Collision tumor composed of Merkel cell carcinoma and lentigo maligna melanoma.

We report the case of an immunocompetent 79-year-old white man with a history of melanoma in situ on the back with a collision tumor composed of a Merkel cell carcinoma (MCC) and lentigo maligna melanoma on the left cheek. The cells of the MCC expressed cytokeratin 20 (CK 20) in a diffuse cytoplasmic pattern, AE1 and AE3 in a perinuclear dot-like pattern and diffusely with neuron-specific enolase. The tumor cells of the MCC failed to express thyroid transcription factor-1. The atypical melanocytes of lentigo maligna melanoma expressed Melan-A and S-100. At the same visit, a lentigo maligna was diagnosed by excisional biopsy on the right cheek. The variability in expression of CK 20, AE1 and AE3 in MCC are reviewed. Prior reports of MCC in collision with non-melanoma skin cancers are reviewed. Additionally, the role of immunosuppression in the development of MCC is considered.