Transgender and non-binary individuals and breast cancer risk

A recent study has found trans women who undergo hormone treatment have an increased risk of breast cancer compared to cis men and lower risk compared to cis women (1).

The study also found trans men who undergo hormone treatment have a reduced breast cancer risk compared to cis women but are at higher risk compared to cis men. Many trans men undergo a double mastectomy, which significantly reduces breast cancer risk (2).

Further studies are needed to assess the potential risks of long-term hormone treatment in trans and non-binary individuals.

High breast density and oestrogen increase breast cancer risk

Everyone is susceptible to breast cancer. Two important factors that increase risk are lifetime exposure to high levels of sex hormones, particularly oestrogen, and having a high breast density, as seen on a mammogram (3).

Dense breasts have relatively higher amounts of glandular tissue (milk ducts and milk lobules) and supportive tissue and lower amounts of fatty tissue (4). Most breast cancers develop in ducts and lobules. Cis males have less breast tissue, which is mainly composed of fatty and fibrous tissue with only small amounts of glandular tissue (5).

Hormone replacement therapy (HRT), which includes synthetic oestrogens with (combined) or without progesterone, increases breast cancer risk in cis women, especially during long-term use of combined HRT (6).

Breast cancer in trans women

Trans women often receive hormone therapy which helps breast tissue development. Cis women (aged between 50-79 years) who receive HRT are known to be at an elevated risk of breast cancer (6). Those taking the contraceptive pill, which also contains synthetic oestrogen and progesterone, have a slightly increased risk (7). This suggests that trans women who take hormone therapy may also have an increased risk of developing breast cancer.

Few studies have examined the risk of breast cancer in trans women (1, 8, 9, 10). The most robust study to date was carried out in 2019 by Dutch researchers, who found that breast cancer was more common in trans women (whose average age was 31) than cis men but less common than in cis women (1).

Another Dutch study (11) examined what proportion of breast lesions in trans women was benign or cancerous. They found around 90% were benign (non-cancerous), similar to the percentage found in cis women.

Breast cancer in trans men

Even fewer studies have investigated breast cancer risk in trans men. The most comprehensive was the 2019 Dutch study discussed previously (1), which found the risk to be lower than expected for cis women (whose average age was 23) but higher than expected for cis men. Other studies have identified a lower risk for trans men compared to cis women (12, 13).

Many trans men have breast reduction surgery as part of gender reassignment. However, not all breast tissue is routinely removed.

Trans men are thought to be at low risk due to breast tissue removal. However, not all trans men undergo mastectomies and they remain at higher risk (2).

The impact of high levels of testosterone on breast tissue is unclear. Testosterone therapy in trans men alters the composition of breast tissue by reducing the amount of glandular tissue and increasing the amount of fibrous connective tissue (2). Some studies suggest that in cis women elevated testosterone increases breast cancer risk (14). This is because aromatase (an enzyme found in breast and other tissues) converts testosterone to oestrogen, raising overall oestrogen levels, which increases breast cancer risk.

However, a study of European trans men found taking testosterone reduces oestrogen levels (15). The same study found those who have their ovaries removed as part of their gender reassignment also had reduced levels of oestrogen. Exposure to lower levels of oestrogen helps reduce breast cancer risk (4).

Another study found that taking testosterone for at least 12 months may reduce the risk of certain types of benign breast lesions (abnormal areas of breast tissue) (16).

Screening and breast self-checking

As mentioned, trans men and trans women are at a lower risk of breast cancer compared to cis women. Nonetheless, everyone is susceptible to breast cancer, especially if you have not had a double mastectomy.

We recommend regular self-checking for everyone, regardless of gender. Please see our short video for guidance on how to check your breasts.

The NHS offers screening mammography to trans women and trans men who have not undergone a double mastectomy. Only individuals who are registered as a female with their GP are automatically invited for breast screening. For more information please see NHS population screening: information for trans and non-binary people – GOV.UK (www.gov.uk).

Glossary

Trans men: transgender men who are assigned female at birth and identify as men

Trans women: transgender women who are assigned male at birth and identify as women

Cis men: cisgender men assigned male sex at birth who identify as men

Cis women: cisgender women assigned female sex at birth who identify as women

Non-binary: those who have transitioned but who do not identify with either gender

*Hormone therapy for trans women includes oestrogen and often anti-androgens. It may also include progesterone

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