Synchronous Recurrent Merkel Cell Carcinoma and Primary Cutaneous Melanoma: A Rare Case Report

Merkel cell carcinoma is a rare and aggressive neuroendocrine tumor of the skin, typically affecting elderly patients and arising in sun-damaged areas. It often presents as a painless, firm nodule and has a high recurrence rate. Superficial spreading melanoma is a malignant neoplasm of melanocytes that arises in intermittently sun-exposed areas and progresses through radial and vertical growth phases. While both tumors are individually well-documented, their synchronous occurrence is exceedingly rare. We report a case of a 74-year-old male with a history of Merkel cell carcinoma who presented with two new skin lesions - an ulcerated nodule on the right thigh and a pigmented plaque on the left anterior chest. Histological and immunohistochemical evaluation confirmed the recurrence of Merkel cell carcinoma in the thigh lesion and a primary superficial spreading melanoma in the chest lesion. This case underscores the role of shared pathogenetic factors such as ultraviolet radiation, immunosenescence, and high mutational burden in the development of both malignancies. Diagnostic workup requires comprehensive histopathological and immunohistochemical analysis. Management of both tumors involves wide local excision and sentinel lymph node biopsy, with adjuvant radiotherapy more commonly required in Merkel cell carcinoma due to its high radiosensitivity and immunotherapy playing a central role in advanced disease for both. This case highlights the importance of a multidisciplinary approach for accurate diagnosis and individualized treatment planning.