Rapidly Enlarging Forearm Mass in a 74-Year-Old Male: A Case of Merkel Cell Carcinoma Presenting as a Soft Tissue Tumor.

Merkel cell carcinoma (MCC) is a rare, aggressive cutaneous neuroendocrine malignancy that most commonly presents in elderly patients as a rapidly growing lesion on sun-exposed skin. Because of its nonspecific clinical appearance, it can mimic a variety of benign and malignant soft tissue conditions, making diagnosis challenging. We report a rare presentation of MCC in a 74-year-old male who presented to the orthopedic clinic with a progressively enlarging soft tissue mass on the dorsoradial aspect of his right forearm. The patient reported a six-week history of swelling and constant radial-sided forearm pain that failed to improve with oral antibiotics and corticosteroids. Physical exam demonstrated a firm, deep subcutaneous mass with overlying mild warmth and flushing. Magnetic resonance imaging revealed a heterogeneous, enhancing soft tissue lesion concerning for soft tissue sarcoma, but final pathology confirmed MCC. This case highlights the diagnostic challenges of MCC and the importance of early recognition and tissue diagnosis in rapidly enlarging masses.