10017 Background: Change in size by RECIST (Response Evaluation Criteria in Solid Tumors) has been the standard to assess response to therapy in non-GIST soft tissue sarcomas (STS). Although recent studies have demonstrated that Positron Emission Tomography with F18-fluorodeoxyglucose (FDG-PET) may be used to assess response, there has not been a direct comparison between these modalities. The aim of this study was to prospectively evaluate whether a change in quantitative FDG-PET or a change in size [computed tomography(CT)] was more accurate at predicting histopathologic response to neoadjuvant therapy in patients with high grade STS using a combined FDG-PET/CT scan. Methods: From 1/05 - 12/06 58 patients with resectable biopsy proven high grade STS scheduled to undergo neoadjuvant chemotherapy were prospectively enrolled in this study. Patients underwent FDG-PET/CT prior to and after neoadjuvant treatment (prior to surgery). Tumor FDG-uptake was quantified by standardized uptake values (SUV). Changes in tumor size were quantified according to RECIST. Following tumor resection, response was assessed histopathologically. Patients with = 10% viable tumor cells were classified as responders. To date, 36 patients have completed the study and are the subject of this analysis. Results: In histopathologic responders (n=10, 28%), reduction of tumor FDG-uptake was significantly greater (-64%) than in histopathologic non-responders (-37%), (p=0.005). Using a 50% decrease in tumor SUV as a threshold value resulted in a sensitivity of 90% and a specificity of 58% for assessment of histopathologic response (p=0.01). Response assessment per RECIST showed no significant correlation with histopathologic response (sensitivity 20%, specificity 89%, p=0.4). There was no correlation between changes in tumor size and histopathologic response (area under the ROC curve = 0.6, p=0.1). Conclusions: In patients with high grade STS, quantitative FDG-PET was significantly more accurate than size based criteria for assessment of histopathologic response to neoadjuvant therapy. FDG-PET should be considered as a modality to monitor treatment response is patients with high grade STS. No significant financial relationships to disclose.
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