Few studies have investigated the incidence of breast cancer in transgender people. [1,2] The studies that have been done were retrospective, owing to the fact that it takes a while before the side effects of hormone treatment can reliably be studied. Although these studies provide new information about the risk of developing breast cancer for transgender people, the researchers encountered some obstacles for which they were unable to account for. For instance, transition status was not always clearly stated in the registries used, potentially leading to unnecessary exclusion of suitable participants. Other studies struggled with the fact that transgender people were treated in different healthcare centres for diagnosis other than gender dysphoria. This has often led to an underestimation of incidence numbers. Some of the studies lost a large number of people at follow-up.
When we were planning our recent study investigating breast cancer risk in transgender people, we struggled with some of these obstacles as well. Even though our healthcare centre provides transgender healthcare to more than 95% of all transgender people in the Netherlands, many of our study participants were lost to follow-up, especially those who started their treatment decades ago.
Although the Netherlands is a small country, it has a lot of hospitals. Transgender healthcare is mostly centralized and performed in our centre, but other healthcare, including breast cancer diagnosis and treatment, is carried out in local hospitals. As we wanted to obtain as much data as possible about the people in our cohort, we started searching for other ways to gain the information that we needed. 
We found an article about the National Pathology Database (Nationwide Network and Registry of Histopathology and Cytopathology in the Netherlands, PALGA).  We contacted PALGA to explore whether we could join forces. By linking our cohort of transgender people to their database of pathology diagnoses, we were able to obtain breast cancer diagnoses of the people in our cohort. Since maintaining the privacy of the people in our cohort is our number one priority, we used a trusted third party to link the data from both sources. Using this method, the researchers were unable to identify the people behind the breast cancer cases.
We wanted to calculate follow-up times as precisely as possible. One challenge that we faced, was following up people who did not attend our clinic anymore after several years. By linking to the PALGA database, we knew whether or not they have had a diagnosis of breast cancer.
We also needed more reliable data about participants who had died. For this information, we linked our cohort to Statistics Netherlands (CBS)5, which registers mortality data of all residents in the Netherlands. Again by using this link, we made sure that the privacy of the study participants was guaranteed. Moreover, CBS has a strict policy regarding data sharing. All data that are published are checked to make sure the anonymity of the people in the study is granted. With this combined data-collection method, we were able to answer our research question on the incidence of breast cancer in transgender people receiving hormone treatment.
For us, the search for other sources of (diagnostic) information led to valuable collaborations with PALGA and CBS. With these collaborations, more data became available and this led to new research possibilities. Other centres searching for information could consider collaborating with (national) registries as well, to answer their research questions which are otherwise hard to answer.
Christel de Blok is a PhD Candidate at the Centre of Expertise on Gender Dysphoria of the Amsterdam UMC.
Chantal Wiepjes is a doctor at the outpatient clinic of the Centre of Expertise on Gender Dysphoria of the Amsterdam UMC.
Nienke Nota is a medical doctor and researcher at the Centre of Expertise on Gender Dysphoria of the Amsterdam UMC.
Martin den Heijer is a professor in endocrinology, epidemiologist, and head of the Centre of Expertise on Gender Dysphoria of the Amsterdam UMC.
Competing interests: See research paper
- Gooren LJ, van Trotsenburg MA, Giltay EJ, et al. Breast cancer development in transsexual subjects receiving cross-sex hormone treatment. J Sex Med 2013;10(12):3129-34. doi: 10.1111/jsm.12319
- Brown GR, Jones KT. Incidence of breast cancer in a cohort of 5,135 transgender veterans. Breast Cancer Res Treat 2015;149(1):191-8. doi: 10.1007/s10549-014-3213-2
- Wiepjes CM, Nota NM, de Blok CJM, et al. The Amsterdam Cohort of Gender Dysphoria Study (1972-2015): Trends in Prevalence, Treatment, and Regrets. J Sex Med 2018;15(4):582-90. doi: 10.1016/j.jsxm.2018.01.016
- Casparie M, Tiebosch ATMG, Burger G, et al. Pathology databanking and biobanking in The Netherlands, a central role for PALGA, the nationwide histopathology and cytopathology data network and archive. Cellular Oncology 2007;29:19-24.
- . Statistics Netherlands (CBS). https://www.cbs.nl/en-gb/our-services/customised-services-microdata/microdata-conducting-your-own-research [accessed 24 April 2019]