Familial aggregation of testicular cancer has been reported consistently, but it is less clear if there is any association between risk of testicular cancer and other cancers in the family. We conducted a population based case-control study to examine the relationship between risk of testicular cancer and 22 different cancers in first-degree relatives. We included 3297 cases of testicular cancer notified to the Danish Cancer Registry between 1991 and 2003. 6594 matched controls were selected from the Danish Civil Registration System, which also provided the identity of 40,104 first-degree relatives of case and controls. Familial cancer was identified by linkage to the Danish Cancer Registry, and we used conditional logistic regression to analyse whether cancer among first-degree relatives was associated with higher risk of testicular cancer. Rate ratio (RR) for testicular cancer was 4.63 (95% CI: 2.41-8.87) when a father, 8.30 (95% CI: 3.81-18.10) when a brother and 5.23 (95% CI: 1.35-20.26) when a son had testicular cancer compared with no familial testicular cancer. Results were similar when analyses were stratified by histologic subtypes of testicular cancer. Familial Non-Hodgkin lymphoma and oesophageal cancer were associated with testicular cancer; however these may be chance findings. The familial aggregation of testicular and possibly other cancers may be explained by shared genes and/or shared environmental factors, but the mutual importance of each of these is difficult to determine.
