Real-world estrogen receptor alpha 1 (ESR1) testing patterns and results for ER+/HER2- metastatic breast cancer in the United States, 2018-2024.

Authors

Jane Meisel, Tim Pham, Clara Chen, Ayush Kris, James Roose

Published in

Breast cancer research and treatment. Volume 219. Issue 2. Sep 09, 2026. Epub Sep 09, 2026.

Abstract

To understand historical and recent ESR1 testing rates, and when ESR1 mutations emerge during first-line (1 L) treatment.
This retrospective, observational cohort study used the Flatiron Health Research Database (FHRD) and the Flatiron Health-Foundation Medicine metastatic breast cancer (mBC) Clinico-Genomic Database (CGDB). Adult patients with a confirmed diagnosis of hormone receptor-positive/human epidermal growth factor receptor 2-negative mBC from 1/1/2018 to 6/30/2024 were included. ESR1 testing patterns and test results were descriptively analyzed.
Among 7772 patients with mBC in the FHRD who initiated 1 L therapy, tumor ESR1 mutation status was evaluated for 222 (3%) patients at baseline (≤ 90 days before 1 L) and 1355 (17%) during 1 L. The percentage of patients who had an ESR1 test result reported during 1 L increased over time (11% in 2018-19, 19% in 2020-21, 22% in 2022-24). Median time from 1 L start to first ESR1 test was 7.4 months (mos) among tested patients. A positive test result was reported for 29/222 (13%) patients tested at baseline and 240/1355 (18%) tested during 1 L. Most (60%) tests during 1 L used tissue specimens, while the remaining 40% were liquid biopsies, and the median time from specimen collection to result reporting in 1 L was 28 (IQR:10-84) days. Focusing on time periods wherein specimens were provided, ESR1 test positivity was 6.7% (76/1,127) for specimens provided at baseline, 23% (15/65) for specimens provided 9 to 12 months into 1L therapy, 38% (26/69) for those provided 15 to 18 months into 1L, and 40% (38/94) for those provided from 18 to 24 months into 1L.
ESR1 mutations can be detected at any time interval during 1L.
Not applicable.

PMID:
42714697
Bibliographic data and abstract were imported from PubMed on 10 Sep 2026.

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