Serial circulating tumor DNA (ctDNA) analysis as a prognostic marker and a real-time indicator of adjuvant chemotherapy (CT) efficacy in stage III colon cancer (CC)

3516Background: Adjuvant CT in stage III CC prevents recurrence by eradicating minimal residual disease (MRD) not visible on imaging. However, many patients (pts) will not have MRD and not all pts with MRD will benefit from standard CT. In this study, we determined (i) if the presence of ctDNA following surgery was predictive of recurrence following CT; (ii) if ctDNA could be used to determine the effectiveness of CT during treatment and (iii) if the presence of ctDNA following CT completion was predictive for later recurrence. Methods: Serial plasma samples from stage III CC pts planned for adjuvant CT were collected post-surgery, during CT and at treatment completion. Somatic mutations in individual tumors were identified via massively parallel sequencing of 15 genes commonly mutated in colorectal cancer. Personalized Safe-SeqS assays to quantify ctDNA in plasma samples were designed. Clinicians were blinded to ctDNA results. Results: 95 pts were enrolled from Nov-2014 to May-2017, median age was 64 yea...