On pre- or postnatal diagnostic X-rays as a risk factor for childhood leukaemia

A review of the types of childhood cancer associated with a medical X-ray examination of the pregnant mother

PURPOSE

For 65 years the interpretation of the statistical association between the risk of cancer in a child and a prior diagnostic X-ray examination of the abdomen of the pregnant mother has been debated. The objections to a direct cause-and-effect explanation of the association vary in their strength, but one of the most notable grounds for controversy is the finding from the first and largest case-control study reporting the association, the Oxford Survey of Childhood Cancers (OSCC), of an almost uniformly raised relative risk (RR) for nearly all of the types of cancer that are most frequent in children. Here we compare the antenatal X-ray associations found in the OSCC for different types of childhood cancer with the results of all other case-control and case-cohort studies appropriately combined in meta-analyses, and we also review the findings of the few cohort studies that have been conducted.

CONCLUSIONS

From the case-control/case-cohort studies other than the OSCC there are consistent and clear elevations of risk for all types of childhood cancer combined, all leukemia, and all cancers except leukemia combined. This compatibility of the findings of the OSCC with those of the combined other studies is less clear, or effectively absent, when some categories containing smaller numbers of incident cases/deaths are considered, but lack of precision of risk estimates due to sparse data presents inferential challenges, although there is a consistent absence of an association for bone tumors. Further, more recent studies almost certainly address lower intrauterine doses, with an anticipated decrease in estimated risks, which could be misleading when comparisons involve a limited number of studies that are mainly from later years, and a similar problem arises when having to employ all types of antenatal X-ray exposures for a study because data for abdominal exposures are absent. The problem of low statistical power is greater for cohort studies, and this, together with other shortcomings identified in the studies, limits the interpretational value of results. The findings of non-medical intrauterine exposure studies are constrained by sparse data and make a limited contribution to an understanding of the association. Certain aspects of the various studies require a need for caution in interpretation, but overall, the appropriate combination of all case-control/case-cohort studies other than the OSCC lends support to the inference that low-level exposure to radiation in utero proportionally increases the risk of the typical cancers of childhood to around the same level.

Risk of Hematologic Malignancies in the Offspring of Female Workers of the Mayak Nuclear Facility in the Southern Urals, Russian Federation

Long-term effects of in utero exposure to ionizing radiation remain poorly quantified in humans. In this study, the risk of hematologic malignancies was investigated in offspring of female workers of the Mayak Production Association, a large Russian nuclear facility. Excess relative risks (ERR) for exposure to gamma radiation and plutonium were estimated in a cohort of 8,466 offspring who were born between January 1, 1948 and December 31, 1988 and followed until 2009. An unstable linear ERR of 1.12 (95% CI 0.11-3.44) per 100 mGy gamma exposure in utero was estimated based on 32 incident hematologic malignancies in 277,002 person-years under risk. The ERR was increased in the dose category 20-79 mGy gamma exposure in utero (1.75, 95% CI 0.04; 5.63), while the other dose categories showed decreased or unstable estimates. Leukemia showed an ERR of 1.76 (95% CI 0.01-8.33) per 100 mGy based on 13 cases. There was no consistent association with plutonium exposure. While an increased risk of hematologic malignancies after gamma exposure in utero was suggested, the small numbers prevented more definitive conclusions.

The risk of childhood leukaemia following exposure to ionising radiation--a review

Since the early years of follow-up of the Japanese atomic-bomb survivors, it has been apparent that childhood leukaemia has a particular sensitivity to induction by ionising radiation, the excess relative risk (ERR) being expressed as a temporal wave with time since exposure. This pattern has been generally confirmed by studies of children treated with radiotherapy. Case-control studies of childhood leukaemia and antenatal exposure to diagnostic x-rays, a recent large cohort study of leukaemia following CT examinations of young people, and a recent large case-control study of natural background γ-radiation and childhood leukaemia have found evidence of raised risks following low-level exposure. These findings indicate that an ERR/Sv for childhood leukaemia of ~50, which may be derived from risk models based upon the Japanese atomic-bomb survivors, is broadly applicable to low dose or low dose-rate exposure circumstances.

Childhood cancer risk from conventional radiographic examinations for selected referral criteria: results from a large cohort study

OBJECTIVE

Little is known about the long-term effects of exposure to diagnostic ionizing radiation in childhood. Current estimates are made with models derived mainly from studies of atomic bomb survivors, a population that differs from today's patients in many respects.

