Abstract

Merkel cell carcinoma (MCC) is a rare, aggressive, primary cutaneous neuroendocrine neoplasm with propensity for local recurrence and distant metastasis. MCC is predominantly a dermal tumor and epidermal involvement only occurs in about 18% of cases. Exclusively intraepidermal MCC, also known as MCC in situ (MCCis), is extremely rare and most cases are associated with other neoplasms. We present a 78-year-old male, with history of rheumatoid arthritis for which he had been on tofacitinib for three years, presented with a five-month history of nonhealing lesion on his left temple. Physical examination revealed an 8.0 x 7.0 mm scaly pink papule on the left temple. Histopathologic examination of shave biopsy revealed exclusively intraepidermal tumor composed of uniform, small, round to oval cells with vesicular nuclei and small nucleoli, with frequent mitoses and focal necrosis. Dermal tumor was not identified. Tumor cells were positive for cytokeratin-20, cytokeratin-7, epithelial membrane antigen (EMA), CAM5.2 and synaptophysin, also weakly positive for pankeratin and focally positive for chromogranin; while they were negative for Melan-A, BerEP4 and p63. Histologic and immunohistochemical findings were consistent with MCCis. The Merkel Cell Poylomavirus (MSPyV) immunostain was negative. The patient underwent wide local excision with clear margins which confirmed MCCis and negative sentinel lymph node. This patient developed MCCis while on tofacitinib with no prior history of extensive UV exposure or any coexisting premalignant lesions, suggesting that MCCis may develop of immunosuppression even in the absence of MCPyV integration, likely due to decreased immune surveillance of tumor cells. Our case is the first case developed MCCis while on long term JAK-inhibitor (tofacitinib). We recommend that patients on long-term immunosuppressive medications have regular skin examination by a dermatologist due to their increased risk of cutaneous malignancies, including MCC.

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