Post-obstructive cyst formation in pancreas and cystic acinar transformation: Are they same?

Pancreatic "acinar cell cystadenoma" (PACA) is a rare benign pancreatic cystic lesion showing acinar cell differentiation. The neoplastic nature of PACA has been questioned and its exact pathogenesis remains unclear. To investigate that acinar cell differentiation is a non-specific metaplastic phenomenon that can occur in pancreatic ductal system, especially when chronically inflamed and dilated, and doesn't necessarily imply an acinar cell neoplasm, we retrospectively analyzed cases diagnosed as PACA and cases with post-obstructive cystic dilatation of pancreatic ducts for acinar cell differentiation using immunohistochemistry for trypsin. The etiology of obstruction was microscopic periductal pancreatic neuroendocrine tumors (PanNET) and pancreatic ductal adenocarcinomas (PDAC). All cases of post-obstructive cystic dilatation showed multiple varying sized cysts distal to the obstruction with histologic findings virtually identical to PACAs. The cysts in both conditions were lined by a single layer of non-dysplastic flattened to columnar ductal-type epithelium with areas of acinar cell differentiation. Trypsin immunohistochemistry confirmed presence of acinar cell differentiation in all cases of post-obstructive cysts and PACAs. Our results suggest that acinar cell differentiation is common in post-obstructive cyst formation, and changes are identical to PACAs. These findings further support the notion that PACA is a benign non-neoplastic cystic lesion with acinar cell differentiation. The findings also suggest that in cases with histology resembling "PACA" or showing diffuse ductal cystic dilation, careful gross examination should be carried out for a proximal obstructive lesion, which can be subtle and easily be missed on initial examination.