Familial risks in prostate cancer between brothers and half-brothers as clues to germline genetic and environmental causes

Swedish family and cancer data constitute the largest source on familial cancer in the world. We analyze here familial risks in prostate cancer (PC) with focus on multiple affected brothers and comparation of full-brothers to maternal and paternal half-brothers. Age-specific incidence and standardized incidence ratios (SIRs) were calculated for PC in brothers. Curves for relative risk (RR) by diagnostic age were plotted for risk distributions. A total of 115,066 PCs were diagnosed in 1.2 million men. Familial SIR for full brothers was 2.23, for maternal half-brothers it was 1.92 and paternal half-brothers was 1.34. Considering SIRs with least possible detection bias (7+ years after first brother's diagnosis) the above SIRs were 2.06, 1.66 and 1.41. SIRs in full brothers increased stepwise by the number of affected brothers reaching 21.33 when 6 brothers were affected. Age-RR curves for two affected brothers declined evenly from RR 2.8 at age 45 to below 2.0 at age 80. When four or more brothers were affected, a discrete high-risk peak (RR 4-7) was detected between ages 60 and 69. Data on full-brothers and half-brothers indicate that familial risk in PC is largely genetic which is also supported by discrete RR peaks in high-risk families at ages matching preferential penetrance age for known predisposition genes of PC. Familial risk increased already when two brothers were affected calling for clinical vigilance concerning family history. Family history should deserve a place as an inclusion criterium in schemes for PC screening.