Health Risks of Ionizing Radiation: - Clark University

24 Medical Exposures
with national rates, continued to be inconclusive.
Although an increased risk persisted in the cohort
as a whole (SIR 1.77, 1.45-2.14), a risk among
patients who had not been examined on suspicion
of a thyroid tumor was not evident (SIR 0.91, 0.64-
1.26). Risk in this cohort appeared to decline over
time 9 and dose-response analysis was inconclusive
(see Figure 3-4).
The radiation-induced risk of thyroid cancer is
known to be much greater in childhood, particularly
early childhood (Ron et al. 1995). The Swedish cohort
was unfortunately not very informative regarding
children since only 7% of the subjects were under
age 20 at the time of exposure. Hahn et al. (2001)
focused on exposures to diagnostic iodine-131 in
a German cohort of adolescent children (median
age 14 years). This study, with only five observed
cases of thyroid cancer, was inconclusive (RR 0.9,
0.1-5.1). Thyroid cancer observations are discussed
further in appendix B.
Thorotrast. Thorotrast was a commercially
prepared alpha-emitting solution that was used for a
variety of different diagnostic procedures 10 from the
late 1920s into the early 1950s; at this time the toxic
properties of the treatments were becoming clear
and its use was discontinued. Thorotrast remains in
the patient’s body for life, thus giving a lifetime of
alpha radiation exposure and high cumulative doses,
particularly to the liver. Although several studies
have addressed these patients 11 we only present two
representative studies here; since these exposures
were unusually high they are of limited applicability
to our low-dose area of interest.
Nyberg et al. (2002) investigated cancer
incidence among patients who had been exposed
to Thorotrast in Sweden and found elevated SIRs
of various cancers 12 . Liver and gallbladder cancer
showed a dramatic 40-fold increase (SIR 39.2, 30.2-
49.9). The dose rates estimated by these authors were
220 mGy per year for the liver and 1-5 mGy per
year for other organs. Dos Santos Silva et al. (2003)
studied Portuguese patients who received estimated
liver doses of 400 mGy per year. In addition to a
high incidence of liver cancer (RR 42.4, 13.9-210)
there was a notable increase in non-CLL leukemia
(RR 10, 1.24-471). The estimated bone marrow
dose rate in this study was 100 mGy per year and the
leukemia risk estimate was compatible with those of
other Thorotrast cohorts.
3.3 Radiotherapy for non-cancer disease
Radiation is still used in some circumstances to
treat non-cancer disease, although not nearly as
Figure 3-6. In the late 1940s chest x-rays were used to
screen for tuberculosis (http://profiles.nlm.nih.gov/RR/B/
B/B/Z/_/rrbbbz.jpg).
9 For the full cohort the central estimates of SIR were 3.07, 2.53, 1.18 and 1.70 for 2-5, 5-10, 10-20 and >20 years
after exposure.
10 For example, cerebral angiography, a process in which arterial pulse movement is tracked through the vascular
system of the brain.
11 See, for example, LB Travis et al. (2001). Mortality following cerebral angiography with or without radioactive
Thorotrast: An international cohort of 3143 two-year survivors. Radiat Res 156:136-50.
12 Again, doses and consequent cancer risks were quite high. SIRs for various cancer sites were: stomach 10.5 (3.9-23),
small intestine 12 (2.4-35.2), pancreas 2.9 (1.1-6.4), kidney 3.4 (1.4-7.0), brain and CNS 3.1 (1.0-7.4), connective
tissue 8.3 (1.7-24.4), leukemia 6.1 (2.9-11.2) and all sites combined 3.0 (2.8-3.7)