Objective To explore the role of diffusion-weighted magnetic resonance imaging(DWI) in forecasting curative effect of preoperative chemoradiotherapy(CRT) on locally advanced rectal cancer (LARC).

Methods We collected 44 cases of LARC patients confirmed by colonoscope biopsy pathology from June 2016 to December 2017. The patients were divided into T-downstaged group(n=24) and T-non-downstaged group(n=20), according to postoperative pathological stage and pretherapeutic clinical stage. We compared ADC value before and after chemoradiotherapy, ADC value between two groups, the ADC variation between groups (ΔADC) and the change rate of ADC (ADC%). According to the ROC curve, we obtained the optimal critical value of ADC for forecasting curative effect.

Results Among 44 cases of LARC patients, 7(15.9%) patients achieved pathological complete response. There was statistically significant difference in ADCs before and after CRT (P=0.000); before CRT, the ADC value in T-downstaged group was significantly lower than that in the T-non-downstaged group (P=0.007); after CRT, the ADC value in T-downstaged group was significantly higher than that in T-non-downstaged group (P=0.005); after CRT, ΔADC and ADC% in T-downstaged group were both significantly higher than those in T-non-downstaged group (Z=-5.53, P=0.000; P=-5.09, P=0.000). We defined the ADC value 0.87×10^-3mm²/s before the treatment as the critical value for forecasting whether the T stage would be reduced or not. The area of the ROC curve was 0.697 (95%CI: 0.539-0.855), the sensitivity of the curative effect forecasting was 87.5%, and the specificity was 55.0%.

Conclusion The quantitative analysis of ADC was able to forecast the sensitivity of rectal cancer patients in early stage to CRT, with certain value in the pre-CRT judgement.