As receiving testosterone replacement therapy has an unknown effect on risk of prostate cancer, we excluded these men identified by data from the Swedish Prescribed Drug Register in a sensitivity analysis. Prescription of testosterone at any time led to exclusion (336 ICSI treated fathers, 212 IVF treated fathers, and 7495 reference fathers).

Using data from the Swedish Prescribed Drug Register, available from July 2005 to November 2016, we identified men who had received androgen deprivation therapy. This is indicated only in cases of locally advanced or metastatic prostate cancer and not in low risk malignancies. 19 Thus, androgen deprivation therapy can act as a proxy for the severity and clinical significance of malignancy. We identified men receiving androgen deprivation therapy, with the date of their first prescription, by parsing for the following drugs: abiraterone acetate, buserelin, cyproterone acetate, degarelix, enzalutamide, flutamide, goserelin, histrelin acetate, leuprorelin, megestrol acetate, nilutamide, and triptorelin.

To avoid bias introduced by fathers being counted multiple times, we paired the birth record of the first child born within the cohort interval with the father. This resulted in 1 181 490 children born to the same number of fathers ( fig 1 ). Prostate cancer was defined according to ICD-7 (international classification of diseases, 7th revision) diagnosis code (177) and early onset prostate cancer according to European Association of Urology guidelines 18 (that is, diagnosed before the age of 55).

We retrieved data on all children born alive in Sweden during the period 1994-2014 (n=2 108 569), as well as their fathers, from the Swedish Medical Birth Register and the Swedish Multi-generation Register. We excluded children with missing paternal identification numbers. We matched the remaining records with the Swedish National Quality Register for Assisted Reproduction. The Swedish Cancer Registry, the Swedish Register of Education, and the Swedish Cause of Death Register supplied the paternal prostate cancer diagnoses, paternal education data, and date of death, respectively. Reporting of cancer diagnoses to the national Swedish Cancer Registry is mandated by law for all newly diagnosed cancers, with an approximated completeness of 96%, 17 ensuring a complete assessment of prostate cancer diagnoses. Similarly, reporting of fertility treatments is mandatory, in both private and public clinics, with coverage close to 100%. Data on patients undergoing intrauterine insemination, which is an uncommon procedure in Sweden, was not collected. Thus, in this paper, assisted reproductive techniques refers to ICSI and IVF. As ICSI was first used in 1992, virtually all fathers who conceived through ICSI in Sweden are likely to be included in our cohort.

Statistical analysis

We grouped the fathers according to mode of conception of their child; ICSI, IVF, or natural conception (reference group). We constructed Kaplan-Meier survival curves stratified on the aforementioned groups, with accompanying log rank tests. We used Cox regression to estimate hazard ratios. In the Cox regressions analyses, we corrected for paternal age by adjusting for fathers’ age at childbirth (continuous). We followed the fathers from conception of the child until diagnosis of prostate cancer, death, or end of follow-up (31 December 2014). We estimated the date of conception by using gestational length data from the Medical Birth Register. To adjust for socioeconomic status, the Cox model was adjusted for the father’s education level (years of formal education, categorical: ≤10, 11-14, ≥15, or missing data).

We tested the assumption of proportionality of hazards by the significance level of the interaction between prostate cancer and the natural logarithm of time within the full Cox regression model with all covariates. We further investigated any evidence of non-proportionality (P<0.05) by estimating hazard ratios for restricted time intervals.

To investigate whether men achieving fatherhood by assisted means had an altered risk of early onset prostate cancer, we did an analysis in which we defined an event as prostate cancer diagnosed before age 55. Follow-up was as above, with the fathers being right censored when they reached age 55. This analysis was adjusted for the same covariates as before (paternal age and paternal education level). We also combined the fathers treated with ICSI and those treated with IVF into one group so that a combined risk estimate could be obtained for men becoming fathers through assisted reproduction techniques.

As the follow-up for each father started from conception of the child, men with a prostate cancer diagnosis before that point were excluded, leading to nine ICSI treated fathers, one IVF treated father, and 28 naturally conceiving fathers being excluded from the analyses. However, as cancer treatment may cause subsequent fertility problems, we did a separate sensitivity analysis in which fathers who had been diagnosed as having any cancer (ICD-7: 140-207.9) before child conception, not only prostate cancer, were also excluded (ICSI, n=451; IVF, n=171; natural conception, n=5179).

As ICSI is indicated in more severe forms of male infertility (azoospermia, severe oligozoospermia) and IVF is used in female infertility, combined with mild or no male infertility,1516 we tested for a trend between level of infertility and prostate cancer. This assumed an equidistant stepwise function for the level of infertility among fathers conceiving naturally, via IVF, or via ICSI (continuous variable coded: natural conception=0, IVF=1, ICSI=2).

Among all fathers with prostate cancer, we compared the fathers who conceived through ICSI and IVF with those who conceived naturally, to estimate the risk for receiving androgen deprivation therapy after diagnosis of prostate cancer. As prescriptions for androgen deprivation therapy could not be ascertained before July 2005, only prostate cancer diagnoses after this date were included in this analysis. After these exclusions 52 ICSI treated, 68 IVF treated, and 2967 reference fathers remained. This analysis also serves to detect overdiagnosis of clinically insignificant cases of prostate cancer among men undergoing assisted reproduction, owing to their contacts with the healthcare system. This potential bias would likely lead to these men being diagnosed as having prostate cancer at an earlier age, with lower grade, and therefore generally not being treated with androgen deprivation therapy. Conversely, observing an equal or higher risk of androgen deprivation therapy for men undergoing assisted reproduction would indicate no such bias. For this analysis, we constructed a binary logistic regression model, adjusted for the father’s age at prostate cancer diagnosis (continuous) and paternal educational level, yielding odds ratios, with an odds ratio below 1 indicating possible bias resulting in more diagnoses of low grade prostate cancer among the assisted reproduction groups—for example, due to better access to prostate specific antigen screening. Conversely, an odds ratio of 1 or above points to an increase in prevalence of clinically relevant prostate cancer, which would likely be diagnosed regardless of whether those men were in contact with the healthcare system.

We also did sensitivity analyses in which men receiving testosterone were excluded. We calculated risk estimates for prostate cancer and for early onset prostate cancer by using the same Cox regression method as above.

We used SPSS version 25 and R version 3.5.0 with the ggplot2 package for statistical analyses. All analyses were two sided, and we defined P<0.05 as statistically significant.