Pancreatic Serous Cystadenoma Imaging 

Cystic lesions of the pancreas are common, and 80-90% of these lesions are pseudocysts or retention cysts. Cystic neoplasms of the pancreas are less common, accounting for about 10-15% of all cystic pancreatic lesions. True cysts of the pancreas are rare. Pancreatic serous cystadenoma used to be referred to as microcystic cystadenoma or glycogen-rich cystadenoma. These terms are no longer considered appropriate. The preferred name is now serous cystadenoma.^{[1, 2]}

The two most common cystic neoplasms of the pancreas are serous cystadenoma and mucinous cystic neoplasms (formerly called macrocystic neoplasm, a term that is no longer appropriate). Serous cystadenoma is more common than mucinous cystic neoplasm, with a ratio of about 2:1. Intraductal papillary mucinous tumor (IPMT) is a more recently discovered cystic neoplasm that may be a variant of the mucinous cystic neoplasm.

The important point to remember is that serous cystadenoma is benign, whereas the biologic behavior of mucinous cystic neoplasm and IPMT ranges from benign to malignant. Therefore, distinguishing these entities is of the utmost importance. (See the images below.)

Because of the significant overlap in the imaging findings of mucinous and serous pancreatic tumors, these tumors should be followed up with surveillance computed tomography (CT) scanning to assess interval growth if aspiration is not performed.

The diagnosis of a serous cystadenoma should be made with caution unless the lesion has all of the typical findings. If a mucinous cystic lesion is incorrectly diagnosed as a serous cystadenoma and if surveillance CT or magnetic resonance imaging (MRI) scan is not performed, grave consequences could result.

Diagnosis

The main differential diagnoses include mucinous cystic neoplasm, IPMT, pseudocyst, focal pancreatitis, and adenocarcinoma.

The diagnosis of pancreatitis and pseudocyst is generally straightforward, especially in a clinical setting of chronic alcoholism with a history of pancreatitis. If the tumor is large enough, symptoms related to mass effects may predominate. Smaller tumors may be discovered incidentally on abdominal ultrasonogram or CT scan.

Imaging findings are usually confirmatory in difficult cases. Adenocarcinoma is most commonly solid and, therefore, infrequently confused with cystic neoplasms. The other entities can have imaging features that are highly suggestive of one entity rather than another. Communication with the pancreatic duct strongly suggests mucinous cystic neoplasm or IPMT instead of serous cystadenoma.

A central stellate scar with calcification and a grapelike cluster of cysts and external lobulation strongly suggests serous cystadenoma. However, the imaging features of these entities can overlap considerably; therefore, an analysis of the percutaneous cells and cystic fluid is often required for diagnosis. In fact, approximately 10% of all serous cystadenomas have cystic components larger than 2 cm and cannot be distinguished from mucinous cystic neoplasms.

Preferred examination

Ultrasonography can be used to detect, and sometimes to characterize, the features of this tumor, especially when the classic features are present. In comparison, CT is superior in lesion depiction and characterization, and it is the preferred imaging modality in the initial workup of pancreatic lesions. However, the first test performed is usually ultrasonography, because the typical presenting symptom is abdominal pain and/or nausea.^[3]

MRI can be a useful problem-solving adjunct in select cases. For example, when an evaluation of the ducts is needed, magnetic resonance cholangiopancreatography (MRCP) can be useful.

Findings from plain radiography and upper GI series are nondiagnostic, except the finding of a classic sunburst central calcification, which is suggestive.

Limitations of techniques

When stellate calcification or the classic CT scan features of central sunburst are present in a multilocular cystic mass in an older woman, some institutions accept this as a clearly benign finding. In one series, however, CT scanning was useful in depicting the lesions, but it was not useful in differentiating benign and malignant tumors, serous and mucinous tumors, or pseudocysts and neoplasms.

No radiographic abnormalities are associated with serous cystadenoma except those related to a mass that is large enough to displace or obstruct the bowel or those related to a prominent central calcification.

The main mimics of this tumor are pseudocysts and mucinous cystic tumors.

Classically, these lesions have a mean diameter of 5-8 cm (range, 4-20 cm) and a lobulated external contour. They are composed of a grapelike cluster or honeycomb pattern of 6 or more uniformly sized cysts that are 2 cm or smaller. They tend to occur in the head or neck of the gland, although biliary obstruction is present in only about 15% of the cases.^[4]

In about 30% of the cases, a central, stellate, late-enhancing scar is present with calcification. Small septa and internal debris may be seen in individual cysts. Because the capsule of these tumors is poorly developed, there is often poor distinction of the tumor from the surrounding pancreatic parenchyma. No communication occurs with the pancreatic duct, except in rare cases. (See the images below.)

The tumor generally has attenuation similar to that of water on nonenhanced scans, and it typically enhances with contrast in a Swiss cheese–like pattern. Some tumors have a few cysts larger than 2 cm.

Serous cystadenomas are usually hyperintense on T2-weighted images and hypointense on T1-weighted images (see the image below). Occasionally, debris (especially hemorrhage) in the cysts alters this signal intensity pattern. The septa are well depicted on T2-weighted images, but the central scar is not.^[5]

The use of a gradient-echo pulse sequence with a long echo time (TE) may bring out the susceptibility effects from the calcified scar. MRCP is helpful in demonstrating the relationship of the mass to the main pancreatic duct. The main duct is almost never obstructed, but the duct and its branches may be gently splayed and draped.

Gadolinium-based contrast agents (gadopentetate dimeglumine [Magnevist], gadobenate dimeglumine [MultiHance], gadodiamide [Omniscan], gadoversetamide [OptiMARK], gadoteridol [ProHance]) have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the eMedicine topic Nephrogenic Systemic Fibrosis. The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans.

False positives/negatives

Rarely, a fluid-filled diverticulum of the transverse duodenum may mimic a cystic pancreatic mass.

The cluster-of-grapes pattern and external lobulation may be seen. However, when the cysts are small, the mass can be echogenic (because of the large number of acoustic interfaces), and they can appear solid (see the image below). This finding can suggest the presence of an adenocarcinoma. The presence of increased through transmission, even if the mass is fairly echogenic, should suggest the diagnosis.^[6]

Octreotide (OctreaScan) can be used to detect pancreatic tumors that express somatostatin receptors (eg, neuroendocrine tumors), because the agent is avid for these receptors. However, serous cystadenoma does not typically express these receptors.

The ability of positron emission tomographic (PET) scanning to depict these tumors will likely be proven, because it is sensitive to hypermetabolic processes. However, PET is not yet approved for use in the workup of pancreatic lesions. Thallium 201–chloride single photon emission computed tomography (SPECT) can depict malignant, as well as some benign, pancreatic processes.^[7]

Serous cystadenomas typically demonstrate intense peripheral hypervascularity at angiography, although this finding is nonspecific.