Fibular giant cell-rich osteosarcoma virtually indistinguishable radiographically and histopathologically from giant cell tumor-analysis of subtle differentiating features.

Giant cell-rich osteosarcoma by its abundance of osteoclastic giant cells and paucity of tumor osteoid, leads to its easy confusion with giant cell tumor during biopsy interpretation. In this report, we describe a unique case of upper fibular metaphyseal giant cell-rich osteosarcoma in a 12-year-old boy; the radiographic and histopathologic features of the biopsy and initial resected tumor are virtually indistinguishable from conventional giant cell tumor. The tumor rapidly recurred 7 months after resection with metastasis to the groin lymph nodes, was resistant to first-line chemotherapy and pursued an aggressive course, developing disseminated metastasis to the lung, liver, pelvis, scapula and clavicle, and resulted in the death of the patient 21 months after initial presentation. The subtle features alerting one to the possibility of giant cell-rich osteosarcoma are retrospectively evaluated in comparison with cases of metaphyseal conventional giant cell tumors, four from our records and those from literature review. We conclude that the occurrence of a giant cell-rich lesion in the metaphysis of a skeletally immature individual merits careful assessment for the presence of periosteal reaction, permeative infiltrative margins, lacelike osteoid formation, high mitotic activity or Ki67 proliferative index, and extra-tumoral lymphovascular permeation, since the possibility of an aggressive lesion notably giant cell-rich osteosarcoma probably increases with the number of such features.