Comparison of racial differences in childhood cancer risk in case-control studies and population-based cancer registries

Introduction: Although selection bias in case-control studies has been studied extensively, little is known about selection of cases and controls among various ethnic groups. This study compares racial differences in childhood cancer rates as estimated by case-control studies with various design features. It also compares estimates of racial distribution among cases as reported by case-control studies to those observed for an ideal case series with complete ascertainment of cases for these studies or in population-based cancer registries in corresponding geographic regions and calendar periods. Methods: Peer-reviewed publications on childhood leukemia and brain tumors from North America, published between 1980 and 2007, were reviewed. Incidence data by race/ethnicity were compiled from research publications, federal cancer statistics, and cancer registries. Meta-analysis was conducted to assess racial/ethnic differences by study characteristics. Racial distributions of cases from published case-control studies were compared to those of a presumably noncensored case distribution (i.e. include both participating and non-participating cases in a case-control study) or cases recorded by cancer registries. Results: In interview-based case-control studies of childhood cancer, the proportion of Whites compared to non-Whites tended to be higher among controls than among cases; however, the opposite was true for record-based case-control studies. Additionally, the proportion of Whites tended to be higher among the participating cases in the published case-control studies compared to the proportion of Whites among the non-participating cases or in cancer registries. Conclusions: Investigators need to consider differential participation by racial group as a potential source of bias in the interpretation of case-control study results.