronic pancreatitis. Pancreatic pseudocysts can occur in association with chronic pancreatitis as chronic pseudocysts or can result from acute exacerbation of pancreatitis or chronic pancreatitis. In the former case, a distinct clinical history of acute pancreatitis may be lacking and the pseudocyst is often detected incidentally, in comparison with the latter case (,12). The recognition of a pancreatic pseudocyst resulting from chronic pancreatitis is usually easy when there are associated stigmata of chronic pancreatitis such as parenchymal calcifications or ductal stones, ductal dilatation, and atrophy of the parenchyma (,Fig 5,,). However, without these findings, pseudocysts will be very difficult to distinguish from IPMT.
Pancreatic Pseudo-cyst without an Antecedent Episode of Acute Pancreatitis
Detection of incidental pancreatic cysts in an asymptomatic patient poses a diagnostic problem. Incidental pancreatic cysts are smaller than symptomatic cysts and are unlikely to be pseudocysts (,2). Cystic pancreatic neoplasm should be considered in the differential diagnosis of a pancreatic cyst, even in patients with a history of pancreatitis, if no recent episode of acute pancreatitis can be documented on clinical or imaging grounds. For pancreatic pseudocysts without an antecedent episode of acute pancreatitis and radiologic evidence of pancreatitis, US-, CT-, or endoscopic US–guided aspiration or biopsy or at least a follow-up study should be recommended (,Fig 6,).
Cystic pancreatic neoplasms are uncommon but important because they are increasingly being detected and are difficult to distinguish from pseudocysts, which are encountered far more frequently. Cystic pancreatic neoplasms are generally associated with symptoms, as are most pancreatic pseudocysts, but an increasing number of incidental pancreatic cysts are being found (,2). The differential diagnosis of a neoplastic cystic lesion of the pancreas encompasses a wide variety of neoplasms, including serous cystadenoma, mucinous cystic neoplasm, IPMT, solid and papillary epithelial neoplasm (SPEN), and cystic islet cell tumor. Definitive diagnosis is often possible when the lesions show typical radiologic appearances, but in many cases characterization by imaging alone is impossible. Although improved radiologic technology provides more detailed anatomic information on many lesions, it produces more challenges because the average size of detected lesions has steadily decreased and imaging characterization becomes more difficult for smaller lesions.
Serous cystadenoma, also referred to as microcystic cystadenoma, is typically found in women over the age of 60 years with nonspecific complaints of abdominal pain or weight loss or more commonly as an incidental finding. Typical serous cystadenomas are composed of multiple cysts varying in size from 0.2 to 2.0 cm, and the size of the tumors ranges in greatest dimension from 1.4 to 27 cm (,5). A central stellate scar with calcification, which is known to be characteristic of serous cystadenoma, may sometimes be observed (,4). Internally, the cyst has a honeycombed appearance compatible with innumerable cysts (,Fig 7,,). At US, the lesion may appear as a solid echogenic mass due to interfaces produced by the numerous cysts. It may appear as a solid mass at CT, depending on the size of the cysts and the amount of fibrous tissue (,Fig 8,).
Asymptomatic serous cystadenomas do not require surgical excision because they are rarely malignant. Tumors smaller than 2 cm have been reported and are more likely to be serous cystadenomas (,5). Macrocystic or oligocystic serous cystadenoma is a variant of serous cystadenoma that is very difficult to differentiate from mucinous cystadenoma (,Fig 9,,,) (,13). Location in the pancreatic head, lobulated contour, and lack of wall enhancement have been reported to be specific for macrocystic serous cystadenoma in comparison with mucinous cystic tumor. Lobulated contours have been reported to be a specific finding in comparison with pseudocyst (,14).
Mucinous Cystic Neoplasms
Mucinous cystic neoplasms are the most common cystic tumors of the pancreas. The large cystic spaces are lined by tall, mucin-producing columnar cells. Mucinous cystic neoplasms may be unilocular or mutilocular and are commonly detected only after achieving a large size. Solid papillary excrescences sometimes protrude from the wall into the interior of these tumors. The absence of excrescences does not exclude malignancy.
Multiple enhancing septations and solid intramural nodules are typical radiologic findings of mucinous cystic neoplasms (,Fig 10). Peripheral calcification, which can be seen in 10%–25%, is an important characteristic for mucinous cystic neoplasms and can be used to differentiate them from serous cystadenomas, which are known to have central calcification (,4,,15). Endoscopic US can depict the internal architecture of the cystic mass, including internal septa and tiny solid components, better than conventional CT, depending on the location of the tumor and patient body habitus. The tumors are round to oval with a smooth external surface. Secondary cysts along the internal wall are common (,Fig 11). Occasionally, communication between the pancreatic duct and the cystic neoplasm is present.
There is a spectrum of mucinous cystic neoplasms from benign to malignant, but the confident exclusion of malignancy is rarely possible on the basis of imaging findings alone (,Fig 12). Mucinous cystic tumors should always be resected because they are all potentially malignant. When the cyst is small in an asymptomatic patient, cyst aspiration and analysis of the cyst fluid can be helpful in differential diagnosis (,2) (,Fig 13,).
