PET/CT and MRI diagnostic discrepancy in evaluating treatment response (ETR) in patients (pts) with bone and soft tissue sarcomas (BSTS): Initial preliminary reports

1588 Objectives: To evaluate the impact of both PET/CT and MRI studies in ETR in pts with BSTS, and to focus on the discrepancy between PET/CT and MRI-alone images. Methods: 17 pts with different histologic subtypes BSTS enrolled 24 PET/CT and MRI studies in ETR follow-up. Comparison was made between abnormalities in selective corresponding areas of interest from MRI and PET/CT. Findings were confirmed histopathologically or by clinical follow-up. Results: 80 abnormal lesions were detected by both PET/CT and MRI:43(54%)concordant and 37(46%)discordant. 36 TN PET/CT lesions with excellent to complete response(ECR)to therapy were FP on MRI with no reduction in size. One MRI TP brain lesion was FN on PET/CT. In ETR, PET/CT and MRI showed 96.2%vs.100% sensitivity, 100%vs.33.3% specificity, 98.7%vs.55% accuracy, 100%vs.42%PPV and 98.2%vs.100% NPV respectively. When comparing studies for ETR,PET/CT and MRI were concordant in 17 of 24 studies(71%):7 complete,1 partial,3 no trt response and 6 progression of dse. Discordance was in 7 of 24 studies(33%).2 ECR on PET corresponded with no reduction in size or stable dse on MRI. 2 ECR and one stable dse on PET showed MRI dse progression(FP).Mixed trt response was recognized in 2 PET/CT and MRI studies.As regards impact on pts mgmt, PET/CT saved 4 pts from more ChT(FP MRI,TN PET/CT),while MRI initiated more trt in 1 pt with brain metastasis(FN PET/CT,TP MRI). Conclusions: In this preliminary study,18F-FDG PET/CT in ETR is more accurate than MRI alone. In addition PET/CT is more accurate for evaluating tumor chemo-sensitivity.