Health Risks of Ionizing Radiation: - Clark University

28 Medical Exposures
(2.1-28.7). This paper is discussed in greater detail
in appendix B.
3.4 Radiotherapy for cancer
Radiation is an important cancer therapy tool.
Although the benefits of radiation therapy obviously
outweigh the risks in many cases, researchers
and doctors have long been concerned about the
possibility of second cancers in cancer patients.
Two types of potential risk factors converge in these
patients; genetic or environmental predisposition
to cancer, demonstrated by the first cancer, and the
therapy, either chemotherapy or radiation. These
factors make information about cancer patients a hard
body of evidence to compare with other studies. The
underlying genetic cancer risk can be accounted for,
to some degree, by focusing exclusively on cancer
patients and assessing the dose-response patterns or
using cancer patients not treated with radiation as
controls. Doses are typically very high, so we do not
go into these studies in depth and only discuss a few
representative papers. Little et al. (1999) provide
a comprehensive review of the field in relation to
atomic bomb survivor-based risk estimates 28 .
Boice et al. (1987, 1988) have studied radiation
treatment for cancer of the cervix using casecontrol
methods. Doses to bone marrow were on the
order of several Gy from this procedure. The 1987
paper focused on leukemia and found a marginally
significant doubling of risk (RR 2.0, 90% CI 1.0-
4.2). The risk appeared to increase with dose up
to about 4 Gy and the authors fit the data with a
linear-exponential model with compartmentalized
dose estimates (see Weiss et al. 1995, above). In this
case the linear estimate of ERR was 0.88/Gy 29 . The
1988 paper focused on second cancers generally.
Tissues close to the cervix received doses ranging
as high as 200 Gy or more; cell killing is an obvious
consideration in these cases. The incidence of
cancers of the rectum, bladder, and genital organs
was significantly increased. The thyroid received an
average dose of 0.11 Gy and showed a nonsignificant
twofold increase in cancer risk (RR 2.35, 0.6-8.7).
Travis et al. (2000) examined men who had
been treated for testicular cancer with radiation to
see if they had an increased incidence of leukemia.
The mean bone marrow dose in this cohort was 12.6
Gy. Patients who had been treated with radiation and
not chemotherapy were found to have a threefold
leukemia risk (RR 3.1, 0.7-22) compared to patients
who had not received radiation or chemotherapy.
Risk appeared to be significantly related to dose
although the relationship was not quantified.
Gilbert et al. (2003) investigated the lung cancer
risk among Hodgkin’s disease patients. This study
benefited from estimates of radiation dose for
specific locations of the lung where cancer later
developed. The authors were also able to control for
chemotherapy treatments and smoking. There was a
significant dose-response relationship in this cohort
(ERR 0.15/Gy, 0.06-0.39) and even though most
doses were above 30 Gy there was little evidence
for departure from linearity, indicating similar doseresponse
relationships for lower doses 30 . It was
found that the interaction between chemotherapy
and radiation was almost exactly additive while the
interaction between radiotherapy and tobacco use
seemed to be multiplicative. Hancock et al. (1993)
found that women who were treated for Hodgkin’s
disease had an increased risk of breast cancer
incidence and mortality (incidence RR 4.1, 2.5-5.7);
this risk was particularly evident in women who
had been treated before the age of 15 (incidence RR
136, 34-371). Most tumors arose in tissues receiving
~44 Gy. These authors also found evidence for an
increased incidence of thyroid disease in this cohort
(thyroid cancer RR 15.6, 6.3-32.5; Hancock et al.
1991).
28 They found that in almost all cases the atomic bomb survivor-based risk estimates were higher. The authors
speculated that cell sterilization from the scheduled high doses of the medical exposure were largely responsible for
the difference.
29 This is substantially lower than the estimated ERR of 5.18/Gy (0.81-23.63) from the ankylosing spondylitis cohort
(Weiss et al. 1995).
30 The lung cancer incidence pattern in the atomic bomb survivors showed a higher ERR of 0.95/Gy (0.60-1.36;
Thompson et al. 1994). Doses among atomic bomb survivors were acute, single doses about 100 times lower than
the fractionated doses in these Hodgkin’s disease patients.