Mucinous Cystic Neoplasm Misdiagnosed as a Pseudocyst
Owing to partial volume averaging with the hypoattenuating cyst fluid, the fine internal septa and small intramural nodules may not be visible at conventional contrast-enhanced CT. This explains why mucinous cystic neoplasm sometimes is misdiagnosed as a pseudocyst (,4) (,Fig 12). The application of thin-section imaging with multidetector CT has been known to depict internal anatomic details more clearly, which possibly avoids misdiagnosis (,16). The large size of unilocular cysts excludes the diagnosis of IPMT.
Intraductal Papillary Mucinous Tumor
IPMT is characterized by the papillary proliferation of pancreatic ductal epithelium and production of mucin. It is characterized by cystic dilatation of a main or a side branch duct that contains thick mucoid secretions. Patients present with nonspecific abdominal symptoms and sometimes hyperamylasemia. IPMTs typically occur in elderly patients and are more common in men. Although the incidence of IPMT seems to be increasing, the likely explanation is increased use of imaging and awareness of this disease entity.
IPMTs are classified into the main duct type, branch duct type, and combined type. Accordingly, imaging findings vary depending on the type of the tumor. The side branch duct type is the most commonly mistaken for mucinous cystic tumor or pseudocyst. Typical location (uncinate process), typical appearance (grapelike locular appearance), and communication with the duct at endoscopic retrograde cholangiopancreatography (ERCP) usually separate it from other lesions in the pancreas (,17). A markedly dilated uncinate branch filled with mucus is a typical feature of a side branch IPMT.
There has been debate over use of ERCP as an initial imaging modality of choice (,17). ERCP is regarded as the modality of choice in the diagnosis of IPMT for its ability to depict the bulging ampulla of Vater, mucin pouring from the papilla, and communication between the pancreatic duct and the cyst cavity (,17). Communication between the duct and the abnormal cystic structure can be shown with thin-section helical CT (,Fig 14,) or with MR imaging and MR cholangiopancreatography (,Fig 15,) (,16,,18). Although the classic diagnostic role of ERCP has been challenged to some extent by combined use of MR cholangiopancreatography and endoscopic US in the evaluation of IPMT, ERCP has a distinctive diagnostic role where the diagnosis is not clear on cross-sectional images (,Fig 16,).
Solid and Papillary Epithelial Neoplasm
SPENs, also known as solid and pseudopapillary tumors, papillary and cystic tumors, or solid-cystic tumors, are histologically distinctive neoplasms of low malignant potential with a favorable prognosis. SPEN is typically found in young women and has a predilection for Asian and black patients. Most patients present with nonspecific signs and symptoms including nausea, vomiting, and abdominal pain or fullness. The tumor tends to be a large, well-circumscribed, and slowly growing mass. The tumor may have a variety of internal appearances, from purely cystic to completely solid, but is usually surrounded by a thick, well-defined rim (,Fig 17,,). The appearance of the internal architecture typically depends on the degree of hemorrhage and necrosis of the tumor.
Multidetector CT and MR Imaging with MR Cholangiopancreatography
The improved multiplanar capability, thin collimation, and ability to optimize parenchymal enhancement of multidetector CT not only improve the detection rate of the cystic pancreatic lesion but may enhance diagnostic accuracy by depicting fine internal architecture and the anatomic relationship between the main pancreatic duct and the cystic lesions (,19). MR imaging with additional performance of MR cholangiopancreatography can be similar in diagnostic value to multidetector CT with thin sections in the examination of patients with IPMT, especially by depicting communication between the cystic lesions and the main pancreatic duct (,20).
Image-guided Fine-Needle Aspiration Biopsy
Preoperative cyst fluid analysis and biopsy of the solid portion of the cystic mass can be obtained by conventional US- or CT-guided percutaneous needle aspiration or by an endoscopic US–guided technique. Analysis of the cystic contents includes viscosity, enzymes (amylase, lipase), tumor markers (carcinoembryonic antigen [CEA], cancer antigen 19-9 [CA 19-9]), and cytologic findings, which may help differentiate between neoplastic cysts and pseudocysts. False-negative results from sampling errors are not uncommon (,21). Interpretation of the results must be done in conjunction with a careful history and interpretation of radiologic findings (,22,,23). When the nature of the pancreatic cyst cannot be definitively established by fluid analysis or cytologic analysis, surgical resection is indicated.
Endoscopic US and US-guided Fine-Needle Aspiration or Biopsy
Endoscopic US has emerged as a useful diagnostic tool in the evaluation of pancreatic cystic lesions. It provides more detailed anatomic information about the cyst than conventional US and allows the sampling of both cyst fluid and any solid component in smaller lesions (,Figs 18,–,,,,,21,). By allowing incorporation of the morphologic appearance at endosonography, results of cytologic analysis, and results of tumor marker analysis, endoscopic US can provide additional diagnostic clues (,5).
Despite a remarkable increase in the number of cystic lesions of the pancreas detected in clinical practice, the number of cystic neoplasms misdiagnosed as pseudocysts has decreased. The likely explanation is the increasing use of various imaging modalities and image-guided aspiration and biopsy. If the patient has no history of pancreatitis and no history of pancreatic trauma or pancreatic surgery and the findings from imaging do not allow conclusive diagnosis, a follow-up imaging study or image-guided aspiration/biopsy should be recommended.
We thank Brenna C. Bounds, MD, for help in preparing the endoscopic US image material.
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