The importance of full participation: lessons from a national case-control study

Maternal lifestyle characteristics and Wilms tumor risk in the offspring: A systematic review and meta-analysis

BACKGROUND

Little is known about the etiology of childhood Wilms tumor (WT) and potentially modifiable maternal risk factors, in particular.

METHODS

Unpublished data derived from the hospital-based, case-control study of the Greek Nationwide Registry for Childhood Hematological Malignancies and Solid Tumors (NARECHEM-ST) were included in an ad hoc conducted systematic literature review and meta-analyses examining the association between modifiable maternal lifestyle risk factors and WT. Eligible data were meta-analysed in separate strands regarding the associations of WT with (a) maternal folic acid and/or vitamins supplementation, (b) alcohol consumption and (c) smoking during pregnancy. The quality of eligible studies was evaluated using the Newcastle-Ottawa Scale.

RESULTS

Effect estimates from 72 cases and 72 age- and sex-matched controls contributed by NARECHEM-ST were meta-analysed together with those of another 17, mainly medium size, studies of ecological, case-control and cohort design. Maternal intake of folic acid and/or other vitamins supplements during pregnancy was inversely associated with WT risk (6 studies, OR: 0.78; 95 %CI: 0.69-0.89, I2 = 5.4 %); of similar size was the association for folic acid intake alone (4 studies, OR: 0.79; 95 %CI: 0.69-0.91, I2 = 0.0 %), derived mainly from ecological studies. In the Greek study a positive association (OR: 5.31; 95 %CI: 2.00-14.10) was found for mothers who consumed alcohol only before pregnancy vs. never drinkers whereas in the meta-analysis of the four homogeneous studies examining the effect of alcohol consumption during pregnancy the respective overall result showed an OR: 1.60 (4 studies, 95 %CI: 1.28-2.01, I2 = 0.0 %). Lastly, no association was seen with maternal smoking during pregnancy (14 studies, OR: 0.93; 95 %CI: 0.80-1.09, I2 = 0.0 %).

CONCLUSIONS

In the largest to-date meta-analysis, there was an inverse association of maternal folic acid or vitamins supplementation with WT risk in the offspring, derived mainly from ecological studies. The association with maternal alcohol consumption found in our study needs to be further explored whereas no association with maternal smoking was detected. Given the proven benefits for other health conditions, recommendations regarding folic acid supplementation as well as smoking and alcohol cessation should apply. The maternal alcohol consumption associations, however, should be further explored given the inherent limitations in the assessment of exposures of the published studies.

Proximity to overhead power lines and childhood leukaemia: an international pooled analysis

BACKGROUND

Although studies have consistently found an association between childhood leukaemia risk and magnetic fields, the associations between childhood leukaemia and distance to overhead power lines have been inconsistent. We pooled data from multiple studies to assess the association with distance and evaluate whether it is due to magnetic fields or other factors associated with distance from lines.

METHODS

We present a pooled analysis combining individual-level data (29,049 cases and 68,231 controls) from 11 record-based studies.

RESULTS

There was no material association between childhood leukaemia and distance to nearest overhead power line of any voltage. Among children living < 50 m from 200 + kV power lines, the adjusted odds ratio for childhood leukaemia was 1.33 (95% CI: 0.92-1.93). The odds ratio was higher among children diagnosed before age 5 years. There was no association with calculated magnetic fields. Odds ratios remained unchanged with adjustment for potential confounders.

CONCLUSIONS

In this first comprehensive pooled analysis of childhood leukaemia and distance to power lines, we found a small and imprecise risk for residences < 50 m of 200 + kV lines that was not explained by high magnetic fields. Reasons for the increased risk, found in this and many other studies, remains to be elucidated.

Childhood brain tumours, early infections and immune stimulation: A pooled analysis of the ESCALE and ESTELLE case-control studies (SFCE, France)

BACKGROUND

Few studies have investigated whether early infections and factors potentially related to early immune stimulation might be involved in the aetiology of childhood brain tumours (CBT). In this study, we investigated the associations between CBT with early day-care attendance, history of early common infections, atopic conditions (asthma/wheezing, eczema, allergic rhinitis), early farm residence/visits and contact with animals.

METHODS

We pooled data from two nationwide French case-control studies, the ESCALE and ESTELLE studies. Children with a CBT diagnosed between 1 and 14 years of age were identified directly from the French National Registry of Childhood Cancers, while population controls were recruited from telephone subscribers. Odds-ratios (OR) and 95% confidence intervals (CI) were estimated using logistic regression adjusted for potential confounders.

RESULTS

The analyses included 469 cases and 2719 controls. We found no association between attending a day-care centre (OR: 0.9, 95%CI: 0.7-1.2) or having had repeated common infections (OR: 0.9, 95%CI: 0.7-1.2) in the first year of life and the risk of CBT. There was also no association with a history of asthma/wheezing (OR: 0.8, 95%CI: 0.56-1.1). Farm visits (OR: 0.6, 95%CI: 0.5-0.8) as well as contact with pets (OR: 0.8, 95%CI: 0.6-1.0) in the first year of life were inversely associated with CBT.

CONCLUSIONS

Our findings suggest a protective effect of early farm visits and contact with pets, but not with other markers of early immune stimulation. This might be related to immune stimulation but needs further investigation.

