For patients with early stage EGFR-mutant-positive (EGFR-M+) NSCLC, curative surgery followed by adjuvant chemotherapy is considered the standard of care. This study evaluated the feasibility and efficacy of longitudinal monitoring of circulating tumor DNA (ctDNA) as a valuable biomarker for early detection of minimal residual disease (MRD) and provides identification of the group at high risk for recurrence in resected stages I to IIIA EGFR-M+ NSCLC.

Between August 2015 and October 2017, a total of 278 patients with curative resected, stages I to IIIA (American Joint Committee on Cancer seventh version) common EGFR-M+ NSCLC were analyzed. Radiological follow-up was accompanied with longitudinal monitoring of ctDNA using a droplet-digital polymerase chain reaction from baseline (preoperative), 4 weeks after curative surgery, and follow-up per protocol until 5 years. The primary outcomes were disease-free survival (DFS) according to the status of ctDNA positivity at landmark points and the sensitivity of longitudinal monitoring of ctDNA.

Among 278 patients, preoperative baseline ctDNA was detected in 67 (24%) patients: 23% (stage IA), 18% (IB), 18% (IIA), 50% (IIB), and 42% (IIIA) (p = 0.06). Of patients with baseline ctDNA, 76% (51 of 67) had clearance at 4 weeks after surgery (postoperative). Patients were classified into the following three groups; group A, baseline ctDNA negative (n = 211) versus group B, baseline ctDNA positive but postoperative MRD negative (n = 51) versus group C, baseline ctDNA positive and postoperative MRD positive (n = 16). The 3-year DFS rate was significantly different among the three groups (84% for group A, 78% for group B, and 50% for group C, p = 0.02). After adjusting for clinicopathologic variables, ctDNA still remains an independent risk factor for DFS along with stage (p < 0.001) and micropapillary subtype (p = 0.02). With longitudinal monitoring of ctDNA, MRD was detected before radiological recurrence in 69% of patients with exon 19 deletion and in 20% with L858R mutation.

These results suggest that patients with baseline ctDNA-positive or MRD-positive status were associated with poor DFS in curative resected stages I to IIIA EGFR-M+ NSCLC and that longitudinal monitoring of ctDNA, a noninvasive method, might be useful to detect early recurrence before radiological recurrence.