ESR1 mutations are uncommon in newly diagnosed ER-positive breast cancer but emerge during aromatase inhibitor therapy in a substantial share of patients. A tissue result from the original diagnosis therefore does not answer the question at progression.
Testing circulating tumour DNA at each progression on endocrine therapy is the practical way to detect them, and the result now changes which endocrine partner is chosen.
- Order ESR1 testing at progression on endocrine therapy.
- Prefer a blood-based ctDNA test, which captures acquired mutations.
- Do not rely on a diagnostic biopsy result for ESR1 status.
- Repeat testing at later progressions if it would change treatment.
Why it matters
A test done only at diagnosis misses the mutation that now guides second-line choice.
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