Intraepidermal Merkel cell carcinoma with no dermal involvement

Cutaneous Merkel cell carcinoma (MCC) typically involves the dermis. Less than 10% of MCC have epidermal involvement. Only one MCC confined exclusively to the epidermis has been previously reported but was not recognized until the lesion recurred with typical MCC in the dermis. We present a case of a wholly intraepidermal pagetoid MCC without dermal involvement in a 74-year-old man with a 2.0-cm solitary verrucous papule on the left index finger. The initial biopsy and complete excision specimens showed marked epidermal hyperplasia, focal prominent squamous cell atypia, and MCC with florid pagetoid spread through the epidermis. There was no evidence of tumor within the dermis. The pagetoid MCC tumor cells showed diffuse cytoplasmic staining with antibodies to cytokeratin 20, and negative staining for chromogranin, neurofilament, S-100, vimentin, HMB45, leukocyte common antigen, and CD3. The cell of origin of MCC is still debated. The existence of an entirely intraepidermal variant of MCC would lend support to the view that MCC is a neoplastic expression of Merkel cells in at least some cases. Dermal-based MCC is a high-grade primary cutaneous neoplasm, but MCC confined exclusively to the epidermis may have a better prognosis.