Aromatase inhibitors work by preventing the body making the hormone oestrogen. They include drugs like letrozole, anastrozole and exemestane, and are already proven to prevent early-stage hormone-receptor positive breast cancer returning if taken for five years. However their side effects can include hot flushes and sweats, nausea, low libido and nausea, and a higher risk of bone fractures, and experts cautioned that not all women would want to take them for a longer period.
Lead researcher Paul Goss, professor of medicine at Harvard Medical School, said current practice - which is to prescribe the drugs for five years - should now change so that the drugs can be prescribed for longer if women want. He said: "Aromatase inhibitors are now readily available around the world and therefore our results will further improve the outcome of women with breast cancer globally. A reduction in recurrences is a very important finding. We have shown that [aromatase inhibitors] given in a different way than is traditional will remarkably reduce fatal recurrences and recurrences in general, and should therefore become the standard practice."
As well as offering these drugs to women diagnosed after the menopause, those diagnosed before their menopause are offered another drug, tamoxifen, for three to five years. Some of these women subsequently switch to aromatase inhibitors.
Professor Goss said women derived "equal benefit" from staying on aromatase inhibitors for 10 years, regardless of whether they had received tamoxifen beforehand. He also added that women on the study reported no worse quality of life when taking the drugs for longer.
However, the authors of an accompanying editorial in the New England Journal noted that trial participants had already been taking the drugs for five years, and thus were likely to be those who already had low levels of side effects.
The trial, called MA.17R, assigned 1,918 post-menopausal women with early-stage breast cancer into two groups: one took the drugs for five years, followed by a placebo; the other took them for 10 years. 95% of women taking the drugs for 10 years had no recurrence of their original cancer five years later, against 91% in the group taking them for five years. Rates of the disease returning in the other breast - called 'contralateral' breast cancer, which is rare - were also markedly reduced.
But experts said women and their doctors needed to balance the benefits against the side effects. In 2013, a study carried out on nearly 3,400 women in Tayside, Scotland, found that only half who had been offered oestrogen-blocking drugs were still taking the pills at the end of five years.
Analysing the new data, Professor Harold Burstein, a US breast cancer expert based at Harvard University, said women diagnosed with faster-growing or more advanced breast cancers would benefit the most from the switch."In general, I would think that women who had riskier cancers, [diagnosed at a] higher stage ... would look to these data and think they are compelling for continuing longer durations of treatment."
Prof Arnie Purushotham, Cancer Research UK's senior clinical advisor, said: "It shows that extending the use of aromatase inhibitors from five to 10 years could reduce the risk of breast cancer returning, and could also help prevent the rare occurrence of cancer being diagnosed in the other breast. We will need more long-term studies to see if this could also improve survival for breast cancer patients."