Structured magnetic resonance imaging assessment improves diagnosis of pathological complete response in rectal cancer after neoadjuvant chemoradiation

Precise prediction of pathological complete response (pCR) following neoadjuvant chemoradiotherapy (nCRT) in rectal cancer may identify candidates for non-operative management. The optimal selection of diagnostic tools is therefore of major clinical importance. Clinical, laboratory, endoscopic and radiological data of patients with rectal cancer treated with nCRT and surgery at an academic medical center from 2010 to 2020 were retrospectively collected. Pre- and post-nCRT magnetic resonance imaging (MRI) was reviewed with a structured report template and assessed by magnetic resonance imaging tumor regression grade (mrTRG). Two senior radiologists reviewed mrTRG independently to determine the inter-reader agreement. Univariate logistic regression was applied to identify parameters that predict pCR. A multivariate prediction model was developed using L1-penalized logistic regression, with performance assessed by area under the curve (AUC) in the total cohort (apparent AUC) and by cross validation (CV-AUC). A total of 261 patients were identified, of whom 36 achieved pCR. Univariate analysis showed a significant correlation between post-nCRT features with pCR, including radiological T-stage (OR 0.05 [0.02-0.15], p<0.001), mrTRG (OR 0.13 [0.05-0.31], p<0.001) and endoscopic response (OR 0.17 [0.05-0.54], p=0.032). Of those, mrTRG showed the highest AUC of 0.77 with a substantial inter-reader agreement (kappa=0.71, 95% CI: 0.61 to 0.81). The multivariate predictive model selected eight pre- and post-nCRT parameters with apparent AUC of 0.84 and CV-AUC of 0.73. Therapy response assessed by MRI, particularly by mrTRG, strongly predicted pCR. Therefore, mrTRG should be implemented in routine assessment of rectal cancer treated by nCRT.