Primary subcutaneous inguinal malignant melanoma mimicking a soft tissue sarcoma

Malignant melanoma typically arises from cutaneous or mucosal melanocytes and is often recognized through visible pigmentation changes. Rarely, melanoma may present as a deep or subcutaneous soft tissue mass without epidermal involvement, thereby mimicking sarcomas or other mesenchymal tumors. Such atypical presentations are exceedingly rare and may delay diagnosis and treatment.

A 66-year-old woman with well-controlled hypertension presented with a painless, mobile, left inguinal mass that had been present for approximately 10 years but recently increased in size. Physical examination revealed a firm, 6-cm lesion with intact overlying skin, without pigmentation or ulceration. MRI demonstrated a superficial, encapsulated, ovoid mass with heterogeneous signal and hemorrhagic changes, initially suggesting a soft tissue sarcoma. The lesion was surgically excised and histopathology revealed malignant pleomorphic cells, with immunohistochemistry positive for S-100, HMB-45, and Melan-A, confirming malignant melanoma. No epidermal involvement or distant metastasis was identified. Postoperative recovery was uneventful, and the patient was referred for multidisciplinary oncologic management.

Primary superficial inguinal melanoma without cutaneous involvement is exceptionally rare. The long-standing, indolent evolution over 10 years, followed by sudden enlargement, highlights the importance of vigilance in evaluating chronic soft tissue masses. MRI findings can mimic sarcoma, underlining the critical role of histopathology and immunohistochemistry in establishing the diagnosis. This case emphasizes the need to include melanoma in the differential diagnosis of atypical inguinal masses, particularly in older adults.

This case illustrates an unusual presentation of superficial inguinal melanoma mimicking a soft tissue sarcoma. It underscores the importance of histopathologic confirmation in atypical masses and highlights the educational value of considering melanoma in the differential diagnosis of inguinal tumors.