The team, based at the New South Wales Breast Cancer Institute, used the latest data to estimate a woman's individual risk of breast cancer up to age 79 years in relation to hormone replacement therapy. This is known as the cumulative absolute risk.
Until now, only population risk data have been available, so this analysis will help doctors to weigh the benefits and harms of treatment more accurately.
Cumulative absolute risk of breast cancer falls with increasing age in women who do not take hormone replacement therapy. The average baseline risk (from 40-79 years) is about 7.2% (1 in 14), reducing to 6.1% (1 in 16) at 50 years, and 4.4% (1 in 23) at 60 years.
Use of hormone replacement therapy increases a woman's cumulative risk, but only slightly. For instance, use of oestrogen only hormone replacement or short term (about five years) use of combined therapy starting at age 50 hardly affects the cumulative risk (no use 6.1%, oestrogen only 6.3%, combined 6.7%).
Use of combined therapy for about 10 years increases the cumulative risk to 7.7%, while oestrogen only formulations have a minimal effect on risk of breast cancer, even with extended use. Studies estimate one half of Australian, European, or American women taking hormone replacement therapy are taking combined preparations.
The additional breast cancer risk is greater with combined therapy, especially if taken for more than five years. Five years' use, starting at age 55, generates an extra 0.6% breast cancer risk and 10 years a further 1.8% risk. However, once hormone replacement therapy is stopped, a woman's breast cancer risk quickly returns that of a never user of the same age.
"Although we found the additional breast cancer risk with hormone replacement therapy for an individual is very small, the effect on the general incidence of breast cancer would be greater, especially in populations with high levels of use," said Professor John Boyages, the Director of the Institute
"The reasons for taking hormone replacement therapy vary and decisions about its use must be made at an individual level. Our analysis provides women and clinicians with better information to make these choices," they conclude.