Objective:Based on clinical and magnetic resonance imaging data,to explore the risk factors of pathological complete remission after neoadjuvant chemoradiotherapy for locally advanced rectal cancer.Methods:From December 2014 to November 2022,175 patients with locally advanced rectal cancer diagnosed by biopsy and clinical diagnosis who underwent TME after neoadjuvant chemoradiotherapy were retrospectively analyzed,including 117 males and 58 females,aged from 22 to 82 years,with an average of (56.93±11.886) years.There were 122 cases in the training group (pathological complete response group n=30,non-pathological complete response group n=92) and 53 cases in the verification group (pCR group n=10,non-pCR group n=43).Clinical data,imaging examination and pathological data of patients before and after treatment were collected.The differences between groups were compared by independent sample T test or Kruskal Wallis rank sum test .χ2 test or Fisher exact test were used to compare the counting data sets.For all statistical analysis results,P<0.05 is considered as a statistical difference.We use the univariate and multivariate Logistic regression to analyze the efficacy of LARC neoadjuvant chemoradiotherapy.The diagnostic efficacy of predictive factors was evaluated by receiver's operating characteristic (ROC) curve.Patients were randomly divided into testing set and validation set according to the ratio of 7:3.The model was validated,and the area under the curve (AUC),cutoff value,sensitivity and specificity were calculated,and the differences of AUC values were compared by DeLong test.Results: There were significant differences in some clinical features between the training set and the validation set between the pCR group and the non-pCR group (the length and diameter of the tumor before and after treatment,the depth of tumor infiltration,the CEA value after treatment,the number of lymph nodes around the tumor after treatment,extramural vascular invasion before and after treatment,the mrT stage after treatment,mesorectal fascia and mrN).After univariate and multivariate logistic regression analysis,The extramural depth after treatment was an independent risk factor for evaluating the efficacy of neoadjuvant chemoradiotherapy.The extramural depth has a good diagnostic efficiency in predicting the curative effect of nCRT in testing set and verification set.The AUC values of the testing set and the validation set were 0.783 and 0.765,the cutoff values were 0.555 and 0.627,the sensitivity was 0.870 and 0.852,and the specificity was 0.733 and 0.773 respectively.There was no statistical difference after Hosmer-Lemeshow goodness of fit test.Conclusion: Evaluating the extramural depth is helpful to evaluate the pCR rate of locally advanced rectal cancer after nCRT,which provides a convenient and noninvasive diagnostic method for clinic and guides clinical individualized treatment.