Answer: Giant cell tumor of bone

Histology: Radiographic examination reveals a well-defined expansile lytic lesion in the proximal tibia. The lesion involves both the metaphysis and the epiphysis right up to the subchondral bone. A curettage shows cellular areas consisting of osteoclast-like giant cells and mononuclear stromal cells. There is no atypia. However, mitotic figures are present. In addition to areas of giant cells, there are numerous fibrous septae containing reactive bone that is surrounding blood filled lakes.

Discussion: The radiograph is a typical presentation of a giant cell tumor. When in long bones, this neoplasm involves the epiphyseal and metaphyseal region in skeletally mature people. The distal femur and proximal tibia are the most common locations. Sheets of giant cells and mononuclear stromal cells are present. Mitotic figures are often numerous. This particular case has large areas of aneurismal bone cyst transformation. This is a secondary process that is engrafted on the primary neoplasm. Fibrous wall septae containing reactive bone and multinucleated giant cells are a feature of aneurismal bone cysts. Areas of sarcoma characterize malignant giant cell tumors. There are areas of pleomorphic cells and atypical mitotic figures. Malignant giant cell tumor is a biphasic neoplasm with both areas of typical giant cell tumor and areas of high-grade sarcoma. Although there are areas of definite aneurismal bone cysts in this lesion, the radiographic setting and areas of typical giant cell tumor indicate that a giant cell tumor of bone is the primary lesion and the aneurismal bone change cyst is secondary. Telangiectatic osteosarcomas, by contrast, have areas of typical high-grade osteoblastic osteosarcoma admixed with the aneurismal bone cyst-like areas. These atypical cells are not difficult to find.