Comparative analyses of studies of childhood leukemia and magnetic fields, radon and gamma radiation

In this paper we compare the findings of epidemiologic studies of childhood leukemia that examined at least two of ELF magnetic fields and/or distance to power lines, and exposure to radon and gamma radiation or distance to nuclear plants. Many of the methodologic aspects are common to studies of non-ionising (i.e. ELF-MF) and ionising radiation. A systematic search and review of studies with more than one exposure under study identified 33 key and 35 supplementary papers from ten countries that have been included in this review. Examining studies that have looked at several radiation exposures, and comparing similarities and differences for the different types of radiation, through the use of directed acyclic graphs, we evaluate to what extent bias, confounding and other methodological issues might be operating in these studies. We found some indication of bias, although results are not clear cut. There is little evidence that confounding has had a substantial influence on results. Influence of the residential mobility on the study conduct and interpretation is complex and can manifest as a selection bias, confounding, increased measurement error or could also be a potential risk factor. Other factors associated with distance to power lines and to nuclear power plants should be investigated. A more complete and consistent reporting of results in the future studies will allow for a more informative comparison across studies and integration of results.

Representativity of a postal public health questionnaire survey in Sweden, with special reference to ethnic differences in participation

Aim: Non-participation in health surveys is a common phenomenon. When differences between participants and non-participants are considerable, the external validity of the sample survey may decrease and false conclusions might be drawn about the health status of the population. For this reason, the authors aimed to investigate the representativity of a postal questionnaire survey performed in the county of Scania, Sweden, in 1999—2000. The survey, which was based on an 18- to 80-year-old population sample, had a 58% response rate (n = 13 604). Methods: For some variables, the information obtained using the questionnaire was compared with information obtained from a population register that covers all the population in the county (for the 18- to 80-year-old group, n = 850 476). The population register includes, among other data, information on age, gender, educational level, country of birth, and healthcare expenditure. Results: Men, individuals with a low level of education, and immigrants were under-represented in the survey. However, except for immigrants, the under-representation was not large. Among immigrants, particularly those born in former Yugoslavia, the Arabic-speaking countries, and Poland were very significantly under-represented in the study. By contrast, immigrants born in other Nordic countries had responded to almost the same extent as respondents born in Sweden. The survey sample had about the same healthcare utilization costs as did the general population. Conclusions: In summary, the ``Health Survey for Scania, 2000'' seems largely representative of the total Scanian population. A major concern, however, is the under-representation of the immigrant population.

Residential mobility and associated factors in relation to the assessment of exposure to naturally occurring radiation in studies of childhood cancer

Migration, that is the study subjects moving from one residential address to another, is a complication for epidemiological studies where exposures to the agent of interest depend on place of residence [corrected]. In this paper we explore migration in cases from a large British case-control study of childhood cancer and natural background radiation. We find that 44% of cases had not moved house between birth and diagnosis, and about two-thirds were living within 2 km of their residence at birth. The estimated dose at the diagnosis address was strongly correlated with that at the birth address, suggesting that use of just the birth address in this case-control study does not lead to serious bias in risk estimates. We also review other individual-based studies of naturally occurring radiation, with particular emphasis on those from Great Britain. Interview-based case-control and cohort studies can potentially establish full residential histories for study subjects and make direct measurements of radiation levels in the dwellings in question. However, in practice, because of study size and difficulties in obtaining adequate response rates, interview-based studies generally do not use full residential histories, and a substantial proportion of dose estimates often derive from models rather than direct measurements. More seriously, problems of incomplete response may lead to bias, not just to loss of power. Record-based case-control studies, which do not require direct contact with study subjects, avoid such problems, but at the expense of having only model-based exposure estimates that use databases of measurements.

Birth order and risk of hepatocellular carcinoma in chronic carriers of hepatitis B virus: a case-control study in The Gambia

BACKGROUND & AIMS

Early age at infection with Hepatitis B virus (HBV) increases the risk of chronic infection. Moreover, early HBV infection may further independently increase the risk of hepatocellular carcinoma (HCC) beyond its effect on chronicity.

METHODS

The distribution of birth order, a proxy for mode and timing of HBV transmission, was compared in The Gambia between hepatitis B surface antigen (HBsAg)-positive HCC cases recruited from hospitals (n = 72) and two HBsAg-positive control groups without HCC: population-based controls from a community HBV screening (n = 392) and hospital-based controls (n = 63).

RESULTS

HCC risk decreased with increasing birth order in the population-based case-control analysis. Using first birth order as the reference, the odds ratios were 0.52 (95% CI: 0.20-1.36), 0.52 (0.17-1.56), 0.57 (0.16-2.05) and 0.14 (0.03-0.64) for second, third, fourth and greater than fourth birth order respectively (P = 0.01). A similar inverse association was observed in the hospital-based case-control comparison (P = 0.04).

CONCLUSIONS

Compared to controls, HCC cases had earlier birth order, a proxy for young maternal age and maternal HBV viraemia at birth. This finding suggests that in chronic HBV carriers perinatal mother-to-infant transmission may increase HCC risk more than horizontal transmission. Providing HBV vaccine within 24 h of birth to interrupt perinatal transmission might reduce the incidence of HCC in The Gambia.

Childhood leukemia and 50 Hz magnetic fields: findings from the Italian SETIL case-control study

We report on an Italian case-control study on childhood leukemia and exposure to extremely low frequency magnetic fields (ELF-MF). Eligible for inclusion were 745 leukemia cases, aged 0–10 years at diagnosis in 1998–2001, and 1475 sex- and age-matched population controls. Parents of 683 cases and 1044 controls (92% vs. 71%) were interviewed. ELF-MF measurements (24–48 h), in the child’s bedroom of the dwelling inhabited one year before diagnosis, were available for 412 cases and 587 controls included in the main conditional regression analyses. The magnetic field induction was 0.04 μT on average (geometric mean), with 0.6% of cases and 1.6% of controls exposed to >0.3 μT. The impact of changes in the statistical model, exposure metric, and data-set restriction criteria was explored via sensitivity analyses. No exposure-disease association was observed in analyses based on continuous exposure, while analyses based on categorical variables were characterized by incoherent exposure-outcome relationships. In conclusion, our results may be affected by several sources of bias and they are noninformative at exposure levels >0.3 μT. Nonetheless, the study may contribute to future meta- or pooled analyses. Furthermore, exposure levels among population controls are useful to estimate attributable risk.

