CK20/CK7 double-negative Merkel cell carcinoma in situ: A case report.

An 83-year-old male with a history of both melanoma and non-melanoma skin cancers presented with a light pink non-ulcerated slightly raised 0.6 × 0.5-cm papule on his left lower extremity. Biopsy specimen revealed a proliferation of intraepidermal round blue cells. On immunohistochemical staining, CD56, chromogranin, and pancytokeratin were faintly positive within the lesional population, while synaptophysin was strongly positive. CD45, CK5/6, CK7, CK20, Melan-A, SOX10, and TTF-1 stains were negative. There was no dermal component identified. A Merkel cell polyomavirus stain was negative. Distant metastases and other in situ pathologies were excluded and a diagnosis of Merkel cell carcinoma in situ (MMCIS) was made. The majority of MCCIS lesions reported in the literature have been discovered amongst other non-melanoma neoplasms. Our findings of an MCCIS with purely intraepidermal involvements without the association with another squamous cell neoplasm is rare finding.