A case of histiocytoid angiosarcoma mimicking Rosai-Dorfman disease histopathologically.

There are very few documented cases of histiocytoid angiosarcoma in the literature. We report a rare case of histiocytoid angiosarcoma demonstrating emperipolesis, a histopathologic finding that mimics Rosai-Dorfman disease (RDD). A 77-year-old male presented with a subcutaneous nodule on his left forehead. Microscopic examination of the tumor revealed a dense lymphohistiocytic and plasmacytic infiltrate with large epithelioid cells, many of which showed abundant pale eosinophilic to foamy-appearing cytoplasm, and some of which displayed phagocytosis of intact inflammatory cells and erythrocytes. The tumor also showed significant cytologic atypia and pleomorphism. Immunohistochemical stains showed strong staining of the histiocytoid cells and focal anastomosing-like vascular spaces for CD31 and ERG-1, but were essentially negative for CD68, lysozyme, CD163, S100, and CD1a, consistent with a vascular endothelial tumor. This case expands the spectrum of findings that can be identified in angiosarcomas, and should help to prevent potential misdiagnosis as a less aggressive tumor such as RDD.