LAUSR

3569 Background: Neoadjuvant chemoradiotherapy (nCRT) is nowadays the standard of care for the locally advanced rectal cancer (LARC). However, there is no effective method to predict patients’ possible benefits from nCRT and monitor the response to it. Methods: Patients with locally advanced middle and low rectal cancer of stage cT3-4N0M0 or cTanyN+M0 were enrolled from August 2017 to July 2018. All patients received nCRT with long-term radiation plus fluorouracil based chemotherapy, followed by the radical surgery. Serial plasma samples were collected pre-nCRT, during nCRT, and preoperatively (8 weeks after the completion of nCRT). Somatic mutations were detected with next-generation sequencing using a 1021-gene panel with peripheral blood lymphocyte DNA as a germline control. Results: This prospective cohort study enrolled 61 patients with rectal cancer. The pathological complete response (pCR) rate and the downstage rate was 31% (19/61) and 80% (49/61), respectively. ctDNA was detectable in 77% (47/61), 18% (11/61) and 13% (8/61) of blood samples obtained pre-nCRT, during nCRT and preoperatively, respectively. No significant association was observed between pre-nCRT ctDNA status with any clinicopathological factors, including age, gender, differentiation or tumor circumferential extent. Among the 8 patients with detectable ctDNA preoperatively, pathological tumor regression grade (TRG) of CAP 2-3 were observed and hepatic metastasis was found in 4 patients within 2 months. For patients with undetectable pre-operative ctDNA, a higher proportion archived pathological downstaging (85% vs 50%). The correlation between preoperative ctDNA status and achievement of pathological downstage was independent of age, gender or differentiation (p = 0.02). In addition, preoperative ctDNA positivity was associated with the persistently involved lymph node (p = 0.02). However, neither pre-nCRT nor during-nCRT ctDNA status was associated with pathological downstaging or persistently lymph node involvement. Conclusions: Detectable ctDNA after the completion of nCRT is a predicator of unsatisfactory curative effect of patients with LARC, which might indicate novel treatment intensification studies. Clinical trial information: NCT03042000.