An unusual round cell tumor of the tibia with granular cells.

We present the clinical, radiographic and histopathologic findings of an unusual tumor that originated in the diaphysis of the tibia in a 10-year-old boy. Clinical symptoms had been present for approximately 2 years and radiographic abnormalities for > or = 8 months before biopsy and subsequent resection of the neoplasm. The child is doing well 4 1/2 years later. Microscopically, the tumor was not typical of any bone tumor with which we are familiar. It was a round-cell tumor with extensive fibrosis, prominent cytoplasmic granularity, and isolated immunoreactivity for vimentin, epithelial membrane antigen, and antichymotrypsin. Ultrastructural examination uncovered the presence of both well-formed desmosomes and cell-associated basement membrane material in addition to abundant phagolysosomes. Classification of this tumor is a challenge; the differential diagnosis includes atypical adamantinoma, atypical Ewing's sarcoma, and small-cell osteosarcoma. We favor the former interpretation, although we raise the possibility that it may be a unique lesion.