Malignant Melanoma Arising in a Primary Mediastinal Teratoma: Case Report of a Rare Phenomenon and Review of the Literature

Abstract Primary mediastinal germ cell tumors (GCTs) account for a small subset of all GCTs. Teratoma is the most common GCT of the mediastinum and usually occurs in adults from 20 to 40 years of age. Malignant somatic transformation is a rare phenomenon that has been described in both primary and metastatic mediastinal GCTs. The most common types of malignant somatic transformation described in mediastinal GCTs are sarcomas and carcinomas, with very few cases of melanoma found in the literature. We report the case of a 21-year-old male patient who presented with anterior mediastinal mass that on initial biopsy and workup revealed a mediastinal mixed GCT with teratoma and yolk sac components. Following chemotherapy, the resected mass demonstrated residual teratoma with malignant transformation into melanocytic, neuroectodermal, and undifferentiated sarcomatous elements. The patient rapidly developed metastases to the lung, liver, spleen, and spine. Biopsy of the bone marrow and liver confirmed metastatic dissemination by melanocytic transformed GCT. Molecular analysis for BRAF mutation was negative. The rapid multiorgan pattern of metastatic spread indicates a very aggressive phenotype. This case represents the second reported case of malignant melanocytic tumor as part of multilineage malignant differentiation arising from mediastinal GCT.