Geographic clustering of testicular cancer incidence in the northern part of The Netherlands

Geographic variations in testicular cancer incidence may be caused by diffe rences in environmental factors, genetic factors, or both. In the present s tudy, geographic patterns of age-adjusted testicular cancer incidence rates (IRs) in 12 provinces in The Netherlands in the period 1989-1995 were anal ysed. In addition, the age-adjusted IR of testicular cancer by degree of ur banization was evaluated. Cancer incidence data were obtained from the Neth erlands Cancer Registry. The overall annual age-adjusted IR of testicular c ancer in The Netherlands in the period 1989-1995 was 4.4 per 100 000 men. T he province Groningen in the north of the country showed the highest annual IR with 5.8 per 100 000 men, which was higher (P < 0.05) than the overall IR in The Netherlands (incidence rate ratio (IRR) 1.3, 95% confidence inter val (CI) 1.1-1.6). The highest IR in Groningen was seen for both seminomas and non-seminomas. In addition, Groningen showed the highest age-specific I Rs in all relevant younger age groups (15-29, 30-44 and 45-59 years), illus trating the consistency of data. The province Friesland, also situated in t he northern part of the country, showed the second highest IR of testicular cancer with 5.3 cases per 100 000 men per year (IRR 1.2, 95% CI 1.0-1.5, n ot significant). This mainly resulted from the high IR of seminoma in Fries land, Analysis of age-adjusted IRs of testicular cancer by degree of urbani zation in The Netherlands showed no urban-rural differences at analysis of all histological types combined, or at separate analyses of seminomas and n on-seminomas, Geographic clustering of testicular cancer seems to be presen t in the rural north of The Netherlands with some stable founder population s, which are likely to share a relatively high frequency of genes from comm on ancestors including genes possibly related to testicular cancer. Althoug h this finding does not exclude the involvement of shared environmental fac tors in the aetiology of testicular cancer, it may also lend support to a g enetic susceptibility to testicular cancer development. Testicular cancer c ases in stable founder populations seem particularly suitable for searching for testicular cancer susceptibility genes because such genes are likely t o be more frequent among affected men in such populations. (C) 1999 Cancer Research Campaign.