Malignant melanoma presenting as an intrathymic tumor. A primary thymic melanoma?

The identification of malignant melanoma in a visceral organ of nonepidermal origin is not an uncommon occurrence. Frequently, these cases are solitary metastases that present years after a thin epidermal melanoma has been diagnosed (and sometimes forgotten). However, primary visceral melanomas have been reported that have not been preceded by an epidermal lesion. We describe herein a unique case of melanoma presenting as a primary intrathymic tumor. The patient had no previous history of epidermal melanoma, and extensive workup did not reveal evidence for an alternative primary site. The tumor exhibited histologic features characteristic of melanoma, including an abundance of large pleomorphic cells with eosinophilic cytoplasm, prominent nucleoli, and occasional intranuclear inclusions. Tumor cells stained for HMB-4. and S-100 protein and ultrastructural analysis revealed stage II and stage III melanosomes. The patient remained free of disease until intrathoracic recurrence was detected on a computed tomographic scan 14 months later. The lack of clinical history and physical findings of melanoma at presentation, the intrathymic location of the tumor, and the pattern of recurrence suggest that this case likely represents a primary thymic melanoma, a previously unreported entity.