Prevalence of Somatic BReast CAncer Gene (BRCA) 1 and 2 Pathogenic Variants in Portuguese Metastatic Prostate Cancer Patients

Currently, poly adenosine diphosphate ribose polymerase inhibitors are used to treat metastatic prostate cancer (mPC) patients with somatic or germline pathogenic variants in genes related to homologous recombination repair deficiency. Testing for these variants is thus advisable, as test results can have implications for systemic treatment. Of those genes, the most relevant in clinical practice are BReast CAncer gene (<i>BRCA</i>) 1and 2. Despite no published data regarding the prevalence of germline and somatic variants in mPC Portuguese patients, practitioners have long felt that the prevalence of somatic <i>BRCA1/2</i> variants in these patients is much lower than in previously studied populations. To estimate the prevalence of pathogenic <i>BRCA1/2</i> variants, we fit a Bayesian hierarchical model with data from metastatic patients subject to universal testing and data from international cohorts. All 42 patients tested were negative for somatic <i>BRCA1/2</i> pathogenic variants. This posterior estimate for the prevalence is 3.1% (95% credibility interval 0.3-10.3%), and we found a large dispersion between the prevalences of different populations. This estimate is much lower than the estimates in other published cohorts. We believe that testing recommendations should be tailored to country-specific prevalence. As such, we will continue to perform universal testing in an investigational context to decrease the uncertainty in our estimates and better establish the role of universal somatic testing in the Portuguese population.