Cystic tumors of the pancreas: the radiologist.

The increasing use of cross-sectional imaging has led to the realization that the prevalence of cystic neoplasms is higher than initially estimated. 10,37 These lesions are problematic for clinicians because many of them have histologic features of cellular atypia or invasive carcinoma, thus forcing a decision for surgery. The initial imaging descriptions appearing in the North American literature were previously reported by Japanese radiologists in the mid-1980s. 12,13,14 From these early reports, it was evident that cross-sectional imaging findings could be used to classify these lesions based on cyst size, presence and location of calcifications, and relationship to the pancreatic ducts. The imaging findings were concordant with the classic pathology descriptions of Compagno and Oertel. 8,9 The classic microcystic adenoma presents as a pancreatic mass characterized by a dense, internal, lacelike, honeycombed matrix composed of fibrous setae. The setae may be so dense that the cyst components may go unrecognized. When calcifications are present, they are invariably within the central portion of the lesion (Fig. 1). Using thin-section CT scanning or current MR imaging technology, the cystic spaces enmeshed within the fibrous matrix can be delineated. When the mass is sufficiently large, it may result in pancreaticobiliary ductal obstruction. 4,19 At the time this lesion is seen, with all of its classic components, it is considered benign. There have been scattered case reports of synchronous ductal adenocarcinoma in patients with microcystic serous cystadenoma 23 and malignant histologic features within the epithelial lining of these cysts. 26 The author has seen lesions that have encased major peripancreatic vessels and have occurred with hepatic metastases; however, these patients do not display the typical clinical course that characterizes those with ductal adenocarcinoma. Many patients who display the classic findings of a serous microcystic adenoma have been followed up by imaging; none have developed metastatic disease. When a unilocular macrocystic mass, with or without internal septations or mural nodules, is detected, the diagnosis becomes less clear-cut. Unilocular pancreatic masses may be mucinous or serous cystadenoma or cystadenocarcinoma. Most cystic pancreatic masses display this macrocystic morphology (Fig. 2). These macrocystic lesions were initially assumed to be mucin containing; however, correlation with resected specimens has identified that several of these may be serous cystadenoma. 20,30 Imaging classification has also suffered because of a lack of uniform descriptions of these lesions. Specifically, there are microcystic (no individual cyst being > 2 cm in diameter) mucinous and serous lesions with morphology, unlike the "classic" glycogen-rich serous cystadenoma described by Compagno and Oertel. This misclassification has led to reports in the literature that have been critical of the ability of imaging to predict the histologic nature of a cystic pancreatic mass; accuracies range from 20% to 60%. 28,32,40 These results are insufficient to preclude surgical exploration in symptomatic cases. In patients with cystic masses incidentally discovered, the recommendation for surgery or follow-up is less clearly defined. Another group of lesions that have been included in the category of "cystic pancreatic masses" arise within the pancreatic duct system. This classification confuses the understanding of these relatively uncommon lesions. Most intraductal lesions are associated with mucin hypersecretion; however, intraductal papillary neoplasms have been reported. 1 Most authorities refer to this group of lesions as intraductal papillary mucinous tumors. On imaging studies, focal or diffuse dilatation of any portion of the pancreatic duct system is recognized. CT scans and MR images are excellent tests for detection of these lesions that have a high prevalence of malignant epithelial changes and must be regarded as frank neoplasms. Intraductal papillary mucinous tumor (IPMT) was first described on endoscopic retrograde cholangiopancreatography (ERCP) associated with main duct location and marked mucin hypersecretion that produces the characteristic endoscopic findings. 25 As this lesion became more fully understood, four major variants were described: (1) segmental or (2) diffuse involvement of the main pancreatic duct, and (3) macrocystic or (4) microcystic involvement of a branch duct. 31 Careful analysis of histologic features of these lesions reveals that the epithelial changes along the ducts range from hyperplasia to dysplasia, carcinoma in situ, or invasive carcinoma. In fact, all of these epithelial changes may be present in a single lesion. Because of the lack of predictable histology, most authorities advocate resection when these lesions are discovered. On imaging, IPMT has a variety of appearances. The initial descriptions by Itai et al distinguished between mucinous hypersecreting tumor of the main pancreatic duct 12 and mucinous duct ectasia (lesions arising in branch ducts). 14 Since those original descriptions, multiple clinical reviews have been published. 3,18,29,31,33,36 The key finding is segmental or diffuse dilatation of the pancreatic duct system. In the author's experience, branch duct lesions are most common. They are usually visualized within the uncinate process of the gland, although they may occur throughout the gland or may be multiple. Once ductal dilatation is visualized, ERCP is indicated to establish the presence of mucin. As experience has increased with these lesions, MR cholangiopancreatography (MRCP) has been more widely used. Advocates of MRCP state that this technique may be more sensitive than ERCP because filling of side branch ducts may be obscured by intraductal mucin plugs. 27,38,39 Many cases of IPMT have been detected as incidental findings in patients undergoing imaging for other indications. The presence of a small pancreatic mass, therefore, has management implications. Most investigators suggest immediate resection. 6,18,33 Nagai et al 21 reviewed clinical material in 29 patients and found a rough correlation between the size of these lesions and their histology, with larger lesions being more commonly malignant. The investigators postulate an adenoma–carcinoma sequence in these lesions. Kimura et al 15,16 reviewed more than 250 patients and found a high prevalence of small cystic lesions in elderly patients and a low prevalence of malignancy in these small lesions. In a study based on serial ERCPs in nine patients, with a mean follow-up time of 30 months, Obara et al 24 found that lesions arising in branch ducts remain unchanged, whereas lesions arising in the main pancreatic duct progress. At the author's institution, staff have followed up on 30 patients extended with small cystic pancreatic lesions with CT scanning or MR imaging from 3 to 144 months (mean follow-up, 28 months). In 9 cases, there was pathologic or cytologic proof of diagnosis (Figs. 3–5); the other 21 cases were followed up by serial imaging. Seventeen lesions were found in the uncinate process. Almost all small lesions (those 5 The results of the author's histologic correlation study and those of the Japanese investigators have led the author to adopt the following protocol for management of these patients: All patients with suspected IPMT are potential surgical candidates. If a patient has a lesion discovered that is (1) less than 2.5 cm in diameter, (2) confined to the branch ducts, or (3) displays no solid components, then the patient is offered the option of serial follow-up. The author initially follows up the lesions at 6 months and then at 8- to 13-month intervals unless there is a change in clinical symptoms. If any perceptible growth is detected, the lesion is immediately resected. Differential diagnosis of cystic lesions of the pancreas include pseudocyst, cystic islet cell tumor, and periampullary diverticulum. The most common lesion, which may be confused with a cystic neoplasm, is a pseudocyst of chronic pancreatitis. Usually, an appropriate history can be elicited; however, in the absence of such a history, the imaging features may be identical. 7,35 Some investigators have advocated needle aspiration as a method of definitively diagnosing mucinous pancreatic masses. 11 Needle biopsy allows for cytopathologic analysis of cyst fluid, histochemical analysis of mucin type, and assay for the presence of tumor markers. 2,22,34