Biostatistics 5:1-14 (2004)
© 2004 Oxford University Press
Frailty modelling of testicular cancer incidence using Scandinavian data
Tron A. Moger, Odd O. Aalen, Tarje O. Halvorsen. Section of Medical Statistics, University of Oslo, P.O. Box 1122 Blindern, N-0317 Oslo, Norway
Hans H. Storm. Department of Cancer Prevention and Documentation, Danish Cancer Society, Strandboulevarden 49, DK-2100 Copenhagen Ø, Denmark
Steinar Tretli. Department of Heredity, Nutrition and Hormone Research, The Cancer Registry of Norway, Montebello, N-0310, Oslo, Norway
*To whom correspondence should be addressed.
The incidence of testicular cancer is highest among young men, and then decreases sharply with age. This points towards a frailty effect, where some men have a much greater risk of testicular cancer than the majority of the male population. Those with the highest risk get cancer, drain the group of individuals at risk, and leave a healthy male population which has approximately zero risk of testicular cancer. This leads to the observed decrease in incidence. We discuss a frailty model, where the frailty is compound-Poisson-distributed. This allows for a non-susceptible group (of zero frailty). The model is successfully applied to incidence data from the Danish and Norwegian registries. It is indicated that there was a decrease in incidence for males born during World War II in both countries. Bootstrap analysis is used to find the degree of variation in the estimates. In the Armitage–Doll multistage model, the estimated number of transitions needed for a cell to become malignant is close to 3 for non-seminomas and 4 for seminomas in both the Danish and Norwegian data. This paper demonstrates that a model including a frailty effect fits the incidence data well and gives interesting results and interpretations, although this is no proof of the effect's truth.
Keywords: Carcinogenesis; Compound Poisson; Frailty; Susceptibility; Testicular cancer
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