FILAMENTOUS INCLUSIONS IN ACINAR CELL-CARCINOMA OF THE PANCREAS

Acinar cell carcinoma of the pancreas exhibits a spectrum of histologi c appearances. Some tumors can be readily identified by light microsco py, but others resemble endocrine/neuroendocrine neoplasms. Ultrastruc turally, though large zymogen granules of acinar cells are usually dis tinctive, the zymogen granules of neoplastic acinar cells are sometime s abnormally small, overlapping in size with the granules of endocrine /neuroendocrine neoplasms. Six cases of acinar cell carcinoma, two wit h a typical histologic appearance and four that resembled endocrine/ne uroendocrine tumors, were studied ultrastructurally. In addition to zy mogen granules and abundant rough endoplasmic reticulum, all cases of acinar cell carcinoma exhibited pleomorphic, membrane bound inclusions that contained filaments. Similar inclusions were not identified in i slet cell or carcinoid tumors, and several findings indicate that the inclusions represent deranged zymogen granules. In the ultrastructural study of a pancreatic neoplasm with granules, these inclusions may pr ovide a clue for the diagnosis of acinar cell carcinoma.