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

How the effect of maternal age on the risk of childhood leukemia changed over time in Sweden, 1960-2004

BACKGROUND

Previous studies on the association between maternal age and risk of childhood leukemia found inconsistent results.

OBJECTIVES

We aimed to assess whether there is an association between maternal age and risk of childhood leukemia and whether such an association is modified by maternal year of birth.

METHODS

By linking nationwide Swedish registers, we analyzed leukemia incidence among all children between 1 and 5 years of age born between 1960 and 1999. We estimated incidence time trends by child year of birth (overall and stratified by maternal age) and incidence rate ratios (RRs) for maternal age groups stratified by maternal birth cohort. We tested the interaction between maternal age and child year of birth through the likelihood ratio test between nested Poisson regression models.

RESULTS

We observed 1,562 leukemia cases. The overall annual percent change (APC) was 1.00 [95% confidence interval (CI), 0.51 to 1.49]. Stratifying by maternal age classes, APCs decreased from 1.66 (0.68 to 2.65) for mothers <or= 24 years to 0.23 (-0.93 to 1.40) for mothers >or= 35 years at delivery. RRs for children born to the oldest with respect to the youngest mothers were 2.42 (1.31 to 4.67), 1.68 (1.00 to 2.72), 1.34 (0.87 to 2.01), and 0.87 (0.46-1.54) for mothers born in 1930-1934, 1940-1944, 1950-1954, and 1960-1964, respectively.

CONCLUSIONS

Childhood leukemia risk increased with maternal age for mothers born in the past, whereas maternal age had no effect on this risk for mothers born more recently. This finding may explain the inconsistency of previous studies and suggests that leukemia risk may be related to an environmental factor to which women's exposure has changed over time.

Childhood leukaemia following medical diagnostic exposure to ionizing radiation in utero or after birth

A statistical association between childhood leukaemia and an abdominal X-ray examination of the pregnant mother was first reported in 1956 from a case-control study of childhood cancer mortality conducted in Great Britain. This study, later called the Oxford Survey of Childhood Cancers (OSCC), was continued and eventually showed a highly statistically significant approximately 50% proportional increase in the risk of childhood leukaemia associated with antenatal diagnostic radiography. The association has been confirmed by many case-control studies carried out around the world, the appropriately combined results of which show a highly statistically significant increase in risk that is compatible with the OSCC finding. There is no doubt about the reality of the statistical association, but a causal interpretation has been questioned. On balance, however, the evidence points to low-level irradiation of the fetus increasing the risk of leukaemia in childhood, with an excess relative risk coefficient of around 50 Gy(-1) (equivalent to an excess absolute risk coefficient of about 3% Gy(-1)), although the uncertainty associated with these coefficients is considerable and they are likely to be overestimates. In contrast to exposure in utero, the evidence from case-control studies for an association between childhood leukaemia and postnatal exposure to medical diagnostic irradiation is equivocal and sometimes conflicting. Since standard radiation risk models predict that low-level exposure in the early years of life should produce an increased risk of childhood leukaemia that is roughly similar to that arising from fetal exposure, this absence of persuasive evidence is likely to be due to various problems with the studies. This is unfortunate given the rise in relatively high dose diagnostic procedures (e.g. paediatric CT scans) that would be predicted to materially increase the relative risk of childhood leukaemia.

Reduced occurrence of childhood cancer in twins compared to singletons: protection but by what mechanism?

BACKGROUND

Several small studies combined have suggested that twins develop fewer childhood cancers than singletons. The national Swedish Family-Cancer Database contains information on a large population of multiple births providing an unbiased dataset for the estimation of cancer risk in twins. Lifelong cancer incidence in these twins has already been reported as similar to that in singleton births. In contrast, the present paper presents robust estimates of a significantly reduced childhood cancer risk in twins to age 15.

METHODS

Standardised incidence ratios (SIR) were used to measure cancer risk for twins, taking the corresponding rates for singletons as reference. Rates were adjusted for age, sex, period of birth, and residential area. Follow up data cover the period 1958-2002.

RESULTS

Overall childhood cancer risk was significantly reduced in all twins (SIR 0.81 [95% CI: 0.69-0.94]) as was the risk for Wilms tumour (SIR 0.34 [95% CI: 0.09-0.88]). These significant reductions in risk were both driven by effects in same sex twins (overall cancer SIR 0.77 [95% CI: 0.64-0.93], Wilms tumour 0.12 [95% CI: 0.00-0.71]). Leukaemia risk was also significantly reduced for same sex twins (SIR 0.69 [95% CI: 0.47-0.97]).

CONCLUSIONS

Our study provides the evidence that twins experience less childhood cancer than singletons. The risk reduction is most marked for Wilms tumour but may, to a varying extent, be true for a number of childhood neoplasms.

