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 cert ain age. We have constructed a statistical model to fit these observations. The idea of the model is that a subgroup of men is particularly susceptibl e 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 t o a mixture of genetic and environmental effects. The strong increase in in cidence over calendar time points to strong environmental effects, which ar e thought to operate in fetal life, causing damage to the fetus. Based on d ata 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 th e interpretations of our findings.