OBJECTIVETo study the clinicopathologic features and differential diagnosis of giant cell-rich osteosarcoma (GCRO) and giant cell tumor of bone (GCT).

METHODSThe clinical, radiologic, pathologic and immunohistochemical features of 18 cases of GCRO and 118 cases of GCT were evaluated.

RESULTSThe mean age of patients with GCRO was 24.6 years. Fifteen of the 18 cases arose in the metaphysis of long bones. GCRO presented as a large poorly-defined mixed lytic and blastic mass, associated with cortical destruction and formation of large soft tissue component. Histologically, GCRO was characterized by a predominance of numerous osteoclast-like giant cells admixed with scanty osteoid which was formed by neoplastic cells in different levels of anaplasia and pleomorphism. In the 118 cases of GCT studied, the mean age of patients was 34.5 years. Most of them (108 cases) arose in the epiphyseal region of long bones. They usually presented as expansile eccentric and osteolytic lesions. Invasive GCT displayed local cortical destruction. Histologic examination of GCT revealed the presence of large number of osteoclast-like giant cells and mononuclear stromal cells. The mononuclear stromal cells possessed poorly defined cytoplasm, showed little cytological atypia and did not carry atypical mitotic figures. They were positive for p63 (83.9%, 99/118). Reactive bone could be observed at the periphery.

CONCLUSIONSGCRO represents a special form of osteosarcoma which shows overlapping clinicopathologic features with invasive GCT. The presence of nuclear atypia, atypical mitoses and osteoid matrix produced directly by neoplastic cells are more in favor of GCRO. These features however may not be demonstrated in full in limited small biopsy samples. It is thus important to analyze all clinical, radiologic and pathologic features before a definitive diagnosis is made.