Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

328 CHAPTER 7
associated risk coefficients can be determined only by epidemiological studies, that
is, findings from observations of very large population groups that had been exposed
to the study agent in comparison to similar studies on a population group that had
not been exposed. The ICRP recommendations are based mainly on studies made
on the survivors of the atomic bombings in Japan. The U.S. National Academy of
Sciences Committee in the Biological Effects of Ionizing Radiation (BEIR committee)
reported on the health effects of low-level ionizing radiation. The Committee
reviewed the latest findings from the survivors of the Japanese bombings as well as
studies on radiation workers, persons who had been treated medically with radiation,
populations living in high-background areas, and relevant laboratory studies,
and presented its conclusions in what is known as the BEIR VII Report, Health Risks
From Exposure to Low Levels of Ionizing Radiation (2005).
In the BEIR VII report, which deals with somatic effects—mainly cancer—and
genetic effects of low-level radiation exposure, the committee members agreed on
the dose–response relationship at doses on the order of 0.1 Gy (10 rads) or more
delivered at high dose rates. They also found that health effects from low doses
would be difficult to evaluate because the “noise” from other factors would impose
statistical limitations that would make it difficult to separate radiogenic effects from
spontaneous effects. Therefore, estimates of risk coefficients must be made from
interpolations, based on a suitable dose–response model, between zero dose and
the lowest dose at which the effect has been seen.
The functional form of a generalized dose–response curve (Fig. 7-16) is
F (D) = (α0 + α1 D + α2 D 2 )e (−β1 D−β2 D 2 ) , (7.10)
where F (D) is the incidence rate of the effect under consideration (e.g., cancer) at
dose D; α0, α1, α2,β1, and β2 have positive values; and α0 is the “spontaneous” or
“natural” incidence rate of the effect. β1 and β2 are significant only at high doses.
Depending on which of these coefficients becomes insignificant, the generalized
curve reduces to the three simple curves shown in Figure 7-16—namely the linear,
the pure quadratic, and the linear–quadratic curves.
Cancer Risk Estimates
In estimating the risk of radiogenic cancers, the committee found that the data
supported a dose-related increase in the relative risk of a radiogenic cancer. Despite
this relationship, the available empirical data did not allow the committee to choose
decisively among the several dose–response models shown in Figure 7-16. However,
the BEIR VII committee found that for all cancers except leukemia and also for
the genetic effects observed in laboratory studies, the data were not incompatible
with the LNT model. The committee concluded that “the current scientific evidence
is consistent with the hypothesis that there is a linear, no-threshold dose–response
relationship between exposure to ionizing radiation and the development of cancer
in humans.” (Note that this conclusion does not agree with that in the French Joint
Report, which was discussed earlier in this chapter.) The LNT model, therefore, was
chosen as the basis for estimating the risk coefficients for solid tumors from low-dose
radiation. The linear–quadratic model was chosen for estimating risk coefficients for
leukemia. Using these models, together with modifying factors to account for age and
sex (these modifying factors are described in the BEIR VII report), the committee