Answer: Rosai-Dorfman disease

Histology: The pancreatic parenchyma is replaced by sheets of histiocytes with abundant pink cytoplasm. Rare cells demonstrating emperipolesis (the active penetration by one cell into and through a larger cell) can be seen.

Discussion: Rosai-Dorfman disease (Sinus Histiocytosis with Massive Lymphadenopathy) usually presents clinically with massive, painless, bilateral lymphadenopathy in the neck and with fevers. Most patients are in the first or second decade of life and Rosai-Dorfman disease has a predilection for African Americans. Emperipolesis, the engulfment of lymphocytes by large histiocytes, is the hallmark of this disease. The histiocytes are immunoreactive for S-100 protein, and immunolabeling for S-100 can sometimes help highlight the emperipolesis as the phagocytized (which do not label) will be sitting within a cell that does label.