In short, ER-positive breast cancer flourishes in response to estrogen.
All of the patients were given a type of anti-oestrogen therapy, such as tamoxifen or an aromatase inhibitor, for five years as part of their treatment.
Even once the cancer has gone, these drugs are taken daily for 5 years. They had a 40 per cent risk of relapsing.
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.
But there are side effects with hormone treatments which can affect patients' quality of life and cause them to stop taking the pills.
Side effects can include hot flashes, vaginal dryness, osteoporosis and joint pain. Pathologist Dr Done is affiliated with The Campbell Family Institute for Breast Cancer Research at the University Health Network, and an associate professor at the University of Toronto, Canada.
They also showed that the risk of cancer coming back remained the same year on year from when they stopped taking the anti-oestrogen drugs to 15 years later.
"Even though these women remained free of recurrence in the first five years, the risk of having their cancer recur elsewhere (for example in the bone, liver or lung) from years five to 20 remained constant", said Daniel F Hayes, Professor at the University of MI in the US.
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.
"We would urge all women who have had treatment for breast cancer not to be alarmed, but to ensure they are aware of the signs of recurrence and of metastatic breast cancer, and to speak to their GP or breast care team if they have any concerns".
The risk was directly related to the size of the original cancer and the number of lymph nodes that it affected. This risk remains the same even after five years of treatment.
Those who had small, low-grade tumours that had not spread had a 10% risk.
"We know that treatments have improved since then so women who were diagnosed more recently may be reassured that recurrence rates will be somewhat lower for them".
Looking at overdiagnosis in older women, the authors highlighted research that shows benefits outweigh the risks until the patient is 90 years old. He said, "There are several ongoing analyses asking a number of questions".