Health Risks of Ionizing Radiation: - Clark University

174 Discussion
Figure 13-1. Estimated solid cancer mortality risk coefficient over increasing ranges of dose (data of Preston et al.
2003).
epidemiology and biology. The authorship of this
paper included the leaders in the field 19 , and in their
conclusion they stated the following:
In light of the evidence for downwardly curving
dose responses 20 , this linear assumption is not
necessarily the most conservative approach,
as sometimes has been suggested, and it is
likely that it will result in an underestimate
of some radiation risks and an overestimate
of others. Given that it is supported by
experimentally grounded, quantifiable,
biophysical arguments, a linear extrapolation
of cancer risks from intermediate to very
low doses currently appears to be the most
appropriate methodology.
Conclusion. What does this mean for the
traditional linear no-threshold model of cancer risk?
Although a threshold is a possibility, it is a remote
possibility, and we know that it would have to be
much lower than 0.1 Gy because epidemiologic
studies have detected risks at lower doses. The
small possibility of a threshold, as Land (2002)
demonstrates, should not affect radiation protection
because it does not affect our sense of the upperbound
risk estimates at low doses.
We now know from direct observation that
the responses of biological systems to radiation are
not as simple as the linear model predicts, but we
cannot say with any certainty what the net effect
of these dynamic responses are. The linear model
is still the preferred model because it is generally
compatible with epidemiological data and because
it includes assumptions about the average behavior
of biological systems that are reasonable given our
limited understanding.
Perhaps the most important lesson from this
review is that there is not one risk estimate that fits
all circumstances. Different tissues within the body
respond very differently in response to radiation.
Tissues in young people react to radiation in different
ways than tissues in older people. Acute exposures
and chronic exposures can influence the body
differently. The interaction of radiation and other
risk factors is not simple or well understood. All
of these factors make it important to consider both
the general behavior of radiation-induced cancer, as
described by the standard linear risk models, and the
observations of specific situations.
19 The authors included several leaders in the field of biological research as well as leading epidemiologists from the
RERF (Dale Preston) and the NCI (Charles Land, Jay Lubin and Elaine Ron).
20 The “downwardly curving” dose-response pattern mentioned here is a reference to the suggestion of supralinearity
in the atomic bomb survivors.