Interdigitating Dendritic Cell Sarcoma Arising on the Skin: An Exceedingly Rare Tumor With a Poor Prognosis.

Interdigitating dendritic cell sarcoma (IDCS) is a rare tumor derived from dendritic cells and has been reported in slightly more than 100 cases in literature. IDCS of the skin is an exceedingly rare presentation, noted in 8 case reports in published literature. We present a rare case of a 44-year-old man who presented with primary cutaneous IDCS on his posterior neck, who was later discovered to have biopsy-confirmed IDCS lung metastases. Secondary to this, the patient was found to have a superficial spreading melanoma on his left shoulder, consistent with the notion that cutaneous IDCS increases a patient's risk of secondary malignancies. Upon further investigation of his IDCS, bilateral lung metastases of IDCS were identified and the patient received several cycles of chemotherapy. This case contributes to the limited body of knowledge on cutaneous IDCS and its treatment modalities. The poor prognosis associated with metastatic IDCS highlights the importance for further research to identify treatment regimens that lead to better prognosis.