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Abstract
Acinar cell carcinoma of the pancreas exhibits a spectrum of histologic appearances. Some tumors can be readily identified by light microscopy, but others resemble endocrine/neuroen-docrine neoplasms. Ultrastructurally, though large zymogen granules of acinar cells are usually distinctive, the zymogen granules of neoplastic acinar cells are sometimes abnormally small, overlapping in size with the granules of endocrine/neu-roendocrine neoplasms. Six cases of acinar cell carcinoma, two with a typical histologic appearance and four that resembled endocrine/neuroendocrine tumors, were studied ultrastructurally. In addition to zymogen 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 islet 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 provide a clue for the diagnosis of acinar cell carcinoma.