Biomedical Engineering Reference
In-Depth Information
Life Shortening
Numerous experiments have been carried out in which animals are given sub-
lethal doses of whole-body radiation at various levels. The animals apparently re-
cover, but are subsequently observed to die sooner than controls. This decreased
life expectancy was originally described as nonspecific radiation life shortening or
as radiation aging. More thorough studies of the effects of low doses of radiation,
particularly with careful autopsy examinations, showed that the life shortening due
to radiation in animal populations can be attributed to an excess of neoplasia rather
than a generally earlier onset of all causes of death. The preponderance of evidence
indicates that radiation life shortening at low doses is highly specific, being primar-
ily the result of an increased incidence of leukemia and cancer.
Some investigations have reported a longer average life expectancy in animals
exposed to low levels of whole-body radiation than in unexposed controls. Such
reports are offered by some as evidence of
radiation hormesis
—that is, the benefi-
cial effect of small doses of radiation. Radiation hormesis has also been extensively
investigated in plants, insects, algae, and other systems. As with other low-dose
studies of biological effects of radiation, one deals with relatively small effects in a
large statistical background of naturally occurring endpoints. Theoretical grounds
can be offered in support of low-level radiation hormesis—e.g., stimulation of DNA
repair mechanisms that reduce both radiation-induced and spontaneous damage.
Evidence for hormesis has been reviewed by the BEIR VII Committee and other
bodies. (See references in Section 13.15.) The BEIR VII Report summarizes its
judgement in stating, “
...
the assumption that any stimulatory hormetic effects
from low doses of ionizing radiation will have a significant health benefit to hu-
mans that exceeds potential detrimental effects from the radiation exposure is un-
warranted at this time.”
Cataracts
The biological effects discussed thus far in this section are stochastic. In contrast,
a radiogenic cataract is a deterministic effect. There is a practical threshold dose
below which cataracts are not produced; and their severity, when they occur, is re-
lated to the magnitude of the dose and the time over which it is administered.
A cataract is an opacification of the lens of the eye. The threshold for ophthalmo-
logically detectable lens opacification, as observed in patients treated with X rays to
the eye, ranges from about 2 Gy for a single exposure to more than 5 Gy for multi-
ple exposures given over several weeks. This level is also consistent with data from
Hiroshima and Nagasaki. The threshold for neutrons appears to be lower than for
gamma rays. The latent period for radiogenic cataracts is several years, depending
on the dose and its fractionation.
Among the biological effects of radiation, a unique feature of a radiogenic
cataract is that it can usually be distinguished from other cataracts. The site of
the initial detectable opacity on the posterior pole of the lens and its subsequent
developmental stages are specific to many radiation cataracts.
