I recently received some questions about the risk of breast cancer for transgender women on HRT, so I thought I would try to present the current most up-to-date knowledge in this area.
We simply don’t have enough population data to be able to give an accurate risk, but we can look at cohort data.
The trouble with this data is that the women in it were on a variety of different HRT combinations for varying lengths of time, so it’s hard to be able to relate this to a particular person’s risk.
The lifetime background risk for a cisgender woman of developing breast cancer is 1 in 8, and for a cisgender man it is 1 in 1000.
If we look at published cases of breast cancer, it appears that the rate in trans women is not too different to the rate in cis males. But we can go further and look at larger cohort data from the Netherlands, which was presented at the World Professional Association for Transgender Health (WPATH) conference last year.
Here we see a slightly increased rate of breast cancer in trans women compared with cis men and a decreased rate of breast cancer in trans men compared with cis women.
The breast cancers in the trans women had characteristics resembling a more female pattern, were mostly oestrogen-receptor positive – meaning that oestrogen would cause them to grow faster – and their age of diagnosis tended to be younger.
This cohort data did not show a rate of breast cancer in trans women that suggested a need for more intensified screening compared with cis women.
It was not possible to understand the individual hormone contribution to the cancers, as hormone regimens were too variable and the numbers too small.
My questioner asked about the role of progesterone in the development of breast cancer, and the short answer is that we don’t know.
There is simply not enough known even in cis female populations to answer that question, and there are theories both for and against.
With respect to oestrogen levels, it is likely that the higher your oestrogen level, the higher your breast cancer risk might be, and certainly oestrogen is by far the more important hormone when it comes to breast cancer.
Again, there is no data around oestrogen levels and breast cancer incidence, so it is all theoretical. One day I hope we have the answers to such questions!
Dr Fiona Bisshop specialises in LGBTIQ health. Read more by Dr Bisshop on her website here or contact her on Twitter. Send your health question to firstname.lastname@example.org and Dr Bisshop will answer them anonymously.
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