Background The optimal sequencing of targeted therapy and immunotherapy in advanced lung cancer remains a major clinical challenge due to evolving tumor heterogeneity and acquired resistance. Circulating tumor DNA (ctDNA) has emerged as a minimally invasive biomarker capable of capturing real-time tumor dynamics. This study aimed to evaluate the clinical utility of dynamic ctDNA monitoring in guiding treatment sequencing in a real-world prospective cohort of patients with advanced lung cancer. Methods In this prospective multicenter study conducted across five tertiary oncology centers in France, 174 patients with advanced lung cancer were enrolled between January 2021 and June 2024. Serial plasma samples were collected at baseline, early during treatment (week 4), at radiologic assessment, and at progression. ctDNA analysis was performed using high-depth next-generation sequencing and digital PCR. Early molecular response was defined as ctDNA clearance or ≥80% reduction at week 4. Associations between ctDNA dynamics, resistance mechanisms, treatment sequencing, and clinical outcomes were evaluated. Results Baseline ctDNA was detectable in 85.6% of patients. Early ctDNA clearance occurred in 52.3% and was strongly associated with improved progression-free survival (14.6 vs 6.1 months; HR 0.48; 95% CI, 0.34–0.67; P < .001) and overall survival (26.3 vs 13.9 months; HR 0.52; P < .001). ctDNA-based molecular progression preceded radiologic progression by a median of 6.4 weeks. Resistance mechanisms were identified in 47.9% of patients receiving targeted therapy, with EGFR C797S mutation and MET amplification being the most frequent. ctDNA-informed treatment adaptations were implemented in 35.1% of patients and were associated with improved post-progression outcomes (8.7 vs 5.2 months; HR 0.66; P = .01). Conclusions Baseline ctDNA was detectable in 85.6% of patients. Early ctDNA clearance occurred in 52.3% and was strongly associated with improved progression-free survival (14.6 vs 6.1 months; HR 0.48; 95% CI, 0.34–0.67; P < .001) and overall survival (26.3 vs 13.9 months; HR 0.52; P < .001). ctDNA-based molecular progression preceded radiologic progression by a median of 6.4 weeks. Resistance mechanisms were identified in 47.9% of patients receiving targeted therapy, with EGFR C797S mutation and MET amplification being the most frequent. ctDNA-informed treatment adaptations were implemented in 35.1% of patients and were associated with improved post-progression outcomes (8.7 vs 5.2 months; HR 0.66; P = .01).

Circulating tumor DNA; Liquid biopsy; Non–small cell lung cancer; Treatment sequencing; Targeted therapy; Immunotherapy.