Pre-operativechemoradiotherapy (CRT)followed by surgical resection is still the standard treatment for locally advanced low rectalcancer.Nowadays new strategiesare emerging to treat patients with a completeresponse to pre-operative treatment, rendering the optimal management still controversial and under debate.The primary aim of this study was to obtain a snapshot of tumor regression grade (TRG) distribution after standard CRT. Second, we aimed to identify a correlation between clinical tumor stage (cT) and TRG, and to define the accuracy of magnetic resonance imaging (MRI) in the restaging setting. Between January 2017 and June 2019, a cross sectionalmulticentricstudy was performed in22referral centers of colon-rectal surgery including all patients with cT3-4Nx/cTxN1-2rectal cancer who underwent pre-operativeCRT.Shapiro-Wilk test was used for continuous data. Categorical variables were compared with Chi-squared test or Fisher's exact test, where appropriate. Accuracy of restaging MRI in the identification of pathologic complete response (pCR) was determined evaluating the correspondence with the histopathological examination of surgical specimens.In the present study, 689 patients were enrolled. Complete tumorregression rate was 16.9%. The "watch and wait" strategy was applied in 4.3% of TRG4 patients. A clinical correlation between more advanced tumors and moderate to absent tumor regression wasfound (p=0.03). Post-neoadjuvantMRI had low sensibility (55%) and high specificity (83%) with accuracy of 82.8% in identifying TRG4 and pCR.Our data provided a contemporary description of the effects of pre-operative CRT on a large pool of locally advanced low rectal cancer patients treated in different colon-rectal surgical centers.
