Answer: Chondrosarcoma

Histology: none provided

Discussion: Chondrosarcomas are malignant proliferations of cartilage which tend to occur in the axial skeleton of older people. The radiograph in this case is helpful to diagnose a malignant tumor because of its destructive and aggressive permeative pattern. Histologically the cartilage shows atypical nunclei and a permeative growth pattern around native trabeculae. This permeative growth pattern is diagnostic of malignancy in most primary bone tumors. The radiograph, however, is a most important tool leading to the diagnosis of chondrosarcoma. Although metastatic carcinoma is the most common bone tumor in this age group, there is no evidence of epithelial cells in this case. Enchondromas occasionally occur in the pelvis. Occasionally, the cartilage of enchondroma can show atypia. However, the destructive and aggressive radiographic pattern precludes the diagnosis of an enchondroma. Therefore, the radiograph should always be consulted when having to differentiate between benign and malignant cartilage tumors. In addition, the permeative growth pattern of the cartilage in this case is not seen in enchondroma. Chondromyxoid fibromas are frequently misdiagnosed as chondrosarcomas. However, the lobular nature of chondromyxoid fibroma along with the myxoid stroma and stellate cells are absent from this case. Occasionally, there may be cellular atypia in a chondromyxoid fibroma. But the absence of mitotic figures in this case excludes the diagnosis of malignancy.