A new test taken two weeks after starting treatment for breast cancer can tell if the disease is likely to return.
The breakthrough could spare thousands of patients unnecessary therapy while allowing others to receive more intensive care sooner.
Researchers at The Institute of Cancer Research, London, found taking hormone drugs for a fortnight changed the characteristics of some tumours, causing them to shift their subtype.
Patients with the highest risk of relapsing had a type of tumour called Luminal B that did not change after this short-term therapy.
These cases, accounting for 6 per cent of the 213 patients studied, require more intensive treatment that others could avoid.
Experts say the findings, published in the journal eBioMedicine, highlight the benefit of taking hormone therapy before surgery to help guide doctors’ decision making.
The new test works for a type of breast cancer known as oestrogen receptor positive, human epidermal growth factor receptor 2 positive, of which there are around 200,000 cases globally each year.
Study author Dr Maggie Cheang, from the ICR, said: ‘To deliver truly personalised care, we need to refine how we classify breast cancer, so that each patient receives the treatment most likely to benefit them.
‘While current classification relies on hormone receptor and HER2 status, we know that patients within these groups can respond very differently to the same therapy.
‘Our earlier research identified distinct molecular subtypes within HER2-positive, oestrogen receptor-positive breast cancer.
‘In this new study, we’ve shown that these subtypes can shift after just two weeks of hormone therapy.
‘This insight helps us identify which patients are likely to respond well and which may show early signs of treatment resistance, offering the opportunity to tailor treatment strategies sooner.
‘Ultimately, our findings move us closer to more precise, patient-centred care for this overlooked breast cancer subtype.’
Professor Kristian Helin, chief executive of the IRC, added: ‘By decoding the underlying biology of tumours, we can tailor treatments to individual patients.’
Dr Simon Vincent, chief scientific officer at charity Breast Cancer Now, which part-funded the study, said: ‘These findings add to the growing evidence that genomic testing can play a powerful role in helping to predict the risk of a woman’s breast cancer coming back, particularly in people with ER-positive, HER2-positive breast cancer.
‘There’s potential for women to benefit hugely from this research in the future, with it ensuring they avoid undergoing unnecessary treatment and leading to more personalised treatment plans, so that women receive the most effective therapy for their specific type of breast cancer.’
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