IJE Advance Access originally published online on March 2, 2006
International Journal of Epidemiology 2006 35(2):384-385; doi:10.1093/ije/dyl034
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Commentary |
Commentary: Numerous, heterogeneous, and often poor—the studies on childhood leukaemia and socioeconomic status
Swiss Childhood Cancer Registry, Department of Social and Preventive Medicine, University of Berne, Switzerland
* Corresponding author. Department of Social and Preventive Medicine, University of Berne, Finkenhubelweg 11, CH-3012 Berne, Switzerland. E-mail: kuehni{at}ispm.unibe.ch
Accepted 9 February 2006
Inequalities in health between socioeconomic groups are a major public health concern. Numerous epidemiological studies have found higher rates of mortality and morbidity among infants, children, and adults of lower socioeconomic status (SES), defined at an individual or area-level.1–3 Together with allergic diseases, childhood leukaemia seemed to be one of the rare exceptions, being reportedly more common among children of high SES.4 Although the evidence is far from conclusive, such an association has led to speculation about a range of potential aetiological factors linked with affluence and modern lifestyle, which could act via altered host susceptibility or environmental exposures.
In this issue, Poole et al.5 present an extensive review of the literature on the association between SES and childhood leukaemia, summarizing the 47 distinct studies identified according to direction of the association (positive or negative) with SES. They included all papers comparing incidence or mortality for children or young adults and providing enough data on an SES measure to determine at least the direction of the association and a P-value. Owing to the vast differences in SES measures used, and in the social implications of measures over time and place, they wisely abstained from a quantitative meta-analysis. Of the 47 studies included in this review, about half found a positive, and the other half a negative association between leukaemia and SES. The direction of the association depended on the study design and the measure used for SES: if the mother's or father's level of education was considered, the studies showed predominantly negative associations (i.e. higher rates associated with lower SES), while occupational class was usually positively associated with SES (i.e. higher rates associated with higher SES). Registry-based and record-based studies generally produced positive associations, whereas interview-based case–control studies showed negative associations. There was also some temporal heterogeneity, with more positive associations in older studies and negative associations in newer studies.
Selection bias related to SES might be a major cause underlying the heterogeneous results, with case–control studies including most cases, but only a selected subset of all relevant controls, resulting in an overrepresentation of high-SES controls owing to a lower response rate in families of lower SES not affected by leukaemia. In contrast, registry-based studies might suffer from a tendency to underrepresent cases from lower social class owing to differentials in the willingness to take part in such studies. The authors conclude that future studies need to measure and report SES in more detail, including several ‘versions’ of individual and ecological SES measures, and investigate the sensitivity of results in function of different SES measures. This would allow an assessment of the robustness of the association between SES and leukaemia.
The main strength of this review is the very broad and sensitive search strategy, including all papers that have reported on SES measures up to August 2002. Several important studies have been published since, including studies on large populations with low probabilities of selection bias. For instance, a study based on 5240 leukaemia cases from the Canadian cancer registries (which cover at least 95% of all Canadian cancer cases), using a neighbourhood-based SES measure (income quintiles from census data), found a lower risk of childhood leukaemia in the poorest compared with the highest income quintile, with no clear time trends.6 A recent publication from the Danish Cancer Registry calls attention to the fact that SES is an exposure that evolves over time and that SES measured at birth might not mean the same as SES assessed at the time of diagnosis or death.7 Their study found an association between leukaemia and low community income at birth but no association with community characteristics or individual SES measures at the time of diagnosis, thus emphasizing the need to consider the timing of repeated and correlated exposures within the framework of a life course approach.8–10 The associations between the development of chronic disease and SES measures assessed at different points in life has recently been studied by performing stepwise adjustments for these factors in multivariable analyses.11–13
The review by Poole et al. reminds us that the published information on important public health questions is often of poor quality, especially if the exposure of interest is difficult to measure, depends on social context, and cannot be studied in controlled trials. This is all the more sobering considering that the association between SES and childhood leukaemia should be easy to tackle, compared with other associations between SES and childhood diseases. In many countries there are cancer registries with near-complete coverage and socioeconomic measures at different time points (at least birth and diagnosis) available from population registries. In addition, the case definition ‘leukaemia’ is reasonably clear, and case detection in industrialized countries is unlikely to depend on SES of the families. The situation is much more difficult for other conditions such as childhood asthma or musculoskeletal disorders, where there are no registries, and, even worse, there is no diagnostic standard. Case definitions then often depend on parental interpretation and reporting of symptoms, and on the interpretation of these symptoms by physicians, all of which are likely to be strongly associated with SES.14,15 By highlighting these methodological difficulties we do not want to imply that the question of SES and its association with important childhood diseases should not be studied. On the contrary, the review by Poole et al. provides useful information on how future studies need to be designed to provide better answers to this important question.
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