AbstractPurposeDistant metastasis (DM) and neoadjuvant treatment response prediction remain critical challenges in the management of locally advanced rectal cancer (LARC). The aim of this study was to investigate the clinical relevance of viable circulating tumor cells (CTCs) for DM or response in patients with LARC in a neoadjuvant setting.MethodsThe detection of viable CTCs at different stages of treatment was planned for consecutive patients from a prospective trial. The Kaplan–Meier method, Cox proportional hazards model, and logistic regression model were utilized to analyze factors associated with DM or pathological complete response (pCR) and clinical complete response (cCR).ResultsBetween December 2016 and July 2018, peripheral blood samples from 83 patients were collected before any treatment (median follow‐up time, 49.3 months). CTCs were present in 76 of 83 patients (91.6%) at baseline, and more than three CTCs detected in the blood sample was considered high risk. Only the CTC risk group was significantly associated with 3‐year metastasis‐free survival (MFS) (high risk vs. low risk, 57.1% (95% CI, 41.6–72.6) vs. 78.3% (95% CI, 65.8–90.8),p= 0.018, log‐rank test). When all the important variables were entered into the Cox model, the CTC risk group remained the only significant independent factor for DM (hazard ratio (HR), 2.74; 95% CI, 1.17–6.45,p= 0.021). The pCR and continuous cCR rates were higher in patients with a decreased number of CTCs of more than one after radiotherapy (HR, 4.00; 95% CI, 1.09–14.71, P = 0.037).ConclusionsThe dynamic detection of viable CTCs may strengthen pretreatment risk assessment and postradiotherapy decision making for LARC. This observation requires further validation in a prospective study.