Biomedical Engineering Reference
In-Depth Information
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s, radicals produced by a charged-particle track in a biological system
have all reacted. Some biochemical processes are altered almost immediately, in
less than about 1 s. Cell division can be affected in a matter of minutes. In higher
organisms, the time at which cellular killing becomes expressed as a clinical syn-
drome is related to the rate of cell renewal. Following a large, acute, whole-body
dose of radiation, hematopoietic death of an individual might occur in about a
month. A higher dose could result in earlier death (1 to 2 wk) from damage to the
gastrointestinal tract. At still higher doses, in the range of 100 Gy, damage to mem-
branes and to blood vessels in the brain leads to the cerebrovascular syndrome and
death within a day or two. Other kinds of damage, such as lung fibrosis, for ex-
ample, may take several months to develop. Cataracts and cancer occur years after
exposure to radiation. Genetic effects, by definition, are first seen in the next or
subsequent generations of an exposed individual.
The biological effects of radiation can be divided into two general categories, sto-
chastic and deterministic, or nonstochastic. As the name implies, stochastic effects
are those that occur in a statistical manner. Cancer is one example. If a large popu-
lation is exposed to a significant amount of a carcinogen, such as radiation, then an
elevated incidence of cancer can be expected. Although we might be able to predict
the magnitude of the increased incidence, we cannot say which particular indi-
viduals in the population will contract the disease and which will not. Also, since
there is a certain natural incidence of cancer without specific exposure to radia-
tion, we will not be completely certain whether a given case was induced or would
have occurred without the exposure. In addition, although the expected incidence
of cancer increases with dose, the severity of the disease in a stricken individual is
not a function of dose. In contrast, deterministic effects are those that show a clear
causal relationship between dose and effect in a given individual. Usually there is a
threshold below which no effect is observed, and the severity increases with dose.
Skin reddening is an example of a deterministic effect of radiation.
Stochastic effects of radiation have been demonstrated in man and in other or-
ganisms only at relatively high doses, where the observed incidence of an effect is
not likely due to a statistical fluctuation in the normal level of occurrence. At low
doses, one cannot say with certainty what the risk is to an individual. As a practi-
cal hypothesis, one usually assumes that any amount of radiation, no matter how
small, entails some risk. However, there is no agreement among experts on just
how risk varies as a function of dose at low doses. We shall return to this subject in
Section 13.13 in discussing dose-response relationships.
We outline next some of the principal sources of data on the effects of radia-
tion on humans and then describe the effects themselves. This collective body of
information, which we only briefly survey here, represents the underlying scien-
tific basis for the radiation-protection standards, criteria, and limits that have been
developed. Additional information can be obtained from the references listed in
Section 13.15. Virtually all aspects of standards setting are under continuing evalu-
ation and review.
