Breast cancer can lay dormant for 15 years after successful treatment, a study has found.
Patients can appear cancer-free but the disease may return many years later with tumours spreading through the body, according to the analysis of data from 88 clinical trials.
These trials involved 62,923 women, all of whom were diagnosed with the most common form of breast cancer – that fuelled by the hormone oestrogen – between 1976 and 2011.
Each woman was given treatment such as tamoxifen or aromatase inhibitors for five years and by the end of that time, their cancer was gone.
But their progress was monitored for years afterwards and it was found that more than 11,000 of them had their cancer return in a distant site, such as in bone, liver or lung, up to 15 years later.
Women who had large tumours and whose cancer had spread to four or more lymph nodes had a 40% risk of the cancer coming back in a different part of the body over a period of 15 years after stopping treatment.
Those who had small, low-grade tumours that had not spread had a 10% risk.
The study, by the Early Breast Cancer Trialists' Collaborative Group, an international team of investigators based at the University of Oxford, is published in the New England Journal of Medicine and funded by Cancer Research UK.
Professor Arnie Purushotham, Cancer Research UK's senior clinical adviser, said: "This research shows that stopping hormone treatment at five years leaves women with an ongoing risk of breast cancer coming back in the distant future.
"It's important to point out that since this research began, new drugs are being used to treat breast cancer, and these work in a different way to tamoxifen.
"It's vital that work continues to better predict which cancers might return.
"We also need to know what the difference for women might be in taking hormone therapies for 10 years instead of five, the side-effects and how this affects patients' quality of life."
Some patients stop their post-cancer treatment due to side effects that can include menopausal symptoms, osteoporosis, joint pain or carpal tunnel syndrome.
Professor Richard Gray, one of the lead authors from the University of Oxford, said that treatment has improved since many of the women were diagnosed so recurrence rates would be "somewhat lower" for today's patients.
He added: "This study does show, though, why women with oestrogen receptor positive breast cancer should at least consider taking anti-oestrogen therapy beyond five years and may also help motivate women who are experiencing side-effects to persist with treatment."