De Blok, C.J.M. et. al.

Breast cancer risk in transgender people receiving hormone treatment: nationwide cohort study in the Netherlands

BMJ 2019 May 14;365:I1652
doi: 10.1136/bmj.I1652

Hormone replacement therapy among postmenopausal women is associated with breast cancer. Along the same lines, it is possible that hormone therapy in trans women (people assigned male sex at birth and who identify as female) increases their risk for the disease. Moreover, trans men (people assigned female sex at birth and who identify as male) who take gender-affirming hormones may have a lower risk of breast cancer.

The investigators care for patients at a clinic in Amsterdam that cares for over 95% of the nation’s trans community. They reviewed clinical records and identified 2260 trans women and 1229 trans men who qualified for the study. Most trans women who took hormone therapy were treated with a combination of antiandrogens and estrogens. Most trans men who took hormone therapy were treated with testosterone.

Age at start of hormone treatment, type of hormone treatment and surgical and medical history were collected. Data was cross-referenced with a national database of breast cancer cases in the country to identify all patients diagnosed.

There were 15 cases of invasive breast cancer among the trans women. The median age at diagnosis was 50, and the median length of time of hormone therapy use was 18 years. The majority of the cases were estrogen and progesterone receptor positive (83% and 67%, respectively); few were HER2-positive (8%). The calculated risk of breast cancer in this group was 46 times higher than that of cisgender men (assigned male sex at birth and identifying as male) but lower than that of cisgender women (assigned female sex at birth and identifying as female).

There were four cases of invasive breast cancer among the trans men. The median age at diagnosis was 47, and the median time of hormone therapy use was 15 years. The calculated risk of breast cancer in this group was lower compared to cisgender women.

This study suggests that the gender-affirming hormone therapy used for trans women increases their breast cancer risk. Breast cancer education would be useful for these individuals, and breast cancer screening may eventually be shown to be helpful. At the same time, gender-affirming hormone therapy used for trans men may reduce their breast cancer risk. However, these individuals still must be educated about breast cancer and screened when appropriate.