Intraductal and papillary variants of acinar cell carcinomas: a new addition to the challenging differential diagnosis of intraductal neoplasms.

The recognition and differential diagnosis of pancreatic intraductal neoplasms (IN) have gained importance in the past few years, as the incidence of these tumors (especially intraductal papillary mucinous neoplasms-IPMNs) have risen to >10% of pancreatic resections, and their significance as precursors of invasive cancer is better appreciated. Acinar cell carcinomas (ACCs) are typically solid tumors; however, we have recently encountered 7 ACCs with either intraductal growth and/or a papillary/papillocystic pattern that could be mistaken for IN. The clinicopathologic features of these cases were studied. Four patients were male and 3 female, with a mean age of 59 and mean tumor size of 4.9 cm (as compared with 10 cm in conventional ACCs). Only 1 patient had metastasis at the time of diagnosis (as opposed to 50% in usual ACCs). In 5 cases, the tumors had nodular growth of sheet-forming acinar cells, some of which were within ducts, as evidenced by the polypoid nature of the process, partial ductal lining, and presence of small tributary ducts in the walls. In 3 cases, the tumor had papillary and/or papillocystic growth, at least focally. All cases had cystic areas. No mucin was identified. All expressed trypsin. Markers of ductal differentiation were either absent or focal. A minor endocrine component was present in 3. The main histologic findings that distinguished these tumors from IPMNs were the more sheetlike nature of the nodules (rather than villous or arborizing papillae), cuboidal cells, overall basophilia of the cytoplasm, prominent nucleoli, apical granules, intraluminal crystals or pale, acidophilic secretions (enzymatic condensations), and lack of mucin. In conclusion, some ACCs show intraductal growth or exhibit papillary patterns, which can mimic IN, especially IPMNs. In such cases, attention to morphologic details described above, and immunohistochemistry are helpful. The clinical significance of this variant is difficult to determine; however, it appears that the tumors are relatively small and metastasis at presentation is less common than typically seen in ACCs (1/7 vs. 50%).