A major international breast cancer study is offering hopeful news for patients and doctors: a DNA test may help millions of people safely avoid chemotherapy when it is unlikely to help them.

The research, led by University College London, found that more than two-thirds of participants could be treated with hormone therapy alone, avoiding the difficult side effects often linked to chemotherapy, including nausea, fatigue, hair loss, fertility problems and a weakened immune system.

The study involved more than 4,000 newly diagnosed breast cancer patients over the age of 40 across the UK, Norway, Sweden, Australia, New Zealand and Thailand.

Researchers used a gene test called Prosigna, which examines the activity of 50 genes connected to breast cancer growth. The test helps calculate the likelihood of the cancer returning and identifies which patients are most likely to benefit from chemotherapy.

For patients who received a low score, chemotherapy was not used. Encouragingly, their five-year survival rate was 93.7%, very close to the 94.9% survival rate among those who did receive chemotherapy as part of their care.

Breast cancer treatment usually begins with surgery to remove tumours. Chemotherapy is often recommended afterwards to reduce the risk of the disease coming back, especially for some people with early-stage breast cancer that has spread to nearby lymph nodes.

But University College London said clinicians have been concerned that chemotherapy may offer little benefit to many people with the most common type of breast cancer. Thanks to the trial, the university said more than 5,000 NHS patients each year could potentially avoid the treatment.

For patients, the findings bring a welcome sense of possibility. Karen Bonham, 64, from Cardiff, joined the trial and was able to avoid chemotherapy after taking the Prosigna test. She has instead received radiotherapy and hormone therapy over eight years.

She described the results as an "immense relief" and said they feel "like Christmas".

"Cancer diagnosis and treatment can be shocking," she said.

"It certainly propels you into a world of uncertainty. Life priorities realign - you simply want to survive."

The findings are due to be presented on Saturday at the American Society of Clinical Oncology’s annual meeting in Chicago, the world’s largest cancer conference.

Professor David Miles, a leading cancer specialist, said the results were "practice-changing".

"We can now confidently predict many patients will get no benefit at all, and therefore there's no need for them to have the chemotherapy," he told BBC’s Newshour.

He said the test could help doctors "confidently define a large population of women who simply aren't going to benefit and don't need to go through all that unpleasantness for no benefit at all."

"We used to give chemotherapy to 100 women to benefit 10, knowing that 90 didn't need it," he said.

The news has also been welcomed by people who have experienced chemotherapy themselves. Tanya Hutson, who was diagnosed with breast cancer in 2022 and received chemotherapy as part of her treatment, called the DNA test "absolutely amazing".

"It just proves what happens when money is put into research," she said, adding that chemotherapy had been "brutal".

"For all these people out there who don't need it but are still getting it - it's an absolute game changer."

UCL said it is not yet known whether the results apply to people under 40, and answers for that group are still expected to take several years.