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3 Basic Concepts in Radiation Physics, Biology, and Epidemiology
Pages 43-72

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From page 43...
... The first presents the principles of physics related to ionizing radiation. The second presents the biology necessary for understanding how radiation affects cells and the mechanisms of radiation injury and repair.
From page 44...
... X rays and gamma rays are particles called photons that can create ionization. Microwaves, ultraviolet radiation, visible light, and infrared are also photons, but they do not result in ionization