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In August of this year (2019), the Lancet published a large meta-analysis looking at the type and timing of post-menopausal hormone replacement therapy (HRT) and breast cancer risk. The authors were a collaborative group based in the Cancer Epidemiology Unit at the University of Oxford. Important findings from this study are listed below and overall, oestrogen only HRT is associated with a much lower risk of breast cancer than combined HRT:
- Baseline risk of breast cancer if you do not take HRT: For women of average weigh (BMI 25-30) not taking HRT, their risk of developing breast cancer between the ages of 50-69 is 6.3%. This increases to 7.2% if women are obese (BMI >30) and reduces to 5.1% if they are lean (BMI <25).
- Topical vaginal oestrogen does not increase the risk of breast cancer.
- Risk of breast cancer with combined HRT (oestrogen and daily progesterone): For women of average weight, 5 years of HRT will increase the 20 year risk of breast cancer (age 50-69) to 8.3% and 10 years of HRT will increase the 20 year risk to 10.3%.
- Risk of breast cancer with combined HRT (oestrogen and intermittent progesterone): For women of average weight, 5 years of HRT will increase the 20 year breast cancer risk to 7.7% and 10 years of HRT will increase the risk to 9.2%.
- Risk of breast cancer with oestrogen only HRT: For women of average weight, 5 years of HRT will increase the 20 year risk of breast cancer to 6.8% and 10 years of HRT will increase the risk to 7.4%.
- Using HRT for less than 1 year does not really increase the risk of breast cancer but there is definite extra risk when used for up to 4 years. The risk increases the longer the HRT is used.
- If HRT is started after the age of 60 the risk of breast cancer is lower than if started at a younger age.
- The 20 year breast cancer risk from all types of HRT is relatively higher in lean women (BMI <25) than in obese women (BMI >30). In obese women, oestrogen only HRT is associated with very little increased breast cancer risk.
- After stopping HRT, some excess risk of developing breast cancer persists for more than a decade. Again, the longer the HRT is used the longer the ‘hang-over’ effect on breast cancer risk.
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