Osteogenic melanoma: report of a case mimicking osteosarcoma and review of the literature

A 32-year-old male presented with a right thumb lesion of 11-month duration. A clinical diagnosis of pyogenic granuloma was entertained but the lesion failed to respond to conservative therapy and eventually necessitated amputation of his thumb. MRI of the right thumb showed an ill-defined, heterogeneously enhancing, infiltrating mass within the dorsal soft tissues abutting the distal phalanx and measuring 4.2 x 2.4 x 0.7 cm. Histologically, the tumor was composed of a high-grade osteosarcoma with a chondrosarcomatous component localized within the underlying bone and a more superficial spindle cell component in the overlying soft tissue. The epidermis was focally ulcerated overlying the lesion. The key to the diagnosis was provided by intense staining of the spindle cell component for S100 protein indicative of a spindle cell melanoma. The presence of an associated osteogenic sarcomatous component established a final diagnosis of osteogenic melanoma. The localization of this element to phalangeal bone is distinctly unusual and resulted in the close resemblance to a primary osteogenic sarcoma.