Collision Tumor of Merkel Cell Carcinoma and Chronic Lymphocytic Leukemia/Small Lymphocytic Lymphoma: A Cytological and Flow Cytometric Diagnostic Pitfall.

The coexistence of chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL) and Merkel cell carcinoma (MCC) is a well-established but rare association. Collision tumors involving both entities within a single lymph node are exceptionally uncommon and pose considerable diagnostic challenges, particularly on cytological specimens. Here, we report the case of a 68-year-old male presenting with right axillary lymphadenomegaly. Fine-needle cytology (FNC) demonstrated a monomorphic population of intermediate-sized lymphoid cells, along with occasional larger cells exhibiting plasmacytoid features. Flow cytometric immunophenotyping showed a CD19+/CD5+/CD23dim B-cell population with lambda light chain restriction consistent with CLL/SLL, and a second CD45-/CD138+/CD200+/CD81+ population suspicious for aberrant plasma cells, raising the differential diagnosis of plasmacytoma or plasmacytic differentiation of CLL/SLL. Subsequent lymph node excision and histological examination revealed a collision tumor comprising MCC and CLL/SLL. This case highlights a rare but significant pitfall in lymph node FNC specimens, in which small neuroendocrine carcinoma was misinterpreted as an aberrant hematopoietic population.