Fetal growth and the risk of childhood CNS tumors and lymphomas in Western Australia

The etiology of childhood cancers is largely unknown, although the early age at diagnosis has led to particular interest in in utero and perinatal factors. Birth weight is the most frequently studied perinatal factor in relation to risk of childhood cancers, and results have been inconsistent. We investigated whether the risk of CNS tumors and lymphomas in children was associated with three measures of the appropriateness of intra-uterine growth: proportion of optimal birth weight (POBW), birth length (POBL) and weight for length (POWFL). A cohort of 576,633 infants born in Western Australia in 1980-2004 were followed from birth to diagnosis of a CNS tumor (n = 183) or lymphoma (n = 84) before age 15, death, or December 31, 2005, and analyzed with Cox regression. Overall, there was little evidence of any association between fetal growth and risk of CNS tumors, although risk of ependymoma/choroid plexus tumors was positively associated with POBL and negatively associated with POWFL. The risk of Hodgkin and Burkitt lymphoma increased with increasing fetal growth among boys only, whereas the increased risk observed with non-Hodgkin lymphoma was only in girls. These associations between fetal growth and disease risk were also observed among children not classified as high birth weight, suggesting that accelerated growth is more important than birth weight per se. Results were similar when cases were compared with their unaffected siblings, suggesting that the increased growth associated with cancer risk was not general to the family. The associations we observed are consistent with causal pathways involving fetal growth factors.

High birth weight as an important risk factor for infant leukemia

In this paper, we compared the birth weight distribution among 201 infant leukaemia (IL) cases with that of 440 noncancer controls enrolled in Brazil in 1999–2005. Compared with the general population and the stratum 2500–2999 g as reference, IL cases weighing 3000–3999 g presented an odds ratio (OR) of 1.68 (95% CI: 1.03–2.76), and those of 4000 g or more, an OR of 2.28 (95% CI: 1.08–4.75), P_trend<0.01. Using hospital-based controls, the OR for 4000 g or more, compared to 2500–2999 g, was 1.30 (95% CI: 1.02–1.43) after adjusting for confounders (gender, income, maternal age, pesticide and hormonal exposure during pregnancy). The results suggest that high birth weight is associated with increased risk of IL.

Reproductive history, infertility treatment, and the risk of acute leukemia in children with down syndrome: a report from the Children's Oncology Group

BACKGROUND

Children with Down syndrome (DS) have from 10 to 20 times the risk of developing acute leukemia than the general pediatric population. There is mixed evidence for associations between reproductive history or infertility and acute leukemia among children without DS.

METHODS

The authors conducted a case-control study of acute leukemia among children with DS to investigate possible risk factors in this population. From 1997 to 2002, 158 children aged <20 years with DS who had a diagnosis of acute leukemia (97 children with acute lymphoblastic leukemia [ALL] and 61 children with acute myeloid leukemia [AML]) were enrolled at Children's Oncology Group (COG) institutions. Controls with DS (n = 173) were selected from the cases' primary care clinic and frequency matched to cases on age. Telephone interviews were conducted with mothers of cases and controls assessing reproductive history, infertility, and infertility treatment.

RESULTS

Null results were observed overall and by subtype for reproductive factors, including previous pregnancy outcomes and contraceptive use, and for most infertility outcomes. There was an increased risk of AML among children with DS whose parents had ever tried for >/=1 year to become pregnant (odds ratio [OR], 2.22; 95% confidence interval [95% CI], 1.14-4.33). A 1-year increase in maternal age also was associated with AML (OR, 1.06; 95% CI, 1.01-1.12).

CONCLUSIONS

Although the questionnaire was limited in this area, the results suggested that the risk for AML may be raised in children with DS because of infertility. In that the risk of infertility, along with having a child with DS, increase with age, these results warrant more research.

Childhood leukaemia and infectious exposure: a report from the United Kingdom Childhood Cancer Study (UKCCS)

Data from a national case-control study are used to explore the relationships between childhood leukaemia, infant infection and three markers of infectious exposure - birth order, infant-activity group attendance and area-based deprivation. Amongst controls, clinically diagnosed infection in the first year varied little with birth order and infant-activity group attendance - with 4 in 5 children having at least one infection, and each child averaging around 2.9 (2.8-3.0). Amongst cases of acute lymphoblastic leukaemia (ALL), the levels of infection increased as the indices of infectious exposure increased - for example, odds ratios associated with at least one infection in the first year being 0.9 (95% confidence interval (CI): 0.6-1.4) for birth order one and 1.6 (95% CI: 1.1-2.2) for birth order two or more. By contrast, interview data were misleading, with mothers - particularly case mothers - consistently under-reporting. We conclude that the findings based on clinical data, combined with the markers of infectious exposure, confirm the observation that immune dysregulation among children who develop ALL is detectable from an early age.

Molar pregnancy and childhood cancer: a population-based linkage study from Denmark

We observed a relative risk of 1.40 (95% confidence interval; 0.86-2.16) for cancers diagnosed under the age 20 in 6192 offspring of 3431 mothers with a molar pregnancy, indicating it is not a major determinant of childhood cancer.

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.

