Millions of women with breast cancer could be spared chemotherapy with a groundbreaking genomic test, according to the results of a trial that could transform healthcare guidelines worldwide.

Treatment for breast cancer , the world’s most prevalent form of the disease, involves surgery to remove tumours. Chemotherapy is then usually recommended when doctors believe there is a risk the disease will return.

But chemotherapy’s toxic side-effects, which can include hair loss, rashes, nausea, insomnia and fatigue, are physically and emotionally gruelling for patients. Some women may face life-changing consequences such as infertility, cognitive impairment or early menopause.

For decades, there has been little choice for patients. Now scientists have developed a genomic test that can spot who needs chemotherapy and who doesn’t.…