Standard treatment for oestrogen receptor-positive, or ER-positive, breast cancer includes five years of the endocrine-based treatments tamoxifen or aromatase inhibitors, both of which are taken daily as a pill.
The study published in the New England Journal of Medicine analysed data from 88 clinical trials involving 62,923 women with ER-positive breast cancer.
The patients all received endocrine therapy for five years and were free of cancer when they stopped therapy.
Over the next 15 years, however, a steady number of these women saw their cancer spread throughout the body, as late as 20 years after the initial diagnosis.
"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 Michigan in the US.
The risk of recurrence was directly tied to the original cancer's size and characteristics, and to the number of lymph nodes that were cancerous.
Among patients who were recurrence-free when they stopped taking endocrine therapy after five years, the highest risk of recurrence was for those with originally large tumours and cancer that had spread to four or more lymph nodes.
These women had a 40 per cent risk of a distant cancer recurrence over the next 15 years.
Women with small, low-grade cancers and no spread to the lymph nodes had a much lower 10 per cent risk of cancer spreading distantly during the following 15 years.
"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 nodes," said Hongchao Pan, lead author from the University of Oxford in the UK.
Doctors have long known that five years of tamoxifen reduces recurrence by about half during treatment, and by nearly a third over the next five years.
Aromatase inhibitors, which work only in post-menopausal women, are even more effective than tamoxifen at reducing recurrence and death from breast cancer.
Newer studies have suggested an additional five years of endocrine therapy is even more effective, sparking the question of whether every woman should continue on this therapy for 10 years.
Life-threatening side effects are rare with these therapies, but they can impact patients' quality of life.
"As we look at extending endocrine therapy for 10 years, we wanted to determine whether there were certain subgroups of women whose risk of recurrence was so low they might not need to continue endocrine therapy after five years," Hayes said.
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