Immunohistochemistry in the differential diagnosis of acinar and endocrine pancreatic neoplasms.

Histologic differential diagnosis of acinar cell carcinoma (ACC), mixed acinar-endocrine cell carcinoma (MAEC), and pancreatic endocrine tumors (PET) can be difficult but is important because of differences in their clinical behavior. This study investigates the utility of immunohistochemistry (IHC) in this differential diagnosis using immunohistochemical stains that are available in most laboratories. IHC was performed on paraffin-embedded tissue in ACC (n = 6), MAEC (n = 2), and PET (n = 13), using synaptophysin (SYN), chromogranin (CHR), chymotrypsin (CHY), and alpha-1-antitrypsin (AAT). Electron microscopy (EM) was performed in all cases to confirm the diagnosis. Long-term follow-up and death of disease (DOD) was known in all patients. The ACCs stained as follows: CHY (4/6), AAT (3/6), SYN (4/6); CHR was negative in all cases. Both cases of MAEC stained with CHY, AAT, and SYN (2/2); CHR was negative. PET stained as follows: SYN (13/13), CHR (8/13), CHY (4/13), AAT (5/13). In the ACC/ MAEC group, six of eight patients were DOD at mean follow-up of 11 months. Among the PET, two of 16 patients were DOD at mean follow-up of 37 months. Considerable immunophenotypic overlap exists between ACC, MAEC, and PET. Consequently, one can neither confirm nor rule out a diagnosis of ACC or MAEC using generally available immunohistochemical stains alone. These findings support a role for EM in the evaluation of exocrine and endocrine pancreatic neoplasms.