Serial Circulating Tumor DNA in Predicting and Monitoring the Effect of Neoadjuvant Chemoradiotherapy in Patients with Rectal Cancer: A Prospective Multicenter Study

Abstract Purpose: We investigated the value of circulating tumor DNA (ctDNA) in predicting tumor response to neoadjuvant chemoradiotherapy (nCRT), monitoring tumor burden, and prognosing survival in patients with locally advanced rectal cancer (LARC). Experimental Design: This prospective multicenter trial recruited 106 patients with LARC for treatment with nCRT followed by surgery. Serial ctDNAs were analyzed by next-generation sequencing at four timepoints: at baseline, during nCRT, presurgery, and postsurgery. Results: In total, 1,098 mutations were identified in tumor tissues of the 104 patients being analyzed (median, seven mutations/patient). ctDNA was detected in 75%, 15.6%, 10.5%, and 6.7% of cases at the four timepoints, respectively. None of the 29 patients with pathologic complete response (ypCR) had preoperative ctDNA detected. The preoperative ctDNA-positive rate was significantly lower in the well-responded patients with pathologic tumor regression grade of ypCAP 0–1 than ypCAP 2–3 group (P < 0.001), lower in ypCR than non-ypCR group (P = 0.02), and lower in pathologic T stage (ypT) 0–2 than ypT 3–4 group (P = 0.002). With a median follow-up of 18.8 months, 13 patients (12.5%) experienced distant metastasis. ctDNA positivity at all four timepoints was associated with a shorter metastasis-free survival (MFS; P < 0.05). Multivariate analyses showed that the median variant allele frequency (VAF) of mutations in baseline ctDNA was a strong independent predictor of MFS (HR, 1.27; P < 0.001). Conclusions: We show that ctDNA is a real-time monitoring indicator that can accurately reflect the tumor burden. The median VAF of baseline ctDNA is a strong independent predictor of MFS.