Barriers in participant recruitment of diverse ethnicities in the state of Kuwait

Background

High rejection rates of subject recruitments for research studies have been reported in immigrants in many countries. However, the barriers in recruiting members of the expatriate population in Kuwait have not yet been investigated. This study was therefore designed to identify barriers in recruiting expatriates for research studies in the state of Kuwait.

Methods

A population-based cross-sectional study was conducted on expatriate subject’s aged 18 years and older living in Kuwait. Difference between groups of continuous independent variables was analyzed using the t-test. Different categories such as ethnicity and gender were compared using the chi-square test.

Results

3460 (85.1%) participants were recruited and 617 (14.2%) refused to participate in the study while 2530 (38%) calls were unreachable from the total of 6607 calls placed. Younger subjects (mean age 41.1 years) were more hesitant to be part of the study compared to older participants. The rejections among South Asians was (41.8%), Arabs (32.6%), Southeast Asians (18.9%) while the others (6.6%) category was least to refuse among all the nationalities. Gender was not significantly associated with refusal.

Conclusion

There is an acute lack of appropriate recording of the problems faced while recruiting the participants. The findings suggest important messages for the decision makers in the area of expatriate recruitments, to understand the challenge and design new strategies to overcome the problem of recruitment in the state of Kuwait for research studies.

Fetal growth and childhood acute lymphoblastic leukemia: findings from the childhood leukemia international consortium

Positive associations have been reported between the measures of accelerated fetal growth and risk of childhood acute lymphoblastic leukemia (ALL). We investigated this association by pooling individual-level data from 12 case-control studies participating in the Childhood Leukemia International Consortium. Two measures of fetal growth-weight-for-gestational-age and proportion of optimal birth weight (POBW)-were analysed. Study-specific odds ratios (ORs) and 95% confidence intervals (CIs) were estimated using multivariable logistic regression, and combined in fixed effects meta-analyses. Pooled analyses of all data were also undertaken using multivariable logistic regression. Subgroup analyses were undertaken when possible. Data on weight for gestational age were available for 7,348 cases and 12,489 controls from all 12 studies and POBW data were available for 1,680 cases and 3,139 controls from three studies. The summary ORs from the meta-analyses were 1.24 (95% CI: 1.13, 1.36) for children who were large for gestational age relative to appropriate for gestational age, and 1.16 (95% CI: 1.09, 1.24) for a one-standard deviation increase in POBW. The pooled analyses produced similar results. The summary and pooled ORs for small-for-gestational-age children were 0.83 (95% CI: 0.75, 0.92) and 0.86 (95% CI: 0.77, 0.95), respectively. Results were consistent across subgroups defined by sex, ethnicity and immunophenotype, and when the analysis was restricted to children who did not have high birth weight. The evidence that accelerated fetal growth is associated with a modest increased risk of childhood ALL is strong and consistent with known biological mechanisms involving insulin-like growth factors. © 2013 UICC.

Children's Oncology Group's 2013 blueprint for research: epidemiology

Investigators worldwide have for over 40 years conducted case-control studies aimed at determining the causes of childhood cancer. The central challenge to conducting such research is the rarity of childhood cancer, thus many studies aggregate cases through clinical trials organizations such as COG. Rarity also precludes the use of prospective study designs, which are less prone to recall and selection biases. Despite these challenges a substantial literature on childhood cancer etiology has emerged but few strong environmental risk factors have been identified. Genetic studies are thus now coming to the fore with some success. The ultimate aim of epidemiologic studies is to reduce the population burden of childhood cancer by suggesting preventive measures or possibly by enabling early detection. Pediatr Blood Cancer 2013; 60: 1059-1062. © 2012 Wiley Periodicals, Inc.

Risk of childhood acute lymphoblastic leukaemia following parental occupational exposure to extremely low frequency electromagnetic fields

Background:

Earlier studies have reported moderate increases in the risk of acute lymphoblastic leukaemia (ALL) among children whose mothers have been occupationally exposed to extremely low frequency (ELF) electromagnetic fields. Other studies examining parental occupational exposure to ELF and ALL have reported mixed results.

Methods:

In an Australian case–control study of ALL in children aged <15 years, parents were asked about tasks they undertook in each job. Exposure variables were created for any occupational exposure before the birth of the child, in jobs 2 years before birth, in jobs 1 year before birth and up to 1 year after birth.

Results:

In all, 379 case and 854 control mothers and 328 case and 748 control fathers completed an occupational history. Exposure to ELF in all time periods was similar in case and control mothers. There was no difference in exposure between case and control fathers. There was no association between maternal (odds ratio (OR)=0.96; 95% CI=0.74–1.25) or paternal (OR=0.78; 95% CI=0.56–1.09) exposure to ELF any time before the birth and risk of childhood ALL.

Conclusion:

We did not find an increased risk of ALL in offspring of parents with occupational exposure to ELF.

Maternal vitamin and iron supplementation and risk of infant leukaemia: a report from the Children's Oncology Group

Background:

Prenatal supplementation has been inversely associated with childhood, but not with infant, leukaemia.

Methods:

Mothers of 443 cases of infant leukaemia diagnosed during 1996–2006 and 324 frequency-matched controls completed interviews. Associations were evaluated by unconditional logistic regression.

