Cancer risks in Nordic immigrants and their offspring in Sweden
Introduction
Most migrant studies have shown that the incidence of cancer moves to the level of the new host population in one or two generations 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11. These studies have covered the major types of cancers and they leave little doubt about the overall importance of changing environmental factors and their effects on cancer. Most of the previous studies have examined migration between continents, cultures and politico-economic systems. The movement of people between approximately the same socio-economic backgrounds and between small geographical distances has rarely been covered and the questions about the possible differences in cancer incidence in ethnically uniform, culturally-mixed populations have not been answered. Migrant studies may also address questions about population differences in the genetic susceptibility to cancer because of the differential distribution of genotypes in the populations 12, 13. In this regard, the Nordic populations will be of particular interest. Scandinavians, Swedes, Norwegians, Danes and Islanders are of Germanic origin and share DNA haplotypes, which are considered evidence for genetic resemblance [14]. Finns, on the other hand, show distinct patterns that resemble haplotypes from other neighbouring Finno-Ugric populations, consistent with the demographic history of Finns 14, 15, 16.
The Nordic countries (Denmark, Finland, Iceland, Norway and Sweden; Iceland will not be discussed further because of its small population) have had an open labour market for decades and there have been no limitations on travel or settlement within their borders. Sweden has been the largest economy in the region and it has attracted a number of Nordic immigrants over the years. A large influx of Finns took place in the 1960s. Even though the Nordic countries have many cultural similarities, the incidence of cancer differs extensively between them (Table 1). The data are based on numbers from 1983 to 1987 and were derived from information in the national cancer registries [17]. It is thus of interest to record the incidence of cancer after migration within the Nordic countries. We used the nationwide Swedish Family-Cancer Database, in which the birth country of each individual is recorded, to study cancer incidence in the Nordic immigrants and their Sweden-born offspring [18]. This is an extension of our previous study which covered all large immigrant populations to Sweden 19, 20. We included all the major cancer sites and paid special attention to marriages between compatriots and parents compared with offspring in order to discriminate between environmental and heritable factors.
Section snippets
Patients and methods
The Swedish Family-Cancer Database was initially created in the middle of the 1990s by linking an administrative family register on all Swedish families to the Swedish Cancer Registry 18, 21. For each child, there are data on both parents at the time of birth. Each person has been assigned a unique technical identification number (which is different from the national identification number, ‘personal number’), allowing construction of families, for example through the mother. The Database
Results
The present study covered data from 1961 to 1998 from the Family-Cancer Database. The Swedish natives and their offspring constituted by far the largest group, which was used as a reference (Table 2). The Finns were the second largest group, 183 394 parents and 277 676 offspring. Norwegians parents numbered 42 112 and their offspring 77 774; Danish parents and their offspring were the smallest Nordic immigrant groups of 35 929 and 63 731 individuals, respectively. Compatriot marriages were
Discussion
Even though numerous migrant studies have been published in the literature, few studies have dealt with migration between similar socio-economic and cultural environments over short distances. The motives for such immigration may be other than purely economical, political or religious, and the selection forces in the immigrant populations may be weaker than those in transcontinental immigration. Another novel aspect of this study is that all the Nordic countries have excellent national cancer
Acknowledgements
The Family-Cancer Database was created by linking registers maintained at Statistics Sweden and the Swedish Cancer Registry. The study was supported by the Swedish Cancer Society and King Gustaf V's Jubilee Fund.
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