Analyzing incidence of testis cancer by means of a frailty model.

There are two striking epidemiological features of testicular cancer. First, the incidence has increased strongly over the past few decades. Secondly, the incidence is greatest among younger men, and then declines from a certain age. We have constructed a statistical model to fit these observations. The idea of the model is that a subgroup of men is particularly susceptible to testicular cancer. In statistical terminology this is called a frailty model, since it focuses on varying frailty of the individuals. The frailty, or susceptibility, is considered as being established by birth, and due to a mixture of genetic and environmental effects. The strong increase in incidence over calendar time points to strong environmental effects, which are thought to operate in fetal life, causing damage to the fetus. Based on data from the Norwegian Cancer Registry we fit a frailty model to incidence data collected during 1953-93. The model gives a good fit and we discuss the interpretations of our findings.