A trial of 10,273 women with the most common form of early breast cancer found it 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 about 3,000 to 5,000 UK women likely to avoid chemotherapy a year as a result.
About 20,000 women a year in the UK are diagnosed with hormone-receptor positive, HER2-negative, node-negative breast cancer.
Around half would historically receive chemotherapy after having surgery to remove their tumour, to prevent recurrence of the disease.
However, the results of the TAILORx trial, published in the New England Journal of Medicine, show only 30 per cent of women with this form of cancer benefit from chemotherapy.
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.”
The TAILORx trial at the Montefiore Medical Centre in New York – thought to be the largest breast cancer treatment trial ever – used the Oncotype DX test, which enables doctors to predict if the breast cancer will return.
A tumour sample is tested after surgery for 21 genetic markers, which indicates if it could grow and spread.
The test has been available on the NHS since 2013, but the National Institute for Health and Care Excellence is currently updating its guidance on whether it should be recommended for use.