Preoperative intra-arterial chemotherapy and limbsalvage surgery for the treatment of osteosarcoma has led to marked improvement in long-term patient survival (1 3). The most accurate prognostic factor is the percentage of tumor necrosis after chemotherapy (4). Purpose : To evaluate the role of enhanced MR imaging in monitoring tumor response to preoperative chemotherapy for osteosarcomas. Materials and Methods : Fo r t y -s even patients (30 males and 17 females, with a mean age 17 years ; range 8 44 years) with osteosarcomas were included in this study. We obtained spin echo T1-, T2-, and enhanced T1-weighted images before and after preo p e r a t i ve chemotherapy and in all patients correlated changes in MR parameters with histopathologic response. We also obtained 19 specimen MR images, correlating these with histopathologic results in order to estimate tissue specific signals. Patients with more than 10% viable tumor in the resected specimen were considered poor responders (n=26), while those with 10% or less viable tumor were considered good responders (n=21).Results : Four distinct patterns of signal intensity corresponded, respectively to dead bone and dense fibrosis (low on T1-and T2-weighted images), viable tumor cells (intermediate on T1-and high on T2-weighted images), necrosis (low on T1-and high on T 2 -weighted images), and hemorrhage (high on T1-and T2-weighted images), but a wide range of overlap was noted. In all four groups, viable tumor cells remained. Increased tumor vo l u m e, stable or increased edema and enhancement were good predictors of poor response (predictive values of 83%, 77%, and 89%, respective l y ) . Decreased enhancement was the only reliable predictor of good response (predictive va l u e, 73%). Changes in tumor margin, homogeneity, signal intensity, and joint effusion did not correlate with histopathologic response. Conclusion : Signal intensities do not reflect histologic nature. Enhanced MR imaging is a useful predictor of tumor response to preoperative chemotherapy.

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To compare the MR imaging findings of ossifying fibroma with the histopathologic findings. Materials and Methods: In eight patients (M:F=1:7; age range, 1 25 years) with pathologically proven ossifying fibroma, plain film and MR images were retrospectively analyzed in terms of signal intensity, homogeneity and patterns of contrast enhancement. The MR imaging findings and histopathology were correlated. Using 1.0-T and 1.5-T MR machines, axial T1 and T2 images and gadolinium-enhanced axial and sagittal T1 images were obtained. Results: In all cases, iso-signal intensity to muscle was observed on T1-weighted images, and high signal intensity on T2-weighted images. After intravenous injection of gadolinium-DTPA in seven cases, intense contrast enhancement was seen in all lesions, which were homogenous on T1, T2, and enhanced MR images. Moderate cellularity of fibrous tissue, with even distribution of osteoid and an absence of secondary changes such as hemorrhage or cystic change were revealed by pathologic examination. Conclusion: Ossifying fibroma shows strong enhancement and homogenous signal intensity on MR images.The homogeneity of the MR signal depends on the even distribution of osteoid and an absence of secondary changes such as hemorrhage or cystic change.

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