Quantitative imaging outperforms molecular markers when predicting response to chemoradiotherapy for rectal cancer

Background and purpose: To explore the integration of imaging and molecular data for response prediction to chemoradiotherapy (CRT) for rectal cancer. Material and methods: Eighty-five rectal cancer patients underwent preoperative CRT. ¹⁸F-FDG PET/CT and diffusion-weighted imaging (DWI) were acquired before (TP1) and during CRT (TP2) and prior to surgery (TP3). Inflammatory cytokines and gene expression were analysed. Tumour response was defined as ypT0-1N0. Multivariate models were built combining the obtained parameters. Final models were calculated on the data combination with the highest AUC. Results: Twenty-two patients (26%) achieved ypT0-1N0 response. ¹⁸F-FDG PET/CT had worse predictive performance than DWI and T2-volumetry (AUC 0.61 ± 0.04, 0.72 ± 0.03, and 0.72 ± 0.02, respectively). Combining all imaging parameters increased the AUC to 0.81 ± 0.03. Adding cytokines or gene expression did not improve the AUC (AUC of 0.72 ± 0.06 and 0.79 ± 0.04 respectively). Final models combining ¹⁸F-FDG PET/CT, DWI, and T2-weighted volumetry at all TPs and using only TP1 and TP3, allowed ypT0-1N0 prediction with a 75% sensitivity, 94% specificity and PPV of 80%. Conclusions: Combining ¹⁸F-FDG PET/CT, DWI, and T2-weighted MRI volumetry obtained before CRT and prior to surgery may help physicians in selecting rectal cancer patients for organ-preservation.