Pancreatic ductal adenocarcinomas account for approximately 85% of all pancreatic tumors. Recently, less common epithelial tumors have been increasingly identified as a result of the frequent use of computed tomography (CT). Solid pancreatic masses with atypical clinical presentations or unusual imaging characteristics may suggest an unusual etiology. At presentation, rare pancreatic conditions can be diagnostically challenging and a thorough differential diagnosis which includes, but is not limited to, pancreatic ductal adenocarcinoma can help to prevent errors in diagnosis. This chapter focuses on unusual solid tumors of the pancreas and discusses optimal diagnostic and therapeutic approaches for management.
ACINAR CELL CARCINOMA
Acinar cell carcinoma (ACC) is a rare pancreatic tumor accounting for less than 1% of pancreatic cancers. In contrast to pancreatic ductal adenocarcinoma, ACC arises from the acinar elements of the exocrine pancreas, not ductal epithelium. As a result, ACC often mimics the growth pattern of normal pancreatic acini and can produce digestive enzymes such as trypsin, chymotrypsin, and lipase. ACC occurs more commonly in men than women (2:1) and primarily affects individuals in the sixth and seventh decades of life. At presentation, approximately 50% of patients are asymptomatic at diagnosis; however, some patients may present with abdominal pain (45%) or weight loss (35%).1,2 Approximately 10% of patients with ACC present with a paraneoplastic syndrome caused by excessive pancreatic enzyme production, which is characterized by the presence of subcutaneous fat necrosis, bony infarcts, arthritis, and eosinophilia. No specific serum or plasma tests exist which are diagnostic for ACC, but serum lipase levels can be elevated in up to 25% of patients.1,3 Serum tumor markers such as carbohydrate antigen (CA) 19-9, alpha fetoprotein (AFP), and carcinoembryonic antigen (CEA) are variably expressed.3 In some patients, the combination of serum lipase and AFP can be quite helpful in assessing tumor burden in response to therapy.
Acinar cell carcinomas tend to be large (mean diameter size of 7.1 cm2 to 10.6 cm2) and unifocal at presentation. The lesions can be entirely solid when small (and look identical to pancreatic adenocarcinoma on CT imaging), but larger tumors often outgrow their blood supply and develop central areas of necrosis. They have been reported to occur predominantly in the head of the pancreas (55%) but can also occur in the body and tail as well.4 Characteristic cross-sectional imaging findings include the presence of a large, exophytic, well-circumscribed mass with capsular enhancement but central hypodensity (Fig. 149-1). There may also be internal foci of calcifications.3 The radiographic differential diagnosis of ACC includes pancreatic ductal adenocarcinoma, pancreatic neuroendocrine tumor, solid pseudopapillary tumors, pancreatoblastoma, and mucinous cystic neoplasms.
Axial image of a contrast-enhanced CT scan from a patient with a large acinar cell carcinoma demonstrating local compression of the duodenum (white arrows) causing gastric distension (S) ...