Rare Primary Mediastinal Melanoma: SVC Obstruction Treated with Endovascular Stenting and Immunotherapy

Primary mediastinal melanoma is exceptionally rare, frequently misdiagnosed as thymic carcinoma, lymphoma, or germ cell tumors—common anterior mediastinal masses. Diagnostic challenge arises from nonspecific imaging features and overlapping morphology. We present a 48-year-old man with progressive neck swelling, voice changes, and orthopnea. Imaging revealed a large, heterogeneously enhancing anterior mediastinal mass (13.5 × 9.3 cm) causing superior vena cava (SVC) obstruction with collateral venous channels. Initial differential included thymic carcinoma and lymphoma. Positron emission tomography–computed tomography showed metabolically active disease (maximum standardized uptake value of 23.3) with mediastinal lymphadenopathy. Biopsy revealed poorly differentiated malignant cells. Immunohistochemistry positive for HMB45, SOX10, and c-KIT established a definitive diagnosis of malignant melanoma. The patient underwent endovascular SVC stenting followed by nivolumab-based immunotherapy. This case underscores diagnostic pitfalls in rare mediastinal masses, emphasizing immunohistochemistry's critical role in distinguishing melanoma from other malignancies, and highlights multidisciplinary management combining endovascular intervention with systemic therapy.