Results:

We observed no associations between prenatal vitamin (odds ratio (OR)=0.79, 95% confidence interval (CI): 0.44–1.42) or iron supplementation (OR=1.07, 95% CI: 0.75–1.52) and infant leukaemia after adjustment for race/ethnicity and income. Similar results were observed for leukaemia subtypes analysed separately.

Conclusion

The observed null associations may be attributable to high supplementation rates and/or national fortification programmes.

Family characteristics as risk factors for childhood acute lymphoblastic leukemia: a population-based case-control study

Background

To date, few risk factors for childhood acute lymphoblastic leukemia (ALL) have been confirmed and the scientific literature is full of controversial “evidence.” We examined if family characteristics, particularly maternal and paternal age and number of older siblings, were risk factors for childhood acute lymphoblastic leukemia (ALL).

Methodology/Principal Findings

In this population-based nationwide matched case-control study, patients 0–14 years of age with ALL diagnosed 1991–2006 and registered in the Swiss Childhood Cancer Registry were linked with their census records of 1990 and 2000. Eight controls per case were selected from the census. The association between family characteristics and ALL was analyzed by conditional logistic regressions. We found that increasing maternal age was associated with incidence of ALL in the offspring (OR per 5-year increase in maternal age 1.18, 95% CI 1.05–1.31; p = 0.004), remaining stable (trend OR 1.14, 95% CI 0.99–1.31; p = 0.060) after adjustment for other risk factors. The association with paternal age was weaker (OR per 5-year increase 1.14, 95% CI 1.01–1.28, p = 0.032) and disappeared after adjustments. Number of older siblings was not associated with risk of ALL in the overall group of children aged 0–14 years at diagnosis. However, we found a negative trend between number of older siblings and ALL diagnosed at age 0–4 years (OR per sibling 0.85, 95% CI 0.68–1.06; p = 0.141) and a positive trend for ALL diagnosed at age 5–9 (OR 1.34, 95% CI 1.05–1.72; p = 0.019), with some evidence for an effect modification (p-value for interaction  = 0.040).

Conclusions

As in other studies, increasing maternal, but not paternal age was associated with risk of ALL. We found only a weak association with the number of older siblings, suggesting a delay in disease manifestation rather than a decrease in incidence.

The statistical power of epidemiological studies analyzing the relationship between exposure to ionizing radiation and cancer, with special reference to childhood leukemia and natural background radiation

The etiology of childhood leukemia remains generally unknown, although risk models based on the Japanese A-bomb survivors imply that the dose accumulated from protracted exposure to low-level natural background ionizing radiation materially raises the risk of leukemia in children. In this paper a novel Monte Carlo score-test methodology is used to assess the statistical power of cohort, ecological and case-control study designs, using the linear low-dose part of the BEIR V model derived from the Japanese data. With 10 (or 20) years of follow-up of childhood leukemias in Great Britain, giving about 4600 (or 9200) cases, under an individual-based cohort design there is 67.9% (or 90.9%) chance of detecting an excess (at 5% significance level, one-sided test); little difference is made by extreme heterogeneity in risk. For an ecological design these figures reduce to 57.9% (or 83.2%). Case-control studies with five controls per case achieve much of the power of a cohort design, 61.1% (or 86.0%). However, participation bias may seriously affect studies that require individual consent, and area-based studies are subject to severe interpretational problems. For this reason register-based studies, in particular those that make use of predicted doses that avoid the need for interviews, have considerable advantages. We argue that previous studies have been underpowered (all have power <80%), and some are also subject to unquantifiable biases and confounding. Sufficiently large studies should be capable of detecting the predicted risk attributable to natural background radiation.

Birth order and risk of non-hodgkin lymphoma--true association or bias?

There is inconsistent evidence that increasing birth order may be associated with risk of non-Hodgkin lymphoma (NHL). The authors examined the association between birth order and related variables and NHL risk in a pooled analysis (1983-2005) of 13,535 cases and 16,427 controls from 18 case-control studies within the International Lymphoma Epidemiology Consortium (InterLymph). Overall, the authors found no significant association between increasing birth order and risk of NHL (P-trend = 0.082) and significant heterogeneity. However, a significant association was present for a number of B- and T-cell NHL subtypes. There was considerable variation in the study-specific risks which was partly explained by study design and participant characteristics. In particular, a significant positive association was present in population-based studies, which had lower response rates in cases and controls, but not in hospital-based studies. A significant positive association was present in higher-socioeconomic-status (SES) participants only. Results were very similar for the related variable of sibship size. The known correlation of high birth order with low SES suggests that selection bias related to SES may be responsible for the association between birth order and NHL.

Representativeness of child controls recruited by random digit dialling

Recruiting control subjects who are representative of the population from which the cases are drawn is a challenge in case-control studies. This paper examines the performance of random digit dialling (RDD) in obtaining a control sample, and the sample's representativeness of the population with respect to socio-economic status. The study subjects were recruited from 2003 to 2006 for a national, population-based case-control study investigating causes of acute lymphoblastic leukaemia (ALL) in children <15 years of age in Australia. Control families' addresses were linked to Australian Bureau of Statistics Census 2006 Collection Districts and thus to Socio-Economic Indexes for Area scores, which are area-based measures of socio-economic status. These scores were compared with those of all collection districts where families lived. We estimate that 55% of eligible families in the RDD sample agreed to participate in the study. Participation was directly related to socio-economic status with those of highest economic status most likely to participate. Completeness of participation in the components of data collection was similarly related to socio-economic status. This evidence of selection according to socio-economic status indicates that there may also be selection with respect to other factors potentially important in the aetiology of ALL.

