Comparison of 18 F-FDG PET/CT methods of analysis for predicting response to neoadjuvant chemoradiation therapy in patients with locally advanced low rectal cancer

TLDR: Qualitative and semiquantitative evaluations for 18F-FDG PET/CT evaluations for LARC are the optimal approach; a valid parameter for response prediction has still to be established.

Abstract: The aim of this study was to prospectively investigate the predictive value of 18F-FDG PET/CT semiquantitative parameters for locally advanced low rectal cancer (LARC) treated by neoadjuvant chemoradiation therapy (nCRT). 68 patients with LARC had 18F-FDG PET/CT scans twice (baseline and 5–6 weeks post-nCRT). All patients underwent surgery with preservation of the sphincter 8 weeks later. 18F-FDG PET/CT analysis was performed by visual response assessment (VRA) and semiquantitative parameters: SUVmaxbaseline, SUVmeanbaseline, MTVbaseline, TLGbaseline, SUVmaxpost-nCRT, SUVmeanpost-nCRT, MTVpost-nCRT, TLGpost-nCRT; ΔSUVmax and mean and Response indexes (RImax% and RImean%). Assessment of nCRT tumor response was performed according to the Mandard’s Tumor Regression Grade (TRG) and (y)pTNM staging on the surgical specimens. Concordances of VRA with TRG, and with (y)pTNM criteria were evaluated by Cohen’s K. Results were compared by t student test for unpaired groups. ROC curve analysis was performed. VRA analysis of post-nCRT 18F-FDG PET/CT scan for the (y)pTNM outcome showed sensitivity, specificity, accuracy, PPV, and NPV of 87.5%, 66.7%, 83.8%, 92.5%, and 53.3%, respectively. Concordances of VRA with TRG and with (y)pTNM were moderate. For the outcome variable TRG, the statistical difference between responders and non-responders was significant for SUVmaxpost-nCRT and RImean%; for the outcome variable (y)pTNM, there was a significant difference for MTVbaseline, SUVmaxpost-nCRT, SUVmeanpost-nCRT, MTVpost-nCRT, RImax%, and RImean%. ROC analysis showed better AUCs: for the outcome variable TRG for SUVmaxpost-nCRT, SUVmeanpost-nCRT, and RImean%; for the outcome variable (y)pTNM for MTVbaseline, SUVmaxpost-nCRT, SUVmeanpost-nCRT, MTVpost-nCRT, RImax%, and RImean%. No significant differences among parameters were found. Qualitative and semiquantitative evaluations for 18F-FDG PET/CT are the optimal approach; a valid parameter for response prediction has still to be established.