Longitudinal post-radiotherapy plasma Epstein-Barr virus DNA trends inform on optimal risk stratification in endemic nasopharyngeal carcinoma.

• Optimal timing for post-radiotherapy EBV DNA risk stratification is uncertain. • EBV DNA level is dynamic between 0–2 and 8–12 weeks post-radiotherapy. • Patients switching from detectable to undetectable EBV DNA had better prognosis. • Delayed EBV DNA testing at 8–12 weeks better enriched for high-risk patients. To characterize longitudinal changes in Epstein-Barr virus (EBV) DNA post-radiotherapy in nasopharyngeal carcinoma (NPC) patients, and investigate whether an early (0–2 weeks) or delayed (8–12 weeks) EBV DNA result better predicts for disease-free survival (DFS). Histologically-confirmed NPC patients with ≥1 EBV DNA test quantified using the harmonized BamHI-W polymerase chain reaction-based assay at 0–2 and 8–12 weeks post-radiotherapy were included. We identified 302 patients with EBV DNA measured at 0–2 weeks post-radiotherapy; of which, 110 (36.4 %) underwent a repeat test at 8–12 weeks post-treatment. Patients harboring a detectable EBV DNA at 0–2 weeks experienced an inferior DFS (adjusted HR 1-264 copies 1.72 [95 %CI: 1.05–2.83], P = 0.031; AHR ≥265 copies 4.39 [95 %CI: 1.68–11.44], P = 0.002 relative to 0 copies/mL). At 8–12 weeks, we observed substantial shifts in EBV DNA readings from 0 to 2 weeks; 76/110 (69.1 %) and 34/110 (30.9 %) patients at 0–2 weeks versus 90/110 (81.8 %) and 20/110 (18.2 %) at 8–12 weeks recorded undetectable and detectable EBV DNA, respectively. Positive EBV DNA at 8–12 weeks was strongly associated with relapse (73.3 % [11/15] for 1–264; 80.0 % [4/5] for ≥265 subgroups had relapses versus 15.6 % [14/90] for 0 copies/mL). Area under receiver operating curve values for 2-year relapse rates were 0.817 (95 %CI: 0.725–0.909) for stage + EBV DNA 8-12w versus 0.654 (95 %CI: 0.542–0.765) for stage + EBV DNA 0-2w. EBV DNA is dynamic post-radiotherapy, and delayed EBV DNA testing better enriched for higher-risk NPC patients. This implicates trials investigating adjuvant chemotherapy intensification based on early EBV DNA testing. [ABSTRACT FROM AUTHOR]