Merkel cell carcinoma and chronic lymphocytic leukemia (collision tumor) of the arm: a diagnosis by fine-needle aspiration biopsy.

Simultaneous involvement of the same anatomical site by two different primary malignant tumors is rare. Cases of hematopoietic malignancies associated with breast and skin neoplasms have been described. The association of chronic lymphocytic leukemia (CLL) and Merkel cell carcinoma (MCC) has been established, although the cause for this association is still unclear. There are reports of MCC metastatic to lymph nodes involved by CLL. We report the case of a 57-year-old man with history of CLL with concurrent involvement of the arm by CLL and MCC diagnosed on fine-needle aspiration biopsy (FNA). To our knowledge, this is the first reported case of such tumors colliding in a nonlymphoid site, diagnosed by FNA in the English literature.