Clinical validity of ultrasensitive single-digit parts per million ctDNA detection in non–small cell lung cancer.

8017 Background: Circulating tumor DNA (ctDNA) is a promising biomarker for detecting molecular residual disease (MRD) and predicting recurrence after curative treatment in non-small cell lung cancer (NSCLC). Ultrasensitive personalized assays have demonstrated clinical validity at detection thresholds of ~80-100 parts per million (PPM). As this assay can detect ctDNA at concentrations an order of magnitude lower (below 10 PPM), we extend these analyses to explore the clinical outcomes of patients with single-digit PPM ctDNA. In the future, this may guide adjuvant treatment decision-making. Methods: We analyzed longitudinal plasma samples from 431 patients with stage IA–IIIB NSCLC enrolled in the TRACERx study and treated with upfront surgery. We achieved ultrasensitive MRD testing with ctDNA limits of detection as low as 1-3 PPM across 2994 plasma samples using NeXT Personal, a personalized liquid biopsy assay which tracks up to ~1,800 patient-specific somatic variants. Plasma samples were classified as preoperative, postoperative landmark (10–120 days), or longitudinal follow-up. Associations with recurrence-free survival (RFS) and overall survival (OS) were assessed. Results: Preoperative ctDNA detection below 10 PPM was associated with inferior recurrence-free survival (RFS) and overall survival (OS) compared to undetectable ctDNA in individuals with lung adenocarcinoma (<10 PPM vs. undetected: HR: 3.25, 95% CI 1.52-6.94, p = 0.0024 and HR: 3.49, 95% CI 1.24-9.81, p = 0.018 for RFS and OS). Postoperative ctDNA was detected in 21% of patients. Detection below 10 PPM occurred in 16.7%, 15.5%, and 11.5% of stage I, II, and III patients, respectively. ctDNA detection preceded imaging-confirmed recurrence in 120 individuals. In postoperative landmark plasma samples, 18% of ctDNA detections occurred below 10 PPM and were associated with increased risk for recurrence and death versus undetectable ctDNA (RFS: HR 3.49, 95% CI 1.70–7.17, p=0.00067; OS: HR 2.58, 95% CI 1.09–6.14, p=0.032). Within ctDNA-positive patients, <10 PPM was associated with improved outcomes compared with ≥10 PPM (HR: 0.38, 95% CI 0.19-0.78, p = 0.0086, HR: 0.37, 95% CI 0.16-0.86, p = 0.021). Conclusions: Sub-10 PPM ctDNA detection identifies a clinically high-risk subgroup of early-stage NSCLC patients. Notably, this included stage I patients who would not typically be classified as high-risk or considered for adjuvant therapy; ctDNA positivity in this group therefore reveals high-risk disease that would otherwise go unrecognized. Reliable detection of ctDNA at sub-10 PPM levels may refine post-surgical risk stratification beyond conventional staging and imaging, enabling earlier identification of molecular relapse and potentially guiding treatment escalation in future practice.