Collision tumor of primary merkel cell carcinoma and chronic lymphocytic leukemia/small lymphocytic lymphoma, diagnosed on ultrasound-guided fine-needle aspiration biopsy: A unique case report and review of literature

We report an extremely rare case of skin collision tumor between primary Merkel cell carcinoma (MCC) and chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL) first diagnosed on ultrasound-guided fine-needle aspiration biopsy (US-FNA). A 95-year-old female with a history of CLL presented with a slow growing left malar mass was referred to our clinic for US-FNA. US scan showed a well-defined subcutaneous mass (2.78 cm) with complex echogenicity. On-site assessment showed a cellular aspiration which was interpreted as small blue round cell tumor. On further examination, smears and cell block showed dimorphic populations of relatively larger cells with neuroendocrine features and smaller lymphoid cells. Immunocytochemical studies of cell block sections revealed that the larger cells were positive for CD56, Chromogranin, Synaptophysin, CK8/18, CK20 (dot-like pattern); and the smaller cells were positive for CD45. Flow cytometric analysis showed a majority of CD16/CD56 positive cells, 17% of monoclonal Bcells, and 14% of reactive T cells. The immunophenotype of the monoclonal B cells were of CLL/SLL. The diagnosis of a collision tumor composed of primary MCC and CLL/SLL was confirmed. Surgical resection of the mass one month later concurred with the FNA cytological diagnosis. The fact that surgical specimen displayed a solid tumor with both CLL/SLL and MCC components ruled out the possibility that the FNA merely had MCC with peripheral leukemic blood contaminant. No additional MCC lesion was found in the patient, which ruled out the possibility of metastatic MCC to a lymphomatous lymph node.