Malignant melanoma originating in the thymus; a case report

A 50-year-old female was referred to us because of an abnormal mass at the right anterior mediastinum on X-ray film. Chest CT showed a mass 4 cm in diameter adjacent to the ascending aorta. Operation was performed by median sternotomy. The tumor was a black and elastic hard mass, which was located in the lower portion of right lobe of the thymus, and pericardial invasion was noted. The intraoperative histological diagnosis was malignant melanoma (MM) or malignant melanotic schwannoma (MMS). Histological examination showed mainly spindle cells with heavy melanin pigmentation and partially epithelioid cells. The tumor was examined by light microscopy with immunohistochemistry (S-100 protein and HMB-45 antigen) and by electron microscopy, but no evidence was found to distinguish MM from MMS. Through the microscopic findings of the form of the tumor cells, and the frequency of the occurrence of each tumor, a diagnosis of MM was made. Both MM and MMS originate in neural crest and histologically similar, so the differential diagnosis between these two tumors is difficult. Thymus was diagnosed as the primary site, since no primary lesions could be located except in the thymus at postoperative evaluation and microscopically the tumor was surrounded by thymic tissue. This is the fourth reported case of MM originating in the thymus.