Compared with cisgender men, transgender women who receive gender-affirming hormone treatment are at a significantly increased risk for invasive breast cancer, according to study results published in BMJ.
The results also indicated that transgender men had a lower risk for invasive breast cancer compared with cisgender women.
The study included adult transgender women (n = 2260) and adult transgender men (n = 1229) in The Netherlands who received gender-affirming hormone treatment. The primary outcomes were incidence and characteristics of breast cancer in transgender participants compared with the general Dutch population.
Overall, the study included 33,991 person-years for trans women and 14,883 person-years for trans men.
In trans women participants, the researchers identified 18 cases of breast cancer, 15 of which were invasive, with a median duration of hormone treatment of 18 years (range, 7-37 years).
Most of the tumors were of ductal origin and were estrogen- or progesterone receptor-positive. In addition, 8.3% were human epidermal growth factor 2-positive.
In trans men participants, the researchers identified 4 cases of invasive breast cancer, with a median duration of hormone treatment of 15 years (range, 2-17 years).
Compared with cisgender men, trans women participants had a 46-fold higher risk of developing breast cancer (standardized incidence ratio, 46.7). However, trans women and trans men had a lower risk for breast cancer compared with cisgender women (standardized incidence ratios, 0.3 and 0.2, respectively).
“As the risk [for] breast cancer in trans women increased during a relatively short duration of hormone treatment, it would be worthwhile for future studies to investigate in more detail the cause of breast cancer in transgender people receiving hormone treatment,” the researchers wrote.
de Blok CJM, Wiepjes CM, Nota NM, et al. Breast cancer risk in transgender people receiving hormone treatment: nationwide cohort study in the Netherlands. BMJ. 2019;365:I1652.