Identification of imaging biomarkers using 18fluorodeoxyglucose-positron emission tomography (FDG-PET) for the prediction of pathologic response in patients receiving neoadjuvant chemoradiotherapy for locally advanced rectal cancer.

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435 Background: Pre- and post-treatment FDG-PET imaging parameters may be useful for predicting pR following neoadjuvant CRT for LARC Methods: Between 2006 and 2010, 18 patients (pts) receiving preoperative CRT (5FU or capecitabine with concurrent 50.4 Gy conformal radiation) for LARC underwent a pre- (0-14 days before) and post-CRT (4 to 6 weeks after) PET/CT. Total mesorectal excision was performed 6 weeks post CRT. pR was dichotomized into complete (pCRs) and partial responders as reference standard. Pts with pathologic microscopic disease were classified as pCRs for this analysis. Changes in FDG metabolic parameters between pre and post-CRT scans, including maximum Standardized Uptake Value (SUVmax), metabolic tumor volume (MTV) and total glycolytic activity (TGA), were investigated as predictive imaging biomarkers for pR. Fixed SUVmax percentage threshold (30%, 40% and 50%) and gradient-based segmentation (PET Edge) were used to calculate the MTV and TGA. Receiver Operator Curve analysis was performed Results: Median pt age was 57.5 years (31-87); 67% M; 61% AJCC 2010 stage II, 33% III and 6% IV. pCR achieved in 44%. The 30% SUVmax threshold MTV best correlated with PET Edge MTV (Pearson r 0.99). Using ROC analysis, % change in SUVmax and PET edge TGA between pre and post-CRT scans were comparable and the most accurate parameters for predicting pR. SUVmax % change yielded an area under the curve (AUC) of 0.7125 (95% CI 0.4318-0.9932), while the AUC for PET Edge TGA % change was 0.7000 (95% CI 0.4209-0.9791). Based on ROC analysis, a cutoff of 78.9% change of PET Edge TGA had a likelihood ratio of 2.4, with a sensitivity of 90% and a specificity of 62.5% for predicting pCR. A cutoff of 88.7% change of PET Edge MTV yielded a likelihood ratio of 2.00 with a sensitivity of 50% and a specificity of 75%. Conclusions: The % change pre- to post-CRT in SUVmax and PET Edge Total Glycolytic Activity appears to be the most suitable FDG metabolic parameters for predicting pR following neoadjuvant CRT for LARC. As such, these PET-FDG biomarkers of CRT response should be validated in future prospective trials.