P-117 Feasibility of tumor informed circulating tumor DNA in neoadjuvant rectal cancer

Circulating tumor DNA (ctDNA) is emerging as a biomarker to detect molecular evidence of residual disease and predict recurrence in patients with colorectal cancer the adjuvant setting. Treatment paradigms have been evolving stage II III rectal total neoadjuvant therapy (TNT). Prospective evaluation, feasibility utility using ctDNA during clinical practice has limited date. In this retrospective single institution study, data from commercial testing 12 their management was analyzed. A personalized tumor-informed multiplex PCR assay (Signatera TM 16-plex bespoke mPCR NGS assay) used for detection quantification ctDNA. performed at time diagnosis completion TNT. Primary objectives study are determine informed proportion positivity diagnosis. Secondary correlation response pathologic responses. Agreement assessed by concordance 95% CI compared 0.5 null hypothesis. patients; 1 patient did not pre-treatment test. Seven 11 evaluable were females 4 males. Ten out had least T3 or T4 N1 N2 presentation. Tumor-informed detected 100% (11/11) completed 2 points testing. Six underwent surgery while opted watch-and-wait approach. Nine 10 reduction levels therapy. 9 cleared after The clearance (N=6) correlated (95% CI: 0.7-1.0, p=0.004) overall (PR CR), (N=3) histologic treatment effect on definitive surgical pathology. prior initiation Clinical pathological responses This demonstrates Further prospective required correlative assess management.