Patient population

The training cohort was derived from the FOWARC randomized prospective phase III trial exploring mFOLFOX6- and fluorouracil-based preoperative chemoradiotherapy in the locally advanced rectal cancer [17]. The patients were eligible for the present investigation if they fulfilled the following criteria: 1) received neoadjuvant chemoradiation followed by TME resection, 2) baseline CEA level was elevated (≥5 ng/mL), and 3) follow-up CEA values (2nd, 4th, 6th, 8th and 14th week from the beginning of the preoperative treatment) were available during the study. Finally, 71 patients were included in the training cohort (Additional file 1: Figure S1). The validation cohort consisted of 75/587 consecutive rectal cancer patients undergoing fluorouracil-based neoadjuvant chemoradiation and TME resection between 2014 and 2015 at the Xiangya Hospital of Central South University. The inclusion and exclusion criteria were similar to that of the FOWARC trial and those previously described in the training cohort, except that CEA was measured at four time points, including the baseline, 2nd, 6th, and 12th weeks from the start of the preoperative treatment (Additional file 2: Figure S2).