Transgender women undergoing hormone treatment have an increased risk of breast cancer compared to cisgender men, new research suggests.
According to a study carried out by researchers from the University Medical Centre in Amsterdam, trans women (individuals assigned male sex at birth who identify as women) are around 47 times more likely to develop breast cancer than cis men (individuals assigned male sex at birth who identify as men).
But Dr Amar Ahmad, Cancer Research UK’s principal statistician, said that while this might sound like a huge increase, it’s important to remember that breast cancer in cis men is rare.
“This means a small increase in the number of breast cancer cases diagnosed in trans women, as found in this study, is enough to give a large increase in breast cancer risk compared to cis men,” he said.
The study, published in the British Medical Journal, found that trans women were less likely to develop breast cancer than cis women (individuals assigned female sex at birth who identify as women).
It also showed that trans men (individuals assigned female sex at birth who identify as men) have a lower breast cancer risk than cis women.
Researchers studied 2,260 trans women and 1,229 trans men who received gender affirming hormone treatment between 1972 and 2016.
Out of the 2,260 trans women, seventeen developed breast cancer. And around 8 in 10 cancers were driven by the female sex hormones oestrogen and/ or progesterone.
Previous studies have shown hormone replacement therapy (HRT) increases the risk of breast cancer in postmenopausal cis women, causing the researchers to suggest there may be a similar increase in risk for trans women taking hormone treatment.
Dr Alison May Berner, a Cancer Research UK-funded registrar in Medical Oncology and a specialist in gender identity from the Gender Identity Clinic, said this study was a great step forward as “cancer risk in transgender people has been under-researched for a number of years.”
She said the new data would help doctors better advise patients on appropriate screening programmes to take part in.
A UK study is needed
Berner was cautious about applying this Dutch data to UK patients, or to those who are starting hormones now.
“The typical hormones used, particularly for trans women, have changed over the years and some are not the standard used by gender identity clinics in the UK.”
She said work is currently underway to perform a similar study on cancer risk for transgender people in the UK and that NHS screening for transgender people is currently being reviewed.
Lastly, Berner adds that those who took part in the study were fairly young and so there may be more trans women in this group who develop breast cancer later on in life, at a more typical age for cis women.
“Further work is now needed to see how a longer time on hormones impacts the breast cancer risk of trans women,” she said.
Blok, C. J. M. et al. (2019) Breast cancer risk in transgender people receiving hormone treatment: nationwide cohort study in the Netherlands. BMJ doi: https://doi.org/10.1136/bmj.l1652