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High doses of radiation cause cancer.
The
evidence comes both from animal experiments and
from studies of the effects of occupational exposure, radiotherapy for
non-neoplastic conditions, the diagnostic use of certain radioisotopes,
and the atom bomb explosions.
There have been reports that years ago scientists and radiologists tested
their equipment by placing their hands in the path of the beam. As a result they developed basal and squamous
cell carcinomas of the exposed skin. In addition, early instruments were
not properly shielded, and the hazards associated with fluoroscopy were
not appreciated.
The radiologists of that era suffered an unusually high
incidence of leukemia, a situation that has disappeared with the use of
modern shielding and protective equipment.
An unusual occupational
exposure to radiation occurred among workers who painted radium-containing
material onto watches to create luminous dials. These workers were in the
habit of licking their paint brushes, which led to the ingestion of the
radioactive element and its subsequent localization in their bones. As a
consequence, these workers later experienced a high incidence of cancer of
the bone and the paranasal sinuses.
Another example of the result of occupational exposure to a radioactive
element is the high rate of lung cancer in uranium miners who inhaled
radioactive dusts. Since most of these workers also smoke, it is difficult
to distinguish the independent from the synergistic effects of radiation
in the induction of their cancers, but the evidence favors a synergistic
effect.
At one time, thymic irradiation of infants for an ailment known as
"status thymico-lymphaticus" was popular. The
infants showed no perceptible improvement in their overall health, but as
adults they did develop cancer of the thyroid.
Another example of
iatrogenic cancer resulted in the widespread use of
spinal irradiation as a treatment for ankylosing spondylitis. While a
beneficial effect on the course of this disease was claimed, the patients
later developed of aplastic anemia and myelogenous leukemia.
Radiation delivered
by long-lived radioactive isotopes used for diagnostic purposes was also
not without danger.
Thorium dioxide (Thorotrast), a
material ingested by phagocytic cells, was at one time used
for radionuclide imaging. Its
persistence in the liver resulted in the development of a number of
tumours, particularly angiosarcomas.
The survivors of the atom bomb explosions suffered
from a number of cancers. These people exhibited a more than tenfold
increase in the incidence of leukemia, which reached its peak from 5 to 10
years after exposure and subsequently declined to background rates. [ An
increased incidence of leukemia was evident in those exposed to doses as
low as 50 rads in Hiroshima, but required more than 100 rads in Nagasaki.
This difference in sensitivity may reflect the greater neutron
component of the radiation in Hiroshima.] Two-thirds were acute leukemia ; the remainder were of the chronic myelogenous variety. Chronic lymphocytic
leukemia, an uncommon disease in Japan, showed no increase in incidence. The risk of multiple myeloma
increased fivefold and there was a small increment in the incidence of
lymphoma. The risk of the development of solid tumors, although not
as great as that for leukemia, was clearly increased for the breast, lung,
thyroid, gastrointestinal tract and urinary tract. The development of
malignant tumours, including leukemia, showed a dose-response
relationship.
Low-level radiation and cancer
: The key question that needs to be answered is whether there is a threshold
dose of radiation below which there is no increase in incidence of cancer,
or whether any exposure carries a significant risk.
Human epidemiologic studies do not provide data precise enough to permit
an accurate estimate of cancer risk from low-level radiation, except to
suggest that it lies between 0 and some projected upper limit.
Experiments involving the effects of radiation on cells in
culture and in other mammals suggest a dose relation that is less than
linear for x- and gamma irradiation.
Such a conclusion implies that the established permissible exposures to
radiation (which are based on a linear relation) are highly conservative
and may exaggerate the risks.
A discussion of the effects of low-level radiation
must include consideration of naturally occurring background
radiation - that is, the radiation derived from cosmic and terrestrial
radiation and the inhalation and ingestion of natural and man-made
radioactive isotopes.
This background radiation is estimated at about 100 millirads
per year.
Since exposure to this radiation is universal, it is clearly impossible to
determine directly whether this level of exposure contributes to the
spontaneous incidence of cancer in man.
Various attempts to estimate cancer incidence from the linear hypothesis
have yielded conflicting conclusions.
For example, a linear extension back from the observed cancer
incidence and radiation exposure among uranium miners predicts a cancer
rate that is four times higher than the rate actually observed in a
population of nonsmokers.
Cancer mortality has been recorded in two
regions of China that have different levels of background radiation.
In
the low-background region, people are exposed to 72 millirads per year,
while in the high-background region the exposure is almost three times
greater.
Despite this difference, no difference in cancer mortality existed.
Moreover, pilots and cabin crews of commercial airlines, who are exposed
to significantly higher doses of background radiation at high altitude,
have not manifested any increased cancer incidence.
These and other studies suggest that the contribution of background
radiation to the occurrence of human cancer may not be as significant as
many believe.
The argument for a risk of radiogenic cancer from
low-level radiation ( between 1 to 10 rads ) has been indicated by a number
of epidemiologic studies.
It has been reported that children exposed to
radioactive fallout from atmospheric testing of nuclear weapons had a
higher incidence of leukemia than similar children not so exposed.
However, close inspection of the data reveals, this
apparent increase is explained not by an increased incidence of leukemia
relative to the general population, but rather by an unusually low
leukemia rate in the controls.
Another epidemiologic study of radiation
associated with nuclear bomb tests involved military personnel engaged in
exercises during and after the detonation of a nuclear device named
"Smoky" in Nevada in 1957.
When the data from atomic bomb survivors
are subjected to a linear quadratic analysis, the lifetime risk from 1 rad
of whole-body x- or gamma irradiation is 1 excess cancer death per 10,000
persons.
Environmental Pathology - Radiation : click
Whole-Body Irradiation: click
Localized Radiation Injury Associated with
Radiotherapy: click
Cutaneous
lesions after exposure to Radiation: click
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