Breast cancer can resurface after remaining dormant for 15 years following successful treatment, a study has found.
Patients who appear cancer free when they stop therapy may relapse many years later with tumours spreading through their body, new research shows.
Scientists analysed data from 88 clinical trials involving 62,923 women, all of whom had the most common form of breast cancer fuelled by the hormone oestrogen.
Each woman received treatments such as tamoxifen or aromatase inhibitors which block the effects of oestrogen or shut off the hormone's supply for five years, after that time their cancer had gone.
But monitoring the women's progress revealed recurrences of the disease up to 15 years later - 20 years after initial diagnosis.
Lead researcher Dr Hongchao Pan, from Oxford University, said: "It is remarkable that breast cancer can remain dormant for so long and then spread many years later, with this risk remaining the same year after year and still strongly related to the size of the original cancer and whether it had spread to the (lymph) nodes."
Women who started off with large tumours and cancer that had spread to four or more lymph nodes faced the highest risk of recurrence, the study showed.
They had a 40% risk of cancer returning in a different part of the body over a period of 15 years after stopping treatment.
For patients diagnosed with small, low-grade cancers that had not spread the risk was 10%.
Recent research has suggested that extending hormone therapy to 10 years may be more effective at preventing breast cancer recurrence and death.
Doctors have long known that five years of tamoxifen reduces the risk of recurrence by about a third in the five years after stopping treatment.
Aromatase inhibitors, which only work for post-menopausal women, are believed to be even more effective.
The research is published in the New England Journal of Medicine.
Co-author Dr Richard Gray, also from Oxford University, said: "To assess 20-year risks, we had to study women who received their breast cancer diagnosis many years ago. We know that treatments have improved since then, so recurrence rates will be somewhat lower for women who were diagnosed more recently."
Some patients choose to halt hormone treatments early because of side effects such as menopausal symptoms, osteoporosis, joint pain or carpal tunnel syndrome.
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."