In ER-positive metastatic breast cancer, resistance to endocrine therapy can come from acquired changes such as ESR1 mutations, and tumour genotype can steer the next line. When disease progresses on endocrine therapy with a CDK4/6 inhibitor, ask whether molecular testing has been done on a recent tumour sample or on circulating tumour DNA, and check the result before choosing the next treatment.
If testing is not available locally, discuss sending a sample to a reference laboratory. Record the result in the notes so it travels with the patient.
- Ask whether molecular testing was repeated at progression.
- Circulating tumour DNA can be an alternative when biopsy is hard.
- Document mutation results in the record.
- Discuss results at the tumour board.
Why it matters
Treatment choice at progression increasingly depends on what the tumour has acquired.
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