Risk coefficients for childhood cancer after intrauterine irradiation: a review.

Purpose: To review the estimates of the risk of childhood cancer
per unit dose of radiation received in utero derived from the largest
case-control study of obstetric X-ray examinations and to
compare them with the childhood cancer risk coefficients
obtained from the cohorts of Japanese atomic bomb survivors
irradiated either in utero or as young children.
Materials and methods: Data from the Oxford Survey of Childhood
Cancers (OSCC) case-control study of foetal exposure to diagnostic X-rays and from the cohort studies of the Japanese
survivors of the atomic bombings of Hiroshima and Nagasaki
were used, together with associated dose estimates. Excess relative
risk and excess absolute risk coefficients were compared, fully
taking into consideration the various sources of uncertainty.
Results: The excess relative risk coefficient for childhood (v15
years of age) cancer obtained from the OSCC was around
50 Gy21
, leading to an excess absolute risk coefficient for incident
cases of about 8% Gy21
. However, the statistical, dosimetry,
modelling and other uncertainties associated with these risk
estimates are appreciable, and there is reason to believe that
these coefficients could be systematic overestimates. When these
uncertainties and those associated with the equivalent risk
coefficients derived from the Japanese cohort exposed in utero
are taken into account, the risk estimates for childhood cancer
obtained from these two sources are compatible. These
coefficients are consistent with the high relative risk of childhood
leukaemia among the Japanese survivors exposed as children.
The absence of cases of childhood solid tumours among the
Japanese children irradiated after birth in contrast to the
significant excesses found in both intrauterine exposure studies
might be explained by the cells from which these cancers
originate being predominantly sensitive only to exposure in utero.
Conclusions: The consistency of the childhood cancer risk
coefficients derived from the Oxford Survey and from the
Japanese cohort irradiated in utero supports a causal explanation
of the association between childhood cancer and an antenatal
X-ray examination found in case-control studies. This implies
that doses to the foetus in utero of the order of 10 mSv discernibly
increase the risk of childhood cancer. However, uncertainties in
risk estimates are such that it is difficult to conclude reliably from
these epidemiological data what the level of risk at these low doses
might be, beyond the inference that the risk is not zero or has
been grossly underestimated.