The association between atopy and childhood/adolescent leukemia: a meta-analysis

Atopic disease is hypothesized to be protective against several malignancies, including childhood/adolescent leukemia. To summarize the available epidemiologic evidence, the authors performed a meta-analysis of associations between atopy/allergies, asthma, eczema, hay fever, and hives and childhood/adolescent leukemia, acute lymphoblastic leukemia (ALL), and acute myeloid leukemia (AML). They searched MEDLINE literature (1952-March 2009) and queried international experts to identify eligible studies. Ten case-control studies were included. Summary odds ratios and 95% confidence intervals were computed via random-effects models. Odds ratios for atopy/allergies were 1.42 (95% confidence interval (CI): 0.60, 3.35) for 3 studies of leukemia overall, 0.69 (95% CI: 0.54, 0.89) for 6 studies of ALL, and 0.87 (95% CI: 0.62, 1.22) for 2 studies of AML, with high levels of heterogeneity detected for leukemia overall and ALL. Inverse associations were observed for ALL and asthma (odds ratio (OR) = 0.79, 95% CI: 0.61, 1.02), eczema (OR = 0.74, 95% CI: 0.58, 0.96), and hay fever (OR = 0.55, 95% CI: 0.46, 0.66) examined separately. Odds ratios for ALL differed by study design, exposure data source, and latency period, indicating that these factors affect study results. These results should be interpreted cautiously given the modest number of studies, substantial heterogeneity, and potential exposure misclassification but are useful in designing future research.

Fetal growth and risk of childhood acute lymphoblastic leukemia: results from an Australian case-control study

The relation between intrauterine growth and risk of childhood acute lymphoblastic leukemia was investigated in an Australian population-based case-control study that included 347 cases and 762 controls aged <15 years recruited from 2003 to 2006. Information on proportion of optimal birth weight, a measure of the appropriateness of fetal growth, was collected from mothers by questionnaire. Data were analyzed by using logistic regression. Risk of acute lymphoblastic leukemia was positively associated with proportion of optimal birth weight; the odds ratio for a 1-standard-deviation increase in proportion of optimal birth weight was 1.18 (95% confidence interval: 1.04, 1.35) after adjustment for the matching variables and potential confounders. This association was also present among children who did not have a high birth weight, suggesting that accelerated growth, rather than high birth weight per se, is associated with risk of acute lymphoblastic leukemia. Similar associations between proportion of optimal birth weight and acute lymphoblastic leukemia were observed for both sexes and across age groups and leukemia subtypes. Results of this study confirm earlier findings of a positive association between rapidity of fetal growth and subsequent risk of acute lymphoblastic leukemia in childhood, and they are consistent with a role for insulin-like growth factors in the causal pathway.

Host, family and community proxies for infections potentially associated with leukaemia

Three hypotheses have proposed the involvement of infections in the aetiology of childhood leukaemia, suggesting either a specific leukaemogenic infection or a series of common infections that lead to a dysregulation of the immune system. Much of the evidence for the link with infections has been based on epidemiological observations, often using proxy measures of infection. Proxy measures include population mixing, parental occupation, age distribution of incidence, spatial and space-time clustering of cases, birth order and day care during infancy. This paper discusses the proxies used and examines to what extent a commonly used proxy measure, birth order, is a fair representation of either specific infections or general infectious load. It is clear that although leukaemia, and other diseases, may be linked with infections, one needs to (1) measure specific and general infections with more accuracy and (2) understand how proxy measures relate to real infections in the population.

The UK Childhood Cancer Study: maternal occupational exposures and childhood leukaemia and lymphoma

Risks of childhood leukaemia and lymphoma were investigated for specific work-related exposures of mothers in the UK Childhood Cancer Study. Interviews with parents of 1881 leukaemia and lymphoma cases (0-14 years) and 3742 controls collected job histories recording exposure to eight specific agents. Exposure was (1) self-reported and (2) reviewed, based mainly on exposure probability and exposure level. Completeness, consistency and sufficiency evaluated data quality. Of all job exposures which were self-reported as exposed, 33% cases and 34% controls remained classified as exposed after review, with the remainder designated as partially exposed or unexposed. No review of underreporting of exposure was made. Data quality was 'good' for 26% of cases and 24% of controls. For self-reported exposure, significant risks of acute lymphoblastic leukaemia (ALL) were observed for solvents and petrol in all time windows. For reviewed exposure, solvents remained significant for ALL during pregnancy and postnatally. Restricting analyses to good-quality information removed all significant results. Refinement of exposure assessment revealed misclassification of self-reported exposures and data quality influenced risk assessment. Maternal exposure to solvents should further be investigated. These findings must invoke caution in the interpretation of risks reliant on self-reported occupational data.

Atopic dysfunction and risk of central nervous system tumours in children

Risk factors for central nervous system (CNS) tumours in children remain largely unknown. Evidence of an inverse relationship between atopy and tumour development exists in adults but little is known about childhood tumours. This study aims to examine the risk of childhood CNS tumours given a history of eczema and asthma. Cases of children diagnosed with CNS tumours (n=575) and controls (n=6292) from the UK Childhood Cancer Study (UKCCS) were analysed using conditional logistic regression comparing reported histories of allergic disease. Asthma was statistically significantly and negatively associated with all CNS tumours (odds ratios, OR 0.75, confidence of interval, CI(95%): 0.58-0.97), though this was not observed for eczema (OR 0.94, CI(95%): 0.74-1.18). Individuals who had suffered both asthma and eczema showed the most significant reduction in risk (OR 0.48, CI(95%): 0.28-0.81). Analysis by tumour subtype showed the strongest effect for the medulloblastoma/PNET group. These results may have a biological explanation with raised immunosurveillance in atopic individuals protecting against the development of brain tumours. Alternative explanations might include bias, reverse causality or confounding.

