Health Risks of Ionizing Radiation: - Clark University

frequently or as intensively as it was in the early and
middle part of the 20 th century. This section reviews
studies of treatment of various diseases including
tuberculosis, ankylosing spondylitis, and childhood
skin conditions (hemangiomas and tinea capitis).
Tuberculosis. Fluoroscopy, a form of x-ray
examination, was frequently used in the past to
both diagnosis and treat tuberculosis. Although
exposures varied, this procedure resulted in average
doses on the order of 1 Gy to the chest (lungs, breast,
esophagus), and ~0.1 Gy to the bone marrow and
stomach organs.
Davis et al. (1989) looked at a cohort of
Massachusetts fluoroscopy patients and found
evidence for an increased risk of mortality from
cancer of the breast (SMR 1.4) and esophagus (SMR
2.1). A largely overlapping cohort was revisited
by Boice et al. (1991) to assess the risk of breast
cancer in more detail. This study found a significant
dose-response relationship that depended on age
at exposure; the estimated relative risk for women
exposed to 1 Gy at age 15 was 2.0; corresponding
estimates for exposure at ages 20, 35 and 45 were 1.7,
1.2 and 1.1, respectively. Howe (1995) investigated
the lung cancer risk associated with fluoroscopies
administered in Canada. There was no evidence for
such a risk in this cohort, and the estimated RR at
1 Gy was 1.00 (0.94-1.07), significantly lower than
the estimate from the atomic bomb survivors of 1.60
(1.27-1.99).
The fact that these observations differ markedly
from atomic bomb survivor data has been explained
as an effect of dose fractionation. Although
cumulative doses averaged ~1 Gy, the fluoroscopic
regimen included an average of 90 separate exposures
received at a rate of two per month. Regarding breast
cancer, Little and Boice (1999) showed that although
Medical Exposures 25
the ERR estimate from the fluoroscopy group was
roughly half that from the atomic bomb survivors,
the absolute risks were essentially the same 13 .
This implies that the induction of breast cancer by
radiation is independent of the underlying breast
cancer risk, which is much lower in Japan. Brenner
(1999), however, observed that fluoroscopy radiation
should be more biologically effective (damaging)
than the atomic bomb radiation 14 . He concluded that
the dose fractionation of the fluoroscopic treatment
probably reduced the risk per dose.
Ankylosing spondylitis. Ankylosing spondylitis
is a chronic, progressive inflammation of the
vertebrae. Over 14,000 patients in the UK, between
1935 and 1954, were treated for this disease with
high doses of x-rays 15 . Weiss et al. (1994, 1995)
studied the cancer mortality in this cohort through
1991 16 . The researchers found that there was
increased risk of cancer mortality (RR 1.30, 1.24-
1.35), particularly non-CLL leukemia mortality (RR
3.11, 2.37-4.07), which peaked in the 1-5 years after
first treatment. The dose-response relationship for
cancer mortality was best described with an ERR
of 0.28/Gy 17 (the atomic bomb survivors estimate is
0.47/Gy; Preston et al. 2003).
Leukemia results were analyzed by Weiss et
al. in a 1995 paper. The dose-response relationship
for leukemia showed a maximum risk in the 0.01-
1.0 Gy dose range (RR 6.58, 2.22-15.98). This is
likely to be a result of cell killing at higher doses- if
cells that might otherwise develop into a cancer are
killed then the risk per unit dose can be less 18 . Weiss
et al. (1995) modeled leukemia mortality with a
complicated linear-exponential model that allowed
for this effect and also calculated total risk as the
sum of the risks in different bone marrow regions.
This compartmental component of the model took
13 The EAR estimates were 5.48/10,000 PY-Sv (0.90-10.43) for the fluoroscopy cohort and 4.95/10,000 PY-Sv (3.37-
6.71) for the atomic bomb survivors (Little and Boice 1999).
14 Fluoroscopic x-rays and gamma radiation from the atomic bombs are types of photons that have different energies.
Fluoroscopic x-rays have a lower energy but a higher linear energy transfer, or LET. At low doses these photons are
expected to be 1.6 to 1.9 times more damaging than the gamma radiation from the atomic bombs (Brenner 1999).
15 Weiss et al. (1994) report mean total body and bone marrow doses of 2.64 and 5.10 Gy.
16 An earlier analysis through 1982 was published by Darby et al. (1987).
17 Although an estimated ERR of 0.1/Gy (0.04-0.18; all cancers except leukemia) was derived with a simple linear
model, the linear term in a linear-quadratic model was reported to be 0.28/Gy (0.07-0.48); this would be a more
appropriate value to use when considering low doses.
18 It was estimated that the ERR at 1 Gy was reduced by 47% (17-79%) due to the effect of cell killing.