Abstract Background: Leveraging circulating tumor DNA (ctDNA) to assess treatment response and emergence of resistance alterations is coming to the forefront in breast cancer, particularly for patients receiving endocrine therapy. However, little is known about real-world clinical utilization. Here we describe the real-world clinical use and results of a tissue-free ctDNA monitoring assay in patients with breast cancer. Methods: The clinical laboratory developed test, FoundationOne®Monitor (F1Monitor), was available in a limited pilot. Enrollment was rolling. The timing and number of tests were determined by each physician as appropriate for their practice. Therapy abstraction was requested but not required. ctDNA was detected and quantified by ctDNA tumor fraction (TF); variants were tracked with variant allele frequency (VAF) and the origin predicted algorithmically. Results: F1Monitor was ordered by 26 physicians for 193 patients (239 tests) with breast cancer between 9/30/2024-04/30/2025. Median turnaround time was 8 days. Therapies were abstracted for 39 patients, including 16 (41%) on endocrine therapy ± CDK4/6 inhibitor. At data cutoff, the median follow-up was 71 days. 33 patients had ≥2 tests. The median interval between the first two tests was 42.0 days (Interquartile Range IQR: 34.0, 60.7). The median cadence of serial testing (3+ tests, 7 patients) was 57 days (IQR: 46.8, 64.5). CtDNA TF was detected in 35% (68/193) of patients overall and 41% (16/39) known to be on therapy. Physicians were more likely to pursue serial monitoring if the first test had ctDNA TF detected. The median and maximum ctDNA TF was 2.8% and 84%. The lowest ctDNA TF and reportable variant VAF values quantified were 0.19% and 0.08%, respectively. ctDNA TF dynamic changes were observed in 55% (18/33) of patients during their course of testing (timepoint results: newly detected: 0; any increase: 9; 50% decrease: 4; 50-89% decrease: 4; 90-99% decrease: 5; clearance: 4). A median of 3 reportable alterations were detected per test with 97% (91/94) of tests with a ctDNA TF detected result having ≥1 copy number variant or predicted tumor somatic short variant and 71% (67/94) having ≥1 predicted clonal hematopoiesis (CH) alteration. Of tests with ctDNA TF not detected, 91% (132/145) had no variants or only alterations predicted to be CH or germline reported. Clinically actionable variants in breast cancer were detected in 31% (60/193) of patients overall and 70% (48/68) with ctDNA TF detected, with VAFs down to 0.1% for each gene: PIK3CA n = 32; ESR1 n = 18; BRCA1 n = 9; BRCA2 n = 9; PTEN n = 6; ERBB2 n = 6; AKT1 n = 4. In patients with ≥2 tests, mutations in clinically actionable genes in breast cancer were persistent in 45% (15/33), cleared in 15% (5), and newly emerging in 15% (5). Conclusion: In real-world tissue-free ctDNA-based monitoring in breast cancer, F1Monitor demonstrated high sensitivity, detecting ctDNA TF as low as 0.19% and VAF as low as 0.08% including alterations in clinically actionable genes such as ESR1 down to 0.1%. Further, ctDNA TF demonstrated high specificity, with 91% of “ctDNA TF not detected” results having no alterations or only CH or germline predicted alterations. Importantly, ctDNA TF was detected in 41% of patients on known active therapy with observed dynamic changes for 55% of patients with longitudinal testing, indicating utility for monitoring treatment response and emerging resistance. In the limited follow up time, physicians chose to monitor patients approximately every 6-8 weeks with a tendency for repeat testing when ctDNA TF was detected at the first test. Citation Format: D. K. Marks, C. B. Tambaoan, H. Tukachinsky, L. W. Pasquina, A. Gasco, M. Childress. Real-world utilization of tissue-free ctDNA monitoring in patients with breast cancer abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS2-06-06.
Marks et al. (Tue,) studied this question.