Health Risks of Ionizing Radiation: - Clark University

Appendix B 184
cities and 2,729 settlements in Russia and Belarus
and found an ERR of 23/Gy (8.6-82). The dose-response
relationship in this study appeared linear and
a significant risk was observed in the lowest dose
group (< 0.1 Gy, mean thyroid dose of 0.05 Gy).
Ivanov et al. (2003) studied adolescents and adults
(aged 15-69 at the time of the accident) in the Bryansk
district of Russia. There was no evidence of increased
thyroid cancer risk in the total cohort, which
is consistent with other studies. However, the subcohort
aged 15-29 at the time of the accident showed a
significant ERR of 8.65/Gy (0.81-11.47).
These risk estimates left a possible confounding
issue unaccounted for- a relatively low intake
of natural iodine in the diet of children near Chernobyl
may have increased their thyroid cancer risk
by increasing the amount of radioiodine sequestered
by the thyroid. This possibility was addressed in
another study of Bryansk children (ages 6-18) by
Shakhtarin et al. (2003). These researchers found
that dietary iodine did play a role in radiation-induced
cancer risk so that although the overall ERR
was 18.1/Gy (11.3-26.9), the ERR in areas with sufficient
dietary iodine was 13/Gy (-11-71.2). Another
important consideration in interpreting these studies
is the fact that follow-up was for a small number of
years after exposure. Since background cancer risk
is low in children, relative risk estimates based on
childhood cancer incidence may overestimate lifetime
risks.
Medical exposures
Iodine-131 has been used at low doses to diagnose
thyroid disease and at high doses to treat hyperthyroidism
by killing off part of the thyroid. Results of
an ongoing cohort follow-up in Sweden have been
published a few times (Holm et al. 1988, Hall et al.
1996, Dickman et al. 2003). The study participants
had received average doses of ~1 Gy of 131I between
1951 and 1962. Holm et al. found an SIR of
1.27 (0.94-1.67) and a positive dose-response relationship
that was not quantified. Hall et al. confirmed
the elevated SIR (1.35, 1.05-1.71) and quantified
the dose-response relationship for patients
younger than 20 (ERR 0.25/Gy, 0-2.7). Dickman et
al. did not report either an elevated SIR or a significant
dose-response relationship (although the data
suggest a positive dose-response), but note the fact
that only 300 subjects in their study were under the
age of 10, the period of highest observed sensitivity
in other studies. Another study of diagnostic 131I,
again with an average dose of ~1 Gy, was carried
out in Germany (Hahn et al. 2001). This study found
no increase in risk (RR 0.86, 0.14-5.13) but, as in
the case of the Swedish cohort, was largely limited
to adults.
Radiation treatment for hyperthyroidism involves
131I doses in the tens or hundreds of Gy.
A cohort of Swedish patients showed an elevated
SMR (1.95, 1.01-3.41)) and a nonsignificant doseresponse
pattern with thyroid cancer mortality (Hall
et al. 1992). A UK study found an SIR of 3.25 (1.69-
6.25) and an SMR of 2.78 (1.16-6.67) (Franklyn
et al. 1999). A U.S. study found an SMR of 3.94
(2.52-5.86) and noted a marginally significant doseresponse
relationship (Ron et al. 1998). Although
these three studies generally agree with each other in
terms of the magnitude of mortality risk that hyperthyroid
patients face with 131I treatment, they offer
little insight on the question of low doses to healthy
people because the thyroid was severely damaged
and there was an underlying thyroid disease in all of
the study subjects.
B.4 Non-cancer Thyroid Disease
Non-cancer effects of radiation have been observed
in the exposed populations described above, although
they haven’t been examined with the same
quantitative detail as cancer. The atomic bomb survivors
have shown an excess of thyroid disease, a
general category that includes hypothyroidism, thyroiditis,
goiter and thyrotoxicosis. It has been estimated
that 16% of the thyroid disease in the Adult
Health Study can be attributed to a-bomb radiation
exposures (Wong et al. 1993). A significant dose-response
relationship was observed in this study with
an ERR of 0.30/Gy (0.16-0.47); this was attributed
to the effects of exposure at younger ages. Hypothyroidism
was also specifically analyzed in this cohort
by Nagataki et al. (1994), who found a concave
dose-response curve with a maximum incidence of
hypothyroidism (OR ~2.5) occurring with a dose of
~0.7 Sv. Below 0.5 Sv the dose-response relationship
was linear with an apparent ERR of ~3/Sv.
Autoimmune thyroid disease is a condition
where the immune system attacks the thyroid. Evi-