Abstract
PURPOSE: Although detection of ctDNA weeks after surgery is linked to recurrence for other solid tumors, the optimal time point for ctDNA assessment as a prognostic biomarker following chemoradiation for anal cancer is undefined. EXPERIMENTAL DESIGN: Patients with stages I to III anal cancer treated with chemoradiation between December 2020 and March 2024 were evaluated for human papillomavirus (HPV) ctDNA status at baseline, at the end of chemoradiation, and during surveillance using a droplet digital HPV ctDNA PCR assay, targeting HPV E6 and E7 oncogenes for 13 oncogenic HPV types. Median recurrence-free survival (RFS) according to HPV ctDNA status was estimated via Kaplan-Meier and compared using a log-rank test. RESULTS: Detection of HPV ctDNA at ≥3 months after chemoradiation was associated with recurrence (80% vs. 2%; OR, 168; 95% confidence interval (CI), 13.6-2,080; P < 0.0001) and inferior RFS [4.9 months vs. not reached; HR, 39.2; 95% CI, 4.6-330; P < 0.0001] relative to HPV ctDNA-negative status. Sensitivity and specificity for recurrence according to HPV ctDNA detection were 89% and 95%, respectively, with positive and negative predictive values of 80% and 98%, respectively. Differences in RFS according to HPV ctDNA status were not observed at the end of treatment (median RFS, not reached for both; HR, 1.6; 95% CI, 0.35-7.4; P = 0.48). CONCLUSIONS: With a novel, highly sensitive assay, detection of HPV ctDNA at least 3 months after chemoradiation was associated with unfavorable survival. Future clinical trials should incorporate this 3-month post-treatment time point to identify patients with HPV-positive anal cancer at elevated recurrence risk according to HPV ctDNA status. See related commentary by Bercz et al., p. 2261.