MRI findings in parosteal osteosarcoma: correlation with histopathology.

PURPOSE To assess the role of magnetic resonance imaging (MRI), particularly signal intensity changes, in predicting the dedifferentiation of parosteal osteosarcoma, and to evaluate other factors that may affect grading on MRI. MATERIALS AND METHODS MRI of 12 patients with parosteal osteosarcoma diagnosed on plain radiography were reviewed with regard to size, location, extent, soft tissue component, intramedullary invasion, and signal characteristics. The findings are correlated with histopathologic results. RESULTS By histopathological examination there were 6 Grade I, 3 Grade II, and 3 Grade III tumors. Average size was 11 cm. All cases had a soft tissue component. Intramedullary extension was evident in 3/6 of the Grade I cases, 2/3 of the Grade II cases, and all (3/3) of the Grade III cases. T1-weighted images revealed lesions of marked hypointensity. Signal intensity on T2-weighted images varied with the presence of necrosis and hemorrhage in relation to size, regardless of the grade of the tumor. Contrast-enhanced images revealed enhancement of the solid components; no enhancement was observed in the necrotic or hemorrhagic parts. CONCLUSION High and heterogeneous signal on T2-weighted images of Grade I, II, and III tumors is not specific for the dedifferentiated component, due to hemorrhage and necrosis in large masses. Therefore, high signal intensity on T2-weighted images is not always a reliable way to predict the grade of the tumor. Contrast enhanced T1-weighted images can be valuable to show the solid component in the heterogeneous areas on T2-weighted images, and can be useful in guiding the biopsy.