Health Risks of Ionizing Radiation: - Clark University

vical cancer. The doses to these patients were quite
high, averaging 7 Gy. Risk was greater in younger
patients and highest in the first few years after exposure.
The estimated ERR at lower doses was 0.88/
Gy (Boice et al. 1987).
Childhood medical exposures
Infants and young children are not often exposed to
radiation but there is some evidence for a leukemia
risk associated with such exposures. A study of postnatal
x-rays in Quebec found that 1 x-ray increased
the risk of childhood ALL with an OR of 1.13 (0.84-
1.50) and that 2 or more x-rays increased the OR
to 1.47 (1.11-1.97) (Infante-Rivard 2003). Infants
treated with external radium for hemangiomas, a
birthmark-like skin condition, showed a non-significant
elevation in leukemia risk (SIR 1.54, 0.82-
2.63); bone marrow doses were not estimated but
other organs received doses ranging from 5-15 cGy
(Lindberg et al. 1995). Shore et al. (2003) studied
children treated with x-rays for ringworm of the
scalp; this procedure resulted in skull marrow doses
of ~4 Gy. The rate ratio for total leukemia was 4.7
(0.8-107).
Medical exposure in utero
The pioneering work of Dr. Stewart with childhood
leukemia cases first drew attention to the possible
risk of prenatal x-rays in 1956. This marked the beginning
of the Oxford Survey of Childhood Cancers
(OSCC), a cohort that came to include over 15,000
case-control pairs. The most recent estimate of the
childhood leukemia mortality risk associated with
prenatal x-ray exposure in this cohort is a RR of
1.39 (1.30-1.49), a value remarkably consistent with
all other prenatal x-ray studies combined (RR 1.37,
1.22-1.53; Doll and Wakeford 1997). The doses received
from these x-rays are uncertain, but they have
been declining over the past 50 years and the leukemia
risk has been declining in parallel. The best current
estimate of the average dose per x-ray over the
years is around 3 or 4 mGy and the corresponding
ERR estimate is ~50/Gy.
Although no leukemia cases were diagnosed
among the atomic bomb survivors who were exposed
in utero, less than 1 case of leukemia would
have occurred spontaneously in this cohort. The es-
177 Appendix A
timated ERR is negative but is very uncertain with
an upper 95% confidence limit of 50/Gy; the atomic
bomb survivor data are therefore roughly compatible
with the OSCC and other prenatal x-ray studies
(Wakeford and Little 2002, 2003).
Radiology occupations
Cohorts of radiologists and radiologic technologists
are limited by the fact that the doses they received
are unknown or very uncertain. It is known that average
annual doses have been decreasing over the
years as occupational standards evolve and leukemia
risk associated with these occupations has been
declining in parallel (Mohan et al. 2003, Berrington
et al. 2001), but dose-response inferences would be
purely speculative based on the available data.
A.4 Workers
Many studies of nuclear workers have been carried
out over the years and each one was limited by the
size of the study population. In 1995 Cardis et al.
published the results of a study that pooled the data
from worker cohorts in the U.S., Canada and the
U.K. The pooled data set included almost 100,000
workers and allowed for much more precise estimates
of risk. In these workers the mean external
dose was 40.2 mSv and increased leukemia risk was
clearly correlated with external dose: The overall
non-CLL leukemia ERR was 2.18/Sv (0.1-5.7), the
ERR for AML was 3.38/Sv (