Adjuvant and neoadjuvant evaluation of a structural variant-based MRD assay in early breast cancer: A real-world cohort.

Abstract

e12607 Background: Despite curative intent surgery and standard adjuvant therapy, patients (pts) with ER+, node+ stage II–III breast cancer (BC) remain at risk for systemic recurrence. Conventional surveillance uses imaging and clinical assessment, which may detect relapse only after overt disease. Tumor-informed circulating tumor DNA (ctDNA) assays enable detection of molecular residual disease (MRD), providing a molecular surveillance approach that may identify residual disease not captured by standard imaging. Pathlight is an ultrasensitive structural variant (SV)-based, tumor-informed ctDNA assay designed to detect low-level MRD from plasma. This study describes the real-world performance, feasibility, and clinical context of Pathlight MRD testing in routine BC care. Methods: An observational real-world cohort study of up to 100 pts from a single institution between May 1 and Dec 31 2025, through retrospective data analysis (no clinical interventions were made). Adult pts (18+) with histologically confirmed ER+ BC, stage II or III with node+ disease, treated with curative intent surgery with/without adjuvant therapy, who underwent Pathlight MRD testing through routine care were included. Pts with metastatic disease or HER2+ at time of testing were excluded. Personalized ctDNA assays were generated using up to 16 SVs from tumor tissue. Descriptive analysis evaluated MRD detection rates, assay feasibility, and clinicopathologic characteristics. Results: Pathlight MRD testing was performed in 69 unique pts, and 150 total monitoring tests. Detectable MRD was observed in 19% (13/69) pts. Median age was 55 yrs (range 33-80), with 61% (42/69) and 28% (19/69) at stages II and III respectively. 62/69 tests were initiated for adjuvant surveillance monitoring. Median turnaround time for initial SV validation was 20 days (range 6-41) and 3 days for ctDNA monitoring tests (range 1-7). Pts had median 13 SVs tracked (range 6-16). Detectable MRD was observed across all stages; I (8%, 1/13), II (54%, 7/13) and III (38%, 5/13). Tests were initiated for neoadjuvant (54%, 7/13) and adjuvant (46%, 6/13) settings. 92% (12/13) were ER+. All pts with proliferation data available were > 20% Ki-67 (11/11). Tumor fingerprints were generated from both biopsy specimens (61%, 8/13) and surgical specimens (38%, 5/13). Ultrasensitive detection was required in most cases: 77% (10/13) detected tumor fractions < 0.01% (range 0.000013%–0.025%). Conclusions: MRD detection using Pathlight was feasible and operationally efficient. MRD positivity occurred at ultra-low ctDNA levels (most below 0.01% VAF), underscoring the analytical sensitivity required for monitoring. MRD+ cases were enriched for higher-risk clinicopathologic features (stage II–III disease, elevated Ki-67). These real-world findings support further prospective evaluation of MRD surveillance in early BC.