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

BIOLOGICAL BASIS FOR R ADIATION SAFETY 333
Generally, radiation bioeffects fall into one of two categories: deterministic effects
and stochastic effects. Deterministic effects result from exposure to very large doses
of radiation. These effects have a threshold dose and their severity increases with
increasing dose. Examples are the acute radiation syndromes that have a threshold
in the range of 1–2 Gy (100–200 rads) whole-body X- or gamma radiation, skin burns
in the range of 2–3 Gy (200–300 rads), and skin ulceration in the range of 20 Gy
(2000 rads). The LD50/60-day dose for whole-body X- or gamma radiation is believed
to lie in the range of 3–4 Gy (300–400 rads). Deterministic effects are clearly and
unequivocally causally associated with the radiation exposure.
Stochastic effects occur by chance and are seen in unexposed individuals as well
as in exposed individuals and therefore are not unequivocally associated with a radiation
exposure. Stochastic effects include cancer and genetic mutations. Exposure to
radiation increases the probability of a stochastic effect, and this probability increases
with increasing dose. Whereas increased incidence of cancer has been documented
among certain heavily exposed populations—such as the early radiologists, atomic
bomb survivors, and patients who had received radiotherapy—no increased incidence
of heritable changes has ever been observed among any human population
exposed at any dose.
The stochastic effects, either in humans or in animals, that have been observed are
no different in kind from those observed in unirradiated populations. The difference
lies only in the frequency of occurrence. Thus, it is impossible for even the most
highly skilled pathologist to definitely attribute any cancer in an exposed individual
to the exposure. The only thing that can be done is to estimate the probability—
based on the patient’s exposure history—that the cancer can be attributed to the
radiation exposure. Of the several possible models that have been postulated to infer
the stochastic effects of low doses from high-dose cases, the zero-threshold, linear
dose–response relationship, because it is believed to be the most conservative of all
the proposed models, was chosen as the basis for setting radiation safety standards.
This brief overview shows that we know enough about the biomedical effects
of radiation to enable us to set radiation safety standards with a very high level
of confidence that adherence to these safety standards will allow the safe use of
radiation and its sources.
SUGGESTED READINGS
Annals of the ICRP. Pergamon, Elsevier Science, Oxford, U.K.
No. 26. Recommendations of the International Commission on Radiological Protection, 1:(3), 1977.
No. 31. Biological Effects of Inhaled Radionuclides, 4:(1/2), 1979.
No. 41. Nonstochastic Effects of Ionizing Radiation, 14:(3), 1984.
No. 42. A Compilation of the Major Concepts and Quantities in Use by the ICRP, 14:(4), 1984.
No. 48. The Metabolism of Plutonium and Related Elements, 16:(2/3), 1986.
No. 49. Developmental Effects of Irradiation on the Brain of the Embryo and Fetus, 16:(4), 1986.
No. 50. Lung Cancer Risk from Indoor Exposures to Radon Daughters, 17:(1), 1986.
No. 59. The Biological Basis for Dose Limitation in the Skin, 20:(4), 1992.
No. 60. 1990 Recommendations of the International Commission on Radiological Protection, 21:(1–3), 1991.
No. 66. Human Respiratory Tract Model for Radiological Protection, 24:(1–3), 1994.
No. 68. Dose Coefficients for Intakes of Radionuclides by Workers, 24(4), 1994.
No. 70. Basic Anatomical and Physiological Data for use in Radiological Protection: The Skeleton, 25:(2), 1996.
No. 79. Genetic Susceptibility to Cancer, 28:(1/2), 1998.