Collision of Malignant Melanoma and Small Lymphocytic Lymphoma/Chronic Lymphocytic Leukemia: A Case Report and Literature Review

Collision tumors are exceptionally rare. We report here a case of a cutaneous collision tumor composed of metastatic melanoma and small lymphocytic lymphoma (SLL). A 56-year-old man with a prior history of melanoma and chronic lymphocytic leukemia (CLL) presented with a new cutaneous nodule near the previous melanoma excision site on the right lower flank. Imaging identified a separate skin lesion in the right medial thigh. Histopathologic examination of both lesions revealed an infiltrative malignant melanoma in the flank and a melanoma with similar morphology in the medial thigh, adjacent to a dense, monotonous lymphoid infiltrate involving the subcutis and skeletal muscle. Immunohistochemical studies demonstrated that the melanoma cells were positive for SOX10, MART-1 and cyclin D1; the lymphoid population was positive for CD5, CD20, PAX5 and CD23, and negative for CD3, CD10 and cyclin D1. Given the known history, the combined morphologic and immunophenotypic findings support the diagnosis of a cutaneous collision tumor composed of melanoma and SLL/CLL. A review of the literature highlights the rarity of such cases and sheds light on current theories of pathogenesis, diagnostic pitfalls, and the mainstay treatment of this rare collision tumor.