Health Risks of Ionizing Radiation: - Clark University

Radiotherapy for childhood cancer. Tucker et
al. (1987), investigating leukemia after childhood
cancer therapy, found an excess associated with
alkylating agents but not radiation therapy. Garwitz et
al. (2000), on the other hand, concluded that radiation
was the most important treatment-related risk factor
for the development of a second malignant neoplasm
in children who were exposed under the age of 20
for a first malignant neoplasm. The irradiated group
had a RR of 4.3. Chemotherapy appeared to play
only an accessory role of potentiating the effects of
radiotherapy. Loning et al. (2000) found that children
that received cranial radiation for the treatment of
acute lymphoblastic leukemia had a higher risk
for developing secondary neoplasms. The risk of
thyroid cancer was specifically investigated by de
Vathaire et al. (1999b). Cases where the first cancer
had been thyroid cancer were excluded from this
cohort of French and English children. The mean
thyroid dose was quite high (7 Gy), and although
the estimated risk was unusually high, suggesting
a predisposition to cancer, the dose-response
relationship within the cohort was consistent with
radiation-induced thyroid cancer patterns in other
cohorts 31 . Acharya et al. (2003) looked at 33 cases of
thyroid neoplasms in cancer survivors who had been
treated with radiation. Thirteen of these neoplasms
were malignant, more than would be expected based
on a 5% malignancy rate in the general population.
Again, doses had been quite high (10-42 Gy) and
these patients were possibly predisposed to cancer.
Wong et al. (1997) examined retinoblastoma 32
patients; these patients are expected to have a
strong genetic predisposition to cancer. The authors
demonstrated a much higher risk in patients whose
primary cancer was hereditary, as expected, but also
demonstrated a dose-response trend for sarcomas of
bone and soft tissue 33 . The OR for any second cancer
Medical Exposures 29
among nonhereditary retinoblastoma patients was
1.6 (0.7-3.1).
3.5 In utero exposures
Studies of prenatal exposure to radiation, mainly
through obstetric x-rays, have clearly demonstrated
risks at relatively low doses. Alice Stewart and
others published results linking prenatal exposure
and childhood cancer in the 1950s (Stewart et al
1956, 1958), and this study grew into the Oxford
Survey of Childhood Cancers (OSCC). Although the
OSCC contains the majority of the data pertaining
to this association, other studies have generated
consistent results. One of the challenges in analyzing
these data is the lack of good dose information; the
average dose of an x-ray exam has declined over
the years and precise estimates of dose per exam
are not available 34 . The studies discussed below
have attempted to make reasonable inferences about
dose and apply them to observed childhood cancer
outcomes.
The OSCC grew to include all childhood cancers
in Great Britain and in 1981 consisted of 15,276 casecontrol
pairs. Initial observations found increased
risks of childhood leukemia (RR 1.92, 1.12-3.28)
and other childhood cancers (RR 2.28, 1.31-3.97)
after prenatal x-rays of the mother’s abdominal area
(Stewart et al. 1956, Doll and Wakeford 1997). As
prenatal exposure has decreased, so has the estimated
risk. Knox et al. (1987) estimated an average relative
risk of 1.94 over the period 1953-79. This analysis
also suggested that the relative risk was highest for
cancers at age 4-7. Gilman et al (1988) also looked
at the OSCC and argued that the timing of the
exposure in the pregnancy was more important than
the dose in increasing cancer risk. Specifically, these
authors found that x-rays taken in the first trimester
31 With a dose of 0.5 Gy the SIR was 35 (90% CI 10-87). The ERR was stated to be between 4 and 8 per Gy; this can
be compared to the estimate from Ron et al. (1995) of 7.7/Gy
32 Retinoblastoma is a cancer of the eye that typically affects young children and is largely hereditary.
33 The odds ratios for sarcomas (1.9, 3.7, 4.5, 10.7) increased with median dose (7, 20, 40, 98 Gy) compared to the 0-5
Gy dose group.
34 Knox et al. (1987) cite the National Radiological Protection Board estimate that the mean fetal gonadal dose per
exposure in 1977 was 3.5 mGy. The United Nations Scientific Committee on the Effects of Atomic Radiation
estimated that the mean fetal dose per film was 18.0 mGy between 1947 and 1950 and 5.0 mGy between 1958 and
1960. Mole (1990) used an estimated mean dose of 6 mGy for the period 1958-61.