About 70% of women with the most common form of early stage breast cancer can be spared the “agony of chemotherapy”, researchers say.
It follows trials of a genetic test that analyses the danger of a tumour, reports BBC.
Cancer doctors said the findings would change practice in UK clinics on Monday, and meant women in this group could be treated safely with just surgery and hormone therapy.
Charities said the news, affecting 3,000 UK women a year, was “wonderful”.
Chemotherapy is often used after surgery to reduce the chance of breast cancer spreading or coming back.
It saves lives, but side-effects of the toxic drugs range from vomiting, fatigue and infertility to permanent nerve pain.
In rare cases it can lead to heart failure and leukaemia.
This trial of 10,273 women analysed cancers using a genetic test that is already widely available, including on the NHS.
Currently, women who get a low score on the test are told they do not need chemo, those with a high score are told they definitely do.
But most women get an intermediate result meaning they are unclear as to what to do.
Data presented at the world’s biggest meeting of cancer doctors and scientists in Chicago shows these women have the same survival rates with or without chemo.
The nine-year-survival-rate was 93.9% without chemotherapy and 93.8% with chemotherapy.