Serial ctDNA Analysis for Monitoring Molecular Response and Acquired Resistance in Metastatic NSCLC and HR +/ HER2 ‐Negative Breast Cancer: A Real‐World Study

ABSTRACT Circulating tumor DNA (ctDNA) is a promising biomarker for disease monitoring, yet evidence of its clinical utility in routine care remains limited. We evaluated the utility of serial ctDNA monitoring using digital PCR (ddPCR) for detecting acquired resistance (AR) and monitoring response in metastatic non‐small cell lung cancer (NSCLC) and breast cancer (MBC). This real‐world, longitudinal study included 48 metastatic patients (42 with EGFR ‐mutant NSCLC; 6 with HR+/HER2‐negative MBC) treated at a Brazilian oncology reference center. Serial plasma samples were analyzed by ddPCR for activating and resistance mutations at specific time points. In the NSCLC cohort, EGFR T790M was detected in 48.4% of patients who progressed on 1st/2nd‐generation TKIs. For patients on 3rd‐generation TKIs, EGFR C797S was the main AR mechanism (47%), followed by PIK3CA , BRAF mutations, and ERBB2 / EGFR amplifications. Longitudinal analysis anticipated radiological progression by approximately 5 months. Early molecular response, characterized by mutation clearance, preceded radiological response. In the MBC cohort, hotspot ESR1 mutations (D538G, L536R, Y537S) were identified in 50% of patients. Variations in ctDNA fractional abundance predicted disease progression up to 9 months before radiological confirmation. This study provides real‐world evidence that ddPCR‐based ctDNA monitoring enables early detection of resistance and predicts disease progression significantly earlier than standard imaging in both NSCLC and MBC. Despite the limited sample size, particularly in the MBC cohort, these real‐world findings support liquid biopsy as a minimally invasive tool with the potential to guide therapeutic decisions and inform personalized management in precision oncology. Larger prospective studies are needed to confirm these observations.