Time dependency for human papillomavirus circulating tumor DNA detection after chemoradiation as a prognostic biomarker for localized anal cancer.

Background: While detection of circulating tumor DNA (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.

Methods: Patients with stages I-III anal cancer treated with chemoradiation between 12/2020-5/2024 were evaluated for 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% versus 2%; odds ratio 168, 95% CI 13.6-2080; p<.0001) and inferior RFS (4.9 months versus not reached (NR); hazard ratio (HR) 39.2, 95% CI 4.6-330; p<.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 NR for both; HR 1.6, 95% CI .35-7.4; p=.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.