Using circulating tumor DNA (ctDNA) to predict surgical outcome and postoperative recurrence following neoadjuvant chemoradiation (CRT) for borderline resectable/locally advanced rectal cancer (LARC).

562Background: This study was designed to assess the capability of perioperative ctDNA analysis to predict surgical outcome and recurrence following neoadjuvant CRT for LARC. Methods: Thirty-one patients (pts) with newly diagnosed LARC (n = 29) or locally recurrent (non-metastatic) rectal cancer (n= 3) were treated between 7/2013 - 7/2017. Pts received long-course neoadjuvant CRT prior to surgical resection: 50.4 Gy/28 fractions. Serum ctDNA was typically measured at baseline, weekly during CRT, pre-operatively, and post-operatively. Next-generation sequencing was used to identify mutations in the primary tumor, and mutation-specific droplet digital PCR was used to detect mutation fraction in ctDNA. Results: The median age of the cohort was 53 years (IQR 46.5-65.3 y). The overall R0-node negative (R0-NN) resection rate was 66.7%. The rate of R0-NN resection was significantly higher among pts with undetectable preoperative ctDNA (n = 17, 88%) compared to pts with a detectable preoperative ctDNA (n = 10, 30...