Participation Rates in Epidemiologic Studies

Participation rates for epidemiologic studies have been declining during the past 30 years with even steeper declines in recent years. This wholesale decrease in participation rate, or at the very least the increase in refusal, has, quite understandably, occasioned some concern among epidemiologists who have long considered a high study participation rate as one of the hallmarks of a "good" epidemiologic study. In this review we synthesize the issues that are central to epidemiologic thinking around declining study participation rates. We consider the reasons why study participation has been declining, summarize what we know about who does participate in epidemiologic studies, and discuss the implications of declining participation rates. We conclude with a discussion of methods that may help improve study participation rates.

Implications from epidemiologic studies on magnetic fields and the risk of childhood leukemia on protection guidelines

The objective of this review is to discuss the impact of findings in epidemiological studies on magnetic fields and the risk of childhood leukemia on the definition of exposure limits. A large number of epidemiological studies have consistently shown an association between the risk of childhood leukemia and residential extremely low-frequency magnetic field exposures. There is virtually no supportive data from experimental research and, so far, no proposed explanation has reached a level beyond speculation. The contradictory results from epidemiological and experimental research may either be due to methodological limitations creating a spurious association in the epidemiological studies or to a failure of experimental research to examine mechanisms relevant in the complex origin of childhood leukemia. Taking this together, the overall evidence is not strong enough to demand a revision of the current guidelines for public protection. Application of precautionary measures may be an option; however, decision-makers should be advised that these measures are often not straightforward and a careful evaluation of a possible benefit needs to be performed for each individual situation. Undoubtedly there are gaps in research, and no substantial contribution for clarification of the apparent inconsistencies emerges from recent studies. However, there are important lessons to learn, either with respect to the etiology of childhood leukemia or with respect to the need for improving epidemiological methods for the identification of presumably weak associations.

Medication use during pregnancy and the risk of childhood cancer in the offspring

The young age at onset of many cancers in childhood has led to investigations on maternal exposures during pregnancy. Data from a population-based case-control study in Germany (1992-1997) that included 1,867 cases and 2,057 controls was used to investigate this question. Maternal use of vitamin, folate or iron supplementation was associated with a reduced risk of non-Hodgkin lymphoma and tumors and, less clearly, with leukemia, but not with CNS tumors. An increased risk of neuroblastoma was associated most markedly with diuretics and other antihypertensives, but also with vitamin, folate or iron supplementation. No associations were seen with pain relievers, antinauseants or cold medications, nor with delivery by Caesarian section. The strengths of this study are its population base, the large number of cases and the inclusion of different case groups to identify disease specificity of associations. The limitation of this study is an exposure assessment relying on maternal self-reports. In conclusion, these data indicate a potential influence of some maternal medication during pregnancy on the risk of childhood cancer in the offspring; however, no clear picture is seen.

Breastfeeding and risk of childhood CNS tumours

We investigated infant feeding habits in relation to risk of childhood central nervous system tumours among 633 cases in the UK Childhood Cancer Study (UKCCS). No significant effect of breastfeeding was detected overall (odds ratio 1.01, confidence interval: 0.85-1.21) nor in any morphological subgroup. Similarly, no effect for the duration of breastfeeding or any other feeding practices was observed.

Variations in radiation exposures of adults and children in the UK

Members of the UK population receive radiation doses from a number of sources including cosmic radiation, from uranium, thorium and their decay products, particularly radon, and from medical sources. On average, members of the UK population receive an effective dose of about 200 mSv over their lifetime. This results in a risk of fatal cancer of about 1%. However, the radiation dose is not the same to all individuals. Some components give doses that vary systematically from one region to another. Doses may also vary greatly from one individual to another. The rate at which the dose is accumulated may vary as the individual ages. Different organs and tissues do not necessarily receive the same dose. This paper discusses these factors and attempts to quantify them. Cosmic rays deliver doses which vary little across the body or between individuals. Terrestrial gamma rays also deliver more or less uniform whole-body doses, but the difference between individuals can be greater. Radionuclides in food deliver doses which vary both across the body and between individuals. These variations are even more marked in the case of doses from radon and from medical exposures.

Childhood leukaemia and socioeconomic status: fact or artefact? A report from the United Kingdom childhood cancer study (UKCCS)

BACKGROUND

It is widely believed that children of high socioeconomic status (SES) are more likely than those of low SES to develop acute lymphoblastic leukaemia (ALL). Such observations have led to wide-ranging speculations about the potential aetiological role of factors associated with affluence and modernization.

METHODS

Children (0-14 years) newly diagnosed with cancer in the UK between 1991 and 1996 were ascertained via a rapid hospital-based case finding system (n = 4430, of which 1578 were ALL). Children without cancer (controls) were randomly selected from primary care population registries for comparative purposes (n = 7763). Area-based deprivation scores were assigned as markers of SES at two time points - birth and diagnosis. An individual-based marker of SES - social class - was assigned using father's occupation as recorded on the child's birth certificate.

RESULTS

No differences in area-based measures of deprivation were observed between cases and controls at time of diagnosis, either for all cancers combined [n = 4430, odds ratio (OR) = 1.00 (95% confidence intervals (CI) 0.98-1.01)] or for ALL alone (n = 1578 OR = 0.99, 95%CI 0.96-1.01). Findings were similar at time of birth (all cancers, OR = 0.99 95%CI 0.98-1.01, ALL OR = 0.98, 95%CI 0.96-1.00). In addition, no case-control differences were observed when an individual-based measure of SES - social class - based on father's occupation at time of birth was used.

