Merkel Cell Carcinoma Arising in Association with Cutaneous T‐cell Lymphoma: A Potential Diagnostic Pitfall

Merkel cell carcinoma (MCC) is a rare, aggressive cutaneous neuroendocrine carcinoma with increased prevalence in patients with immunosuppression or B‐cell neoplasms. To our knowledge, an association with cutaneous T‐cell lymphoma (CTCL) has not been previously described. In this case we present two cases of MCC arising in the setting of CTCL. The first case was a female in her 70s with previously diagnosed stage IVA1 Sezary syndrome. Biopsy of a scaly patch revealed two distinct abnormal cell populations. The first population consisted of hyperchromatic dermal and epidermotropic lymphocytes, expressing CD3 and CD4 with diminished CD7. The second population consisted of intraepidermal clusters of larger atypical cells that expressed synaptophysin, neurofilament, CK20, and Merkel cell polyomavirus transcript. The combination of findings was consistent with intraepidermal MCC in a background of CTCL. Excision demonstrated residual intraepidermal MCC without dermal involvement. The second case was a male in his 50s with a longstanding history of mycosis fungoides, who presented with a new lesion on his right thigh. Biopsy and excision demonstrated dermal MCC without secondary involvement by CTCL. Our cases demonstrate that MCC may rarely occur in the setting of T‐cell lymphoma, and that intraepidermal MCC may mimic epidermotropic T‐cells.

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