Metastatic interdigitating dendritic cell sarcoma masquerading as a skin primary tumor: a case report and review of the literature.

We report an unusual case of a lymph node interdigitating dendritic cell sarcoma (IDCS), metastatic to skin, in a 73-year-old patient. The patient initially presented as having a primary skin tumor with lymph node metastasis. The metastatic IDCS was initially read as an atypical fibroxanthoma. However, the morphology seen on the lymph node excision, paired with immunohistochemistry and electron microscopy studies, was diagnostic for an IDCS. Additional immunohistochemistry was performed on the shave biopsy, confirming that the skin tumor was a metastasis. IDCS is a rare tumor that belongs to the histiocytic and dendritic cell group of tumors. Diagnosing this entity is difficult without the aid of ancillary testing such as immunohistochemistry and electron microscopy. In the workup of a spindle cell neoplasm, IDCS should be included in the differential diagnosis.