A case of interdigitating dendritic cell sarcoma/histiocytic sarcoma--a diagnostic pitfall.

Interdigitating dendritic cell sarcoma (IDCS) and histiocytic sarcoma (HS) are two distinct rare hematolymphoid neoplasms, and HS derived from a likely pre-existing IDCS has never been reported in the English literature. Diagnosis of such entities in excised specimens is difficult, but becomes more difficult with the scant amount of materials obtained with fine needle aspiration (FNA) and core needle biopsy. Here we present an interesting and unique case of an IDCS located within a mesenteric mass, which was initially diagnosed as IDCS from the cytology of FNA and core needle biopsy specimens. After brief chemotherapy, the patient again developed abdominal pain, and a HS was diagnosed based on the excised segmental small intestinal specimen. While the exact relationship between the IDCS and HS cannot be ascertained, it is most likely that the HS is derived from the IDCS, although co-existing HS in addition to IDCS from the cytology specimen cannot be completely ruled out.