CONCLUSIONS

The comprehensive nature of the data, coupled with complete case-ascertainment and representative population-based controls suggests that SES in the UK is not a determinant of ALL in children. We believe the small effects reported for SES in some past studies may be artefactual.

Mobile phone use and risk of glioma in adults: case-control study

OBJECTIVE

To investigate the risk of glioma in adults in relation to mobile phone use.

DESIGN

Population based case-control study with collection of personal interview data.

SETTING

Five areas of the United Kingdom.

PARTICIPANTS

966 people aged 18 to 69 years diagnosed with a glioma from 1 December 2000 to 29 February 2004 and 1716 controls randomly selected from general practitioner lists.

MAIN OUTCOME MEASURES

Odds ratios for risk of glioma in relation to mobile phone use.

RESULTS

The overall odds ratio for regular phone use was 0.94 (95% confidence interval 0.78 to 1.13). There was no relation for risk of glioma and time since first use, lifetime years of use, and cumulative number of calls and hours of use. A significant excess risk for reported phone use ipsilateral to the tumour (1.24, 1.02 to 1.52) was paralleled by a significant reduction in risk (0.75, 0.61 to 0.93) for contralateral use.

CONCLUSIONS

Use of a mobile phone, either in the short or medium term, is not associated with an increased risk of glioma. This is consistent with most but not all published studies. The complementary positive and negative risks associated with ipsilateral and contralateral use of the phone in relation to the side of the tumour might be due to recall bias.

Childhood Cancer and Social Contact: the Role of Paternal Occupation (United Kingdom)*

OBJECTIVE

To investigate the relationship between childhood cancer (particularly leukaemia) and paternal occupational social contact (a proxy for potential exposure to infections) using the UK Childhood Cancer Study.

METHODS

Using a national population-based case-control study, self-reported occupational data from fathers of 3596 children diagnosed with cancer between 1991 and 1996 under 15 years of age (cases) and fathers of 7011 children without cancer (controls) were analysed. Associations were assessed using odds ratios (OR) calculated by time of exposure (birth, diagnosis), diagnostic group (all cancers, leukaemia, acute lymphoblastic leukaemia (ALL), central nervous system tumours, other cancers), level of occupational social contact (high, medium, low), urban-rural status at diagnosis and occupational title.

RESULTS

From 371 occupations, 75 (20%) were classified as having high levels of occupational social contact, 31 (8%) as medium and the remaining 265 (71%) as low. No associations were apparent for fathers' occupational social contact overall, for any time period or diagnostic group. OR for leukaemia and high levels of paternal occupational social contact at birth and diagnosis being 1.02 (95% confidence interval (CI) = 0.88-1.18) and 0.91 (95% CI = 0.79-1.06), respectively. Analyses by urban-rural status at diagnosis and by occupational title revealed no notable associations.

CONCLUSIONS

The analyses of paternal occupational histories do not support the suggested association between high levels of paternal occupational social contact and an increased risk of childhood leukaemia. However, the role of participation bias should not be discounted.

Day care in infancy and risk of childhood acute lymphoblastic leukaemia: findings from UK case-control study

OBJECTIVE

To test the hypothesis that reduced exposure to common infections in the first year of life increases the risk of developing acute lymphoblastic leukaemia. Design and setting The United Kingdom childhood cancer study (UKCCS) is a large population based case-control study of childhood cancer across 10 regions of the UK.

PARTICIPANTS

6305 children (aged 2-14 years) without cancer; 3140 children with cancer (diagnosed 1991-6), of whom 1286 had acute lymphoblastic leukaemia (ALL).

MAIN OUTCOME MEASURE

Day care and social activity during the first year of life were used as proxies for potential exposure to infection in infancy.

RESULTS

Increasing levels of social activity were associated with consistent reductions in risk of ALL; a dose-response trend was seen. When children whose mothers reported no regular activity outside the family were used as the reference group, odds ratios for increasing levels of activity were 0.73 (95% confidence interval 0.62 to 0.87) for any social activity, 0.62 (0.51 to 0.75) for regular day care outside the home, and 0.48 (0.37 to 0.62) for formal day care (attendance at facility with at least four children at least twice a week) (P value for trend < 0.001). Although not as striking, results for non-ALL malignancies showed a similar pattern (P value for trend < 0.001). When children with non-ALL malignancies were taken as the reference group, a significant protective effect for ALL was seen only for formal day care (odds ratio = 0.69, 0.51 to 0.93; P = 0.02). Similar results were obtained for B cell precursor common ALL and other subgroups, as well as for cases diagnosed above and below age 5 years.

CONCLUSION

These results support the hypothesis that reduced exposure to infection in the first few months of life increases the risk of developing acute lymphoblastic leukaemia.

Perinatal and reproductive factors: a report on haematological malignancies from the UKCCS

The United Kingdom Childhood Cancer Study was designed to examine the potential aetiological role of a range of perinatal and reproductive factors. Our use of clinical records permitted a more exact characterisation of reproductive events than is possible in investigations that rely on self-reporting; and the increased specificity with which antecedent events were measured produced more precise risk estimates, albeit ones based on progressively smaller numbers. Information on the conduct of this component of the study and results for 1485 children with haematological malignancies and 4864 controls are presented. The 'find' rate for obstetric records was high at 86% for cases, with 81% having information on both matched controls. Associations were seen for severe hyperemesis (Odds Ratio=3.6, 95%Confidence Interval=1.3-10.1, for all leukaemias), polyhydramnios (OR=4.0, 95%CI=1.5-10.3, for acute myeloid leukaemia (AML)), anaemia (haemoglobin <10 g, OR=2.6, 95%CI=1.7-4.1, for AML), and pre-eclampsia (OR=1.7, 95%CI=1.1-2.7, for non-Hodgkin's lymphoma). Babies who developed leukaemia were heavier at birth (>4000 g, OR=1.2, 95%CI=1.0-1.4), as were their older siblings (>4000 g, OR=1.4, 95%1.0-1.9). Mothers' whose children developed common B-cell precursor acute lymphoblastic leukaemia (ALL) were more likely to have had a previous molar pregnancy (OR=5.2, 95%CI=1.9-14.7). Gender-specific analysis revealed that findings often differed markedly for boys and girls; and, in common with other reports, strong associations with Down's syndrome were seen for both ALL and AML.

