MRI-Guided Assessment of Rapid Locoregional Progression of Merkel Cell Carcinoma of the Thigh Following Wide Local Excision: A Case Report.

Merkel cell carcinoma (MCC) is a rare and highly aggressive neuroendocrine skin cancer with a marked predilection for elderly and immunosuppressed patients. Although most commonly cutaneous, MCC can present with extensive locoregional soft tissue involvement, and its clinical course is characterized by rapid growth, frequent local recurrence, and early lymphatic and distant spread. We report the case of a 74-year-old woman who underwent wide local excision of a solitary thigh mass, with histopathology confirming MCC and a lateral margin that was uninvolved but immeasurably close. The patient was lost to follow-up shortly after surgery and, as a result, did not receive the recommended adjuvant treatment. She presented three months later with progressive swelling of the limb. Follow-up magnetic resonance imaging (MRI) revealed extensive, confluent cutaneous and subcutaneous masses spanning 32 cm along the thigh, infiltrating the adjacent muscular compartments, encasing the common femoral and common iliac vessels, and extending into the pelvis with intimate contact with the bladder wall and pelvic bones, without evidence of bone lysis. This dramatic progression, within only three months and in the absence of adjuvant therapy, rendered the lesion surgically unresectable. This case illustrates the aggressive natural history of MCC in an elderly patient lost to follow-up, highlights the central role of MRI in characterizing locoregional extension and supporting the assessment of surgical unresectability, and underscores the importance of adjuvant radiotherapy following excisions with close margins to prevent such outcomes.