Thousands of women with breast cancer could be spared chemotherapy every year thanks to a test which has been approved for use on the NHS.
The genetic test could benefit those with early-stage breast cancer facing the intensive treatment, whose side effects include nausea, vomiting, insomnia, hair loss and fatigue.
Studies have suggested that half of women with the most common form of early breast cancer could be spared chemotherapy if they have the test.
The Oncotype DX test involves the examination of genes taken from a sample of a tumour removed during surgery.
It can help doctors decide whether chemotherapy would actually benefit patients and the risk of cancer returning.
Almost 50,000 people are diagnosed with breast cancer every year in the UK and most of these have early-stage disease.
Doctors will use the test in combination with other information, such as the size and grade of the tumour, to work out if chemotherapy could be beneficial.
Current evidence suggests some people are over or under-treated because of uncertainties about whether their cancer will actually spread.
The National Institute for Health and Clinical Excellence (Nice) has approved the Oncotype DX test for people with oestrogen receptor positive (ER+), lymph node negative (LN-) and human epidermal growth factor receptor 2 negative (HER2-) early breast cancer who are at intermediate risk of the cancer spreading.
Professor Carole Longson, Nice health technology evaluation centre director, said: "Breast cancer patients face significant emotional and psychological strain when considering chemotherapy.
"A test that can help to predict better the risk of the breast cancer spreading, and therefore the potential likely benefit of additional chemotherapy, represents a significant step forward for patients."
Some 15 studies on more than 6,000 patients worldwide have taken place using Oncotype DX, showing that around one third of treatment plans are changed as a result of the test.
A 2011 UK study on Oncotype DX found that 46% of patients with early stage disease could be spared unnecessary chemotherapy.
The research was carried out by Simon Holt, a breast surgeon at the Hywel Dda Local Health Board in Wales, on 142 NHS patients with oestrogen receptor positive breast cancer, the most common type.
Commenting on the Nice announcement, Kathi Apostolidis, vice president of the European Cancer Patient Coalition based in Brussels, said: "The decision of whether or not to undergo chemotherapy is an extremely important and difficult one for patients, not only due to the many side effects associated with treatment, but also due to the hardship for the patient and her family and the loss of income due to days off work.
"Allowing patients and their physicians to make more personalised treatment decisions can help improve outcomes for the rising number of early-stage breast cancer patients in Europe."