Abstract
Merkel cell carcinoma (MCC) is an uncommon aggressive neuroendocrine neoplasm affecting the skin. Most commonly, MCC is located in the dermis, often with extension into the subcutaneous fat. Epidermal involvement is reported in ~18% of MCC cases while MCC limited to the epidermis, MCC in situ (MCCIS), is even rarer, with only 26 cases reported. Herein, we describe a case of MCCIS with pagetoid spread. A 74-year-old male with a history of multiple non-melanoma skin cancers presented with an asymptomatic skin lesion on the face, which persisted despite completing a course of 5-fluorouracil six-weeks prior. Examination revealed a pink patch with overlying scale on the right cheek. Shave biopsy demonstrated a proliferation of atypical, hyperchromatic basaloid cells with pagetoid spread within the epidermis. The tumor cells were positive for CK20, CK7, INSM1, chromogranin and synaptophysin, and negative for p63, MCPyV, TTF1, MelanA, ER, and PR. A diagnosis MCCIS was rendered and Mohs micrographic surgery was recommended. This case presentation of asymptomatic erythematous patches on the face in an older male aligns with previous reports of MCCIS. Unlike the 40% five-year overall survival rate of invasive MCC, no MCCIS-associated deaths have been reported. Diagnosing MCCIS can be challenging due to its rarity and mimicry of epidermal tumors with pagetoid spread—including SCCIS, malignant melanoma, and extramammary Paget disease. This case underlines the importance of considering MCCIS in the differential of atypical epidermal neoplasms with pagetoid spread.