Incidental chronic lymphocytic leukaemia in a biopsy of Merkel cell carcinoma.

Chronic lymphocytic leukaemia (CLL) shows cutaneous involvement in 2% of cases. Merkel cell carcinoma (MCC) is a rare primary cutaneous epithelial neoplasm most commonly found in sun-exposed sites in elderly male Caucasians. A 66-year-old man presented with a 2-month history of a purple painless 2 cm tumor on the scalp. Excision biopsy revealed an incompletely excised MCC, and at the periphery of the MCC, a lymphocytic infiltrate interpreted as reactive. A re-excision biopsy showed residual MCC as well as dense aggregates of small lymphocytes within and surrounding the MCC. Immunohistochemistry showed characteristic dot-like cytoplasmic positivity for cytokeratin 20 in the MCC; the lymphocytic infiltrate was positive for CD5, CD20 and CD23, diagnostic of CLL. Subsequent staging revealed widespread lymphadenopathy, and lymph node biopsy showed CLL. Histologically, CLL and MCC are 'round blue cell tumors' and are therefore in the differential diagnoses of each other. Whenever there is a more prominent than expected infiltrate of small lymphocytes surrounding a skin lesion in an elderly patient, immunohistochemistry to rule out CLL is advised. This case adds to the literature suggesting an increased incidence of CLL and other neoplasms in patients with MCC and vice versa.