MATERIALS AND METHODS

We analyzed the cancer incidence among children who underwent diagnostic x-ray exposures between 1976 and 2003 in a large German university hospital. We reconstructed individual radiation doses for each examination and sorted results by groups of referral criteria for all cancers combined, solid tumors, and leukemia and lymphoma combined.

RESULTS

A total of 68 incidence cancer cases between 1980 and 2006 were identified in a 78,527-patient cohort in the German childhood cancer registry: 28 leukemia, nine lymphoma, six tumors of the CNS, and 25 other tumors. The standardized incidence ratio for all cancers was 0.97 (95% CI, 0.75-1.23). Dose-response relations were analyzed by multivariable Poisson regression. Although the cancer incidence risk differed by initial referral criterion for radiographic examination, a positive dose-response relation was observed in five patients with endocrine or metabolic disease.

CONCLUSION

Overall, we observed no increase in cancer risk among children and youths with very low radiation doses from diagnostic radiation, which is compatible with model calculations. The growing use of CT warrants further studies to assess associated cancer risk. Our work is an early contribution of epidemiologic data for quantifying these risks among young patients.

Lauriston S. Taylor lecture: radiation epidemiology--the golden age and future challenges

Epidemiology is the study of the distribution and causes of disease in humans. Studies of human populations exposed to ionizing radiation have been conducted for nearly 100 y during the "Golden Age of Radiation Epidemiology." Radiation epidemiology is now so sophisticated that human studies are the basis for radiation protection standards and for compensation schemes in response to claims of ill health from prior exposures. The studies of exposed human populations are very broad and include not only the Japanese atomic bomb survivors, but also patients given radiotherapy for cancer, patients treated with radiation for nonmalignant disease, patients given diagnostic radiation, persons with intakes of radionuclides, workers exposed to occupational radiation, and communities exposed to environmental sources of radiation. But there is more to be learned, and future knowledge may be advanced from new and continued occupational studies of the early radiation workers, atomic veterans, medically exposed patients, and populations living in areas of high natural background radiation. The interaction between radiation and underlying genetic susceptibilities is an important emerging area of research. It is indeed an honor to be included among the Lauriston S. Taylor Lecturers.

Diagnostic X-rays and risk of childhood leukaemia

BACKGROUND

The association between diagnostic X-ray exposures early in life and increased risk of childhood leukaemia remains unclear.

METHODS

This case-control study included children aged 0-14 years diagnosed with acute lymphoid leukaemia (ALL, n = 711) or acute myeloid leukaemia (AML, n = 116) from 1995 to 2008. Controls were randomly selected from the California birth registry and individually matched to cases with respect to date of birth, sex, Hispanic ethnicity and maternal race. Conditional logistic regression analyses were performed to assess whether ALL or AML was associated with self-reported child's X-rays after birth (post-natal), including number of X-rays, region of the body X-rayed and age at first X-ray, as well as maternal X-rays before and during pregnancy (preconception and prenatal).

RESULTS

After excluding X-rays in the year prior to diagnosis (reference date for matched controls), risk of ALL was elevated in children exposed to three or more post-natal X-rays [odds ratio (OR) = 1.85, 95% confidence interval (CI) 1.12-2.79]. For B-cell ALL specifically, any exposure (one or more X-rays) conferred increased risk (OR = 1.40, 95% CI 1.06-1.86). Region of the body exposed was not an independent risk factor in multivariable analyses. No associations were observed between number of post-natal X-rays and AML (OR = 1.05, 95% CI 0.90-1.22) or T-cell ALL (OR = 0.84, 95% CI 0.59-1.19). Prevalence of exposure to prenatal and preconception X-rays was low, and no associations with ALL or AML were observed.

CONCLUSIONS

The results suggest that exposure to post-natal diagnostic X-rays is associated with increased risk of childhood ALL, specifically B-cell ALL, but not AML or T-cell ALL. Given the imprecise measures of self-reported X-ray exposure, the results of this analysis should be interpreted with caution and warrant further investigation.

Radiodiagnostic imaging in pregnancy and the risk of childhood malignancy: raising the bar

Eduardo Franco and Guy-Anne Turgeon discuss new findings from Joel Ray and colleagues on the cancer risk following prenatal exposure to radiodiagnostic imaging, and where new research needs to be focused.