Epidemiology and genetics of childhood cancer

Childhood cancer is rare everywhere in the world, with age-standardized annual incidence usually between 70 and 160 per million at age 0-14 years. Greater variation is seen between populations for some specific tumour types. Some of the largest variations are geographical and are attributable to environmental factors, whereas variation mainly on ethnic lines seems likely to be a marker of genetic predisposition. A wide range of familial and genetic syndromes is associated with an increased risk of childhood cancer. Virtually all the excess risk of cancer among first-degree relatives of children with cancer can be accounted for by known hereditary syndromes. Studies of weak predisposition and gene-environment interaction have so far shown limited consistency. There are very few established environmental or exogenous risk factors and most of these are infective agents. Many putative risk factors can hardly ever be investigated epidemiologically except by interview or questionnaire studies. Some recent examples illustrate the continuing problems of participation bias and recall bias.

Parental occupation at periconception: findings from the United Kingdom Childhood Cancer Study

AIMS

To study the risk of childhood cancer in relation to parental occupation and related exposures.

METHODS

Self reported occupational data from mothers and fathers of 3838 children with cancer and 7629 control children were analysed. Odds ratios were calculated for 31 "occupational groups" by parent, diagnostic group (leukaemia, acute lymphoblastic leukaemia (ALL), central nervous system tumours, and other cancers) and time of exposure (periconception, birth, and diagnosis).

RESULTS

Findings did not support the hypothesis that occupational exposure of fathers to ionising radiation increases the risk of childhood cancer in their offspring. Specific examination of periconceptual chemical exposures showed small but statistically significant increased risks for leukaemia and ALL among children whose fathers were exposed to exhaust fumes, driving, and/or inhaled particulate hydrocarbons. In the remaining analyses, a fourfold increase in the risk of other cancers was observed among the children of fathers working with leather but based on small numbers. Both maternal and paternal exposure to textile dust was related to an increased risk of other cancers.

CONCLUSION

Results failed to produce any strong evidence to link parental occupational exposures with an increased risk of childhood cancer. No relation was found for paternal periconceptual exposure to ionising radiation. The consistency of the associations observed between childhood leukaemia and paternal occupational exposure to exhaust fumes, driving, and/or inhaled particulate hydrocarbons at periconception suggest a small risk for vehicle related exhaust. However, other explanations cannot be excluded and further research into the nature of the associations is required.

An epidemiologic study of index and family infectious mononucleosis and adult Hodgkin's disease (HD): evidence for a specific association with EBV+ve HD in young adults

Infectious mononucleosis (IM) is an established risk factor for Hodgkin's disease (HD). A substantial minority (33%) of cases of HD have Epstein-Barr virus (EBV) DNA within the malignant cells (are EBV+ve). It is unclear whether risk after IM applies specifically to EBV+ve HD. We report the results of a population-based case-control study of HD in adults (n = 408 cases of classical HD, 513 controls) aged 16-74 years; the case series included 113 EBV+ve and 243 EBV+ve HD. Analyses compared total HD, EBV+ve HD and EBV-ve HD with the controls and EBV+ve HD with EBV-ve HD cases using, mainly, logistic regression. Regression analyses were adjusted for gender, age-group and socioeconomic status, and were performed for the whole age range and separately for young (< 35 years) and old adults (> or = 35 years); formal tests of effect modification by age were included. For the young adults, reported IM in index or relative was strongly and significantly associated with EBV+ve HD when compared to controls (odds ratio [OR] = 2.94, 95% confidence interval [CI]: 1.08-7.98 and OR = 5.22, 95% CI: 2.15-12.68, respectively). These results may be interpreted as indications that late first exposure to EBV increases risk of HD, especially in young adults; this applies primarily to EBV+ve HD.

Parental smoking and childhood cancer: results from the United Kingdom Childhood Cancer Study

There are strong a priori reasons for considering parental smoking behaviour as a risk factor for childhood cancer but case - control studies have found relative risks of mostly only just above one. To investigate this further, self-reported smoking habits in parents of 3838 children with cancer and 7629 control children included in the United Kingdom Childhood Cancer Study (UKCCS) were analysed. Separate analyses were performed for four major groups (leukaemia, lymphoma, central nervous system tumours and other solid tumours) and more detailed diagnostic subgroups by logistic regression. In the four major groups, after adjustment for parental age and deprivation there were nonsignificant trends of increasing risk with number of cigarettes smoked for paternal preconception smoking and nonsignificant trends of decreasing risk for maternal preconception smoking (all P-values for trend >0.05). Among the diagnostic subgroups, a statistically significant increased risk of developing hepatoblastoma was found in children whose mothers smoked preconceptionally (OR=2.68, P=0.02) and strongest (relative to neither parent smoking) for both parents smoking (OR=4.74, P=0.003). This could be a chance result arising from multiple subgroup analysis. Statistically significant negative trends were found for maternal smoking during pregnancy for all diagnoses together (P<0.001) and for most individual groups, but there was evidence of under-reporting of smoking by case mothers. In conclusion, the UKCCS does not provide significant evidence that parental smoking is a risk factor for any of the major groups of childhood cancers.