Does stopping HRT reduce your risk of breast cancer?

An article recently published in the Medical Journal of Australia (Canfell et al 188: 641-4, 2008) suggested that the 6.7% decline in the incidence rate of breast cancer seen between 2001 and 2003 could be attributed to a reduction in the use of hormone replacement therapy (HRT) by Australian women that occurred following release of the findings of the Women's Health Initiative study in July 2002. The Women's Health Initiative (WHI) study reported an increased risk of breast cancer in women using combined (estrogen + progestin) therapy which prompted some women to cease their HRT.

It is important to understand that this recent report is connecting two separate observations and is not strong evidence of cause and effect. The study reports on patterns of use of HT derived from data from the Pharmaceutical Benefits and Repatriation Pharmaceutical Benefits schemes, as these are the only sources of prescription data that are sufficiently consistent to derive trends. People covered by these schemes ( pensioners and health care card holders) are not representative of all Australians so that the trends seen in this data may or may not reflect what has happened in relation to HRT use in the broader community.

The data about breast cancer incidence rates are from the Australian Institute of Health and Welfare and involves data on breast cancer from all state registries.

So the first issue with this new article is that the data is not individualised. This means that we do not know whether the women who stopped the HRT were the ones with the reduced incidence of breast cancer. Secondly, the reduction in the breast cancer incidence seen over the two year period (6.7%) is modest in magnitude and it will be important to see whether the reduction persists or emerges in the light of subsequent data to have been a fluctuation only.

Another concern is that the apparent reduction in breast cancer incidence occurred between 6 and 18 months after the release of the WHI data. This seems be a very short time-frame in which to expect that stopping HRT would affect the development of breast cancer.

One factor that could contribute to the apparent "reversibility" of breast cancer risk following cessation of HRT is that stopping HRT may affect whether or not women undergo breast checks.  Women who are no longer using HRT will most likely have  seen their doctors less often, when they would have had a breast examination, and may also have been less likely to have had their routine mammogram. A change in the pattern of use of mammographic screening could have contributed to the apparent reduction in breast cancer incidence. Although the overall rate of mammographic screening during this time period appears to be relatively stable, the impact of screening on the rate of diagnosis will be affected by the mix of breast cancer risk amongst those women being screened.

Since the publication of the WHI findings HRT has largely been prescribed for women with severe symptoms associated with menopause where, with the woman fully informed of the risks, a judgement is made that, on balance, the benefits of short-term treatment out-weigh the risks. The publication of this new paper will not change this practice.