Integrating baseline ctDNA-derived tumor metrics enhances risk stratification in HR-positive/HER2-negative advanced breast cancer: a real-world multicenter cohort study from Austria

BACKGROUND: Advances in endocrine therapies for hormone receptor (HR)-positive/HER2-negative advanced breast cancer (ABC) continue to transform care and significantly improve patient outcomes. However, the integration of molecular and clinical risk stratification into guiding individualized treatment selection remains a key challenge. We therefore evaluated whether the integration of baseline circulating tumor DNA (ctDNA)-derived tumor metrics enhances risk stratification among patients receiving early lines of treatment of HR-positive ABC in a real-world multicenter cohort of patients in Austria. METHODS: Patients with HR-positive/HER2-negative ABC treated at multiple Austrian centers were included. CtDNA was analyzed using a 77-gene panel (AVENIO ctDNA Expanded Kit). Tumor fraction (TFx) was estimated via two complementary approaches: untargeted aneuploidy assessment using mFAST-SeqS, and the highest variant allele frequency (hVAF) from the AVENIO assay. Somatic variants and single, binary, and three-level composite TFx metrics were assessed for their association with progression-free and overall survival (PFS, OS). RESULTS: We analyzed 225 ctDNA samples from 184 patients [128 before first-line (1L) and 76 before second-line (2L) treatment], including 40 paired samples. Overall TFx was low (median z-score 2.49; range -0.5-208.3), with higher levels in 2L, although the difference did not reach statistical significance (P = 0.058). In contrast, the hVAF was significantly higher in the 2L cohort (P = 0.007). Somatic variant burden was significantly increased in 2L (P < 0.001), with notably more frequent ESR1 mutations (26.7% versus 7.1% in 1L). Median PFS was 29.2 months in 1L and 6.0 months in 2L, while median OS was 57.3 months and 16.1 months, respectively. TP53 and ESR1 mutations, and all ctDNA-based metrics were significantly associated with PFS and OS, with a three-level composite ctDNA variable showing the highest prognostic discrimination. CONCLUSIONS: Our findings demonstrate that integrating baseline ctDNA-derived TFx metrics with established clinical variables significantly improves risk stratification in HR-positive/HER2-negative ABC.