Geographic and environmental factors in pediatric cancer.

It is important to determine the relative contributions of genetic and environmental factors to the etiology of childhood cancer in order to elucidate the pathogenic mechanisms involved and to develop effective means of primary prevention. Geographic differences in cancer incidence as well as changes in incidence over calendar time have long been used to generate clues to possible etiologic agents. The important role of genetic factors in childhood cancer is clear, and is exemplified by the observations in retinoblastoma. The importance of the contributions of environmental factors in general and of specific factors in particular, to the etiology of cancers in children, has proven more difficult to determine. A variety of environmental factors have been implicated to varying degrees in the etiology of different childhood cancers. These factors include physical agents such as radiation, chemical agents such as nitrosamines, and organic solvents, and infectious agents such as the Epstein-Barr virus. The observations that certain compounds may act as teratogens when a prenatal exposure occurs early in pregnancy and as carcinogens when the exposure occurs late in pregnancy, suggests that there may be a continuum of teratogenesis and carcinogenesis. This finding has major implications for the possible biologic mechanisms that could be involved in childhood cancers and for the design of future research of their etiology and prevention. The etiology of childhood cancer should be viewed as an interaction of environmental factors to which the child or his parent were exposed together with varying degrees of genetically determined susceptibility of the child to the carcinogenic effects of these factors.