Thousands of breast cancer patients may be safely spared gruelling chemotherapy following a landmark study.
A trial of more than 10,000 women with the most common form of early breast cancer found the treatment was unnecessary for many after surgery.
The findings will lead to a "fundamental change" in how the disease is treated, a leading oncologist said, with an estimated 3,000 to 5,000 UK women likely to avoid chemotherapy every year as a result.
More than 20,000 women in the UK are diagnosed with hormone-receptor positive, HER2-negative, node-negative breast cancer annually.
Around half of these patients would historically receive chemotherapy after having surgery to remove their tumour, to prevent recurrence of the disease.
However, the results of the TAILORx trial show that only 30% of women with this particular form of early-stage breast cancer benefit from the treatment.
The study, presented at the American Society of Clinical Oncology (ASCO) annual meeting in Chicago, is thought to be the largest breast cancer treatment trial ever conducted.
Dr Alistair Ring, consultant medical oncologist at the Royal Marsden NHS Hospital, in London, said: "I think this is a fundamental change in the way we treat women with early-stage breast cancer and will lead to a considerable number of women no longer needing to have chemotherapy."
Charity Breast Cancer Now described the findings as "practice-changing".
The TAILORx trial used the Oncotype DX test, currently available on the NHS, which allows doctors to predict the likelihood of the breast cancer returning.
A sample of the tumour is tested after surgery for 21 genetic markers, which indicate if it could grow and spread.
Patients with a recurrence score of up to 10 out of 100 have previously been shown not to benefit from chemotherapy, and instead need only hormone treatment.
Those who score 26 or higher on the scale do benefit and currently receive chemotherapy.
However, there was unclear evidence on whether those who fall in between - the vast majority of patients - needed chemotherapy.
The TAILORx study, led by the Montefiore Medical Centre in New York, found women older than 50 with this form of breast cancer and a score of up to 25 did not need chemotherapy.
Under 50s with a score of up to 15 can also be spared the treatment, according to the research.
Dr Ring said the study would likely have an immediate impact on UK practice and represent a significant shake-up in the treatment of early-stage breast cancer.
"It is a significant step because it is about avoiding a treatment that, for most people diagnosed with cancer, is what they all fear being suggested to have," he said.
"I, as an oncologist on Monday in clinic, will offer less chemotherapy that will not be of benefit to patients and that is very reassuring to know that when I am offering patients chemotherapy they are likely to benefit from it."
Baroness Delyth Morgan, chief executive of Breast Cancer Now, said: "It's fantastic news that this landmark study could now enable thousands more breast cancer patients over 50 to be safely spared gruelling chemotherapy.
"This is another significant step towards personalised breast cancer treatment and we hope these practice-changing findings will now help refine our use of chemotherapy on the NHS.
"Chemotherapy is an absolute cornerstone of breast cancer treatment, but with the side-effects being almost unbearable for some we must ensure it is only given to those that will benefit from it."
The Oncotype DX genetic test has been available on the NHS since 2013, but the National Institute for Health and Care Excellence (NICE) is currently updating its guidance on whether it should be recommended for use.
Dr Ring said the publication of the trial results was "timely", adding: "I would be very, very keen that the TAILORX results are incorporated into that evaluation.
"Because this is a huge study. It's exactly the sort of study that we need to make decisions about these genomic tests."
Breast Cancer Now also called for NICE to reflect the findings in its guidance.
"With the recent consultation on tumour profiling tests having already closed, it is crucial that NICE has the opportunity to consider these results as soon as possible," Baroness Delyth said.
The study of 10,273 women, led by Dr Joseph Sparano, associate director for clinical research at the Montefiore Medical Centre, is published in the New England Journal of Medicine.