Supplementary Table S2 from Use of ctDNA in Older Women with ER+ Breast Cancer to Facilitate Surgical De-escalation: A Prospective, Hybrid-Decentralized Trial with Correlative Studies

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Abstract
Overall Representativeness of the Study The study cohort is broadly representative of very elderly (≥75 years), frail patients with ER⁺ breast cancer in the United States. The high proportion of community-based enrollment, distribution of clinical stage, and histology align with SEER data, although the cohort underrepresents Hispanic patients and healthier older adults who may receive more aggressive therapy. This study cohort (n = 43; all ≥ 75 years, median age 86, median RAI 36, and 42% ECOG ≥ 2) closely mirrors the demographic and clinical features of prior primary endocrine therapy (pET) cohorts in older women with ER⁺ breast cancer. Carleton et al. (PMID 38135542) reported a median age of 82 (range up to 97), with ∼82% ER⁺/PR⁺ disease, 85% T1–T2 tumors, and 66% node-negative, which closely parallels our distribution of 83% T1–T2 and 72% N0. Similarly, large observational pET studies (PMIDs 37479944, 36326972) describe patient populations with median ages of 81-83, predominantly frail, with early-stage, unifocal, ER⁺ tumors and low nodal burden. In these series, 70-85% of patients were node-negative, most tumors were histologic grade 1–2, and selection for pET was strongly associated with comorbidity or functional dependence – characteristics directly reflected in our cohort with high RAI scores and ECOG ≥ 2. Collectively, our study population is highly representative of the real-world population of elderly, frail women receiving pET for ER⁺ breast cancer in contemporary practice, supporting external validity for this therapeutic context.