Effects of maternal age and cohort of birth on incidence time trends of childhood acute lymphoblastic leukemia

Several studies report increasing trends in the incidence of childhood acute lymphoblastic leukemia (ALL). Because ALL may generate in utero, this study investigated if maternal age and birth cohort influence ALL temporal trends. Data on 252 ALL cases in children ages 1 to 5 years were extracted from the population-based Childhood Cancer Registry of Piedmont, Italy. Information on cases' maternal age and year of birth was obtained from the registry, whereas population data were obtained for children born in 1980 to 1997. Incidence rates were analyzed using an age-period-cohort approach, in which the period effect was represented by the child year of birth, the age effect by the maternal age at the time of delivery, and the cohort effect by the maternal birth cohort. ALL incidence increased over the study period [annual percentage change 2.49%; 95% confidence interval (95% CI), 0.09-4.93]. A linear effect of the maternal time variables (P = 0.012) was found, which was equally described by maternal age (direct association) and maternal birth cohort (inverse association). The annual percentage change was 1.83% (95% CI, -0.59-4.31), when maternal age was included in the model, and 5.72% (95% CI, 2.29-9.27), when maternal year of birth was included. In conclusion, maternal characteristics substantially affect temporal trends in childhood ALL incidence.

Examination of gender effect in birth weight and miscarriage associations with childhood cancer (United Kingdom)

BACKGROUND

Higher birth weight and maternal history of miscarriage has been associated with an increased risk of childhood leukemia. The possibility that this association may be sex-specific has not been explored in detail in previous studies.

METHODS

In a retrospective case-control study, 732 childhood (< or =14 years) cancer cases from a population-based Registry in Northern England whose hospital birth records could be accessed and 3,723 controls matched for date and hospital of birth to the cases were compared. We examined birth weight for sex-specific associations with childhood cancer. Conditional logistic regression analysis was used for statistical evaluation of associations.

RESULTS

In acute lymphoblastic leukemia (ALL) (225 cases and 1,163 matched controls), birth weight and sex showed a strong interaction (P = 0.003). In boys with ALL, but not in girls, there was a nonlinear association with birth weight (P for trend = 0.008; OR = 3.05 for the highest quintile compared to the second lowest quintile, 95% CI = 1.40-6.64; P = 0.005). When birth weights were adjusted using UK standards for gestational age and sex, the risk associations were similar in statistical significance and magnitude. Maternal history of miscarriage showed an association with all cancers and individually with ALL. The miscarriage association with ALL was statistically significant in boys only (OR = 1.91, 95% CI = 1.07-3.42; P = 0.03). A multivariable model for ALL containing other examined maternal and reproductive variables confirmed the independence of the birth weight and miscarriage associations. There was no birth weight or miscarriage associations in other cancers.

CONCLUSIONS

This study confirmed the risk associations with birth weight and miscarriages in childhood ALL. Statistically significant association of size at birth suggested marked differences in etiology between girls and boys.

Maternal hemoglobin concentration during pregnancy and risk of infant leukaemia: a children's oncology group study

In contrast to the positive association found in three studies between maternal anaemia during pregnancy and childhood leukaemia, no such association was found in infant leukaemia (odds ratio 0.85, 95% confidence interval 0.53–1.37).

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.

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.

Relationships between perinatal and maternal characteristics and hepatoblastoma: a report from the UKCCS

Within the context of a national population-based case-control study--the United Kingdom Childhood Cancer Study (UKCCS)--we aimed to explore relationships between perinatal and maternal factors and childhood hepatic tumours, for participants with data available from medical records. 26/28 children with hepatic tumours (22/24 hepatoblastomas, 4/4 hepatocellular carcinomas (HCC)) and 4753 age- and sex-matched controls were included. Polyhydramnios was associated with 0.9% of control pregnancies and 13.6% of case pregnancies (Odds Ratio (OR)=28.64, 95% Confidence Interval (CI)=6.94-118.21, P<0.0001); eclampsia or severe pre-eclampsia complicated the pregnancies of 16.7% of mothers whose children developed hepatoblastoma compared with 0.5% of control pregnancies (OR=52.50, 95% CI=10.75-257.05, P<0.0001). Three children with hepatoblastoma weighed <1500 g at birth, two of whom weighed <1000 g (OR for birthweight <1500 g=69.00, 95% CI=11.98-397.17, P<0.0001). Of children with hepatoblastoma, 50% (11/22) had records of congenital anomalies, as did two of their mothers. Three mothers of children with hepatoblastoma had diagnoses of cancer--two of papillary carcinoma of the thyroid and one of acute lymphoblastic leukaemia (ALL). Paediatricians and others should be alert to the possibility of familial or genetic syndromes in children with hepatoblastomas. Potential links between maternal pre-eclampsia, low birthweight and subsequent malignancy merit further investigation. Hepatoblastoma is an extremely rare childhood tumour, but understanding the mechanism(s) underlying severe pre-eclampsia and eclampsia may also shed light on factors that contribute to the development of hepatoblastoma.