Periosteal Ewing-like adamantinoma.

We report a Ewing-like adamantinoma of the periosteal region of the right tibia in a 15-year-old boy. The tumour was well demarcated but unencapsulated and showed cortical bone erosion. Histologically, the neoplastic cells were arranged in trabecular and cord-like patterns with fibrous, hyalinized, and myxoid stroma. Cellular atypia was mild, and mitotic figures were rarely seen. Many tumour cells expressed wide keratin, epithelial membrane antigen, leu 7, synaptophysin, Ewing's sarcoma-related antigen O13, and some were positive for neuron-specific antigen, vimentin, and CD68. The tumour was negative for S-100 protein, desmin, alpha-smooth muscle actin, and muscle-specific actin. Flow cytometric analysis showed that the tumour was aneuploid. After wide excision the patient has been well for the 16 months since diagnosis.