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113 INCIDENCE OF TESTICULAR CANCER IN TRANS WOMEN USING GENDER-AFFIRMING HORMONAL TREATMENT 7 Necessity for informed consent was waived because of the retrospective design , t he large study population, and the risk of selection bias (e.g. excluding deceased trans women). Data collection Data on medical history (e.g. testicular cancer, cryptorchidism), age at start of GAHT, documented hormone use, endocrine laboratory results, date of bilateral orchiectomy, date of last visit to our clinic, and data on mortality were collected from the medical files of the participants. This database was linked to PALGA to obtain data regarding testicular cancer histology (germ-cell tumors, sex cord/stromal tumors and germ cell neoplasia in situ) and the date of testicular cancer diagnosis. 126 Data on testicular cancer diagnosis was further validated by comparing notes from the medical files with data obtained from PALGA. Statistical analysis Descriptive analyses were conducted to assess the characteristics of the cohort. Normally distributed data are presented as means with standard deviation and non-normally distributed data as medians with interquartile range (IQR). Mean estradiol and testosterone concentrations were calculated by averaging the results from measurements performed during GAHT. In people who had started GAHT prior to their first visit to our clinic, we used the first known start date of GAHT to calculate the most accurate treatment duration. Follow-up time was calculated as the number of years from the start of GAHT until, either the date of testicular cancer diagnosis, or the date of bilateral orchiectomy, or the date of death, or the date of the last visit to our gender identity clinic. To calculate the age-adjusted standardized incidence ratio (SIR), we used the observed cases and the expected cases of testicular cancer in our cohort. Only testicular cancer cases that occurred after the start of GAHT were included for analysis. For our primary research aim, we only included testicular cancer cases that were discovered due to symptoms (e.g. scrotal mass or infertility), since cis men are similarly diagnosed. Expected cases were calculated based on age-specific incidence rates obtained from the Netherlands Comprehensive Cancer Organization. 15 Since this organization also uses data from PALGA to generate the incidence rates of testicular cancer in the Dutch population, this allows for a reliable comparison. For the sake of clarity, we will refer to the reference population as cis men, although we were not able to verify if this was true for the whole population. The SIR with 95% confidence interval (95% CI) was calculated using a mid-exact P test. Since it remains largely unknown how GAHT during puberty affects testicular architecture in terms of testicular cancer risk, a subgroup analysis was performed for trans women who initiated GAHT when 18 years or older. Furthermore, in order to more accurately assess the effect of long-term hormone use, a subgroup analysis was performed for trans women with a follow-up time of 5 years or more. Lastly, to assess how often testicular cancer was discovered in orchiectomy specimens obtained during gender- affirming surgery, a subgroup analysis was performed for trans women whose testicular tissue was send for histopathological analysis at time of bilateral orchiectomy. STATA Statistical Software, version 15.1 (Statacorp, College Station, TX, USA) and OpenEpi version 3.01 (www. OpenEpi.com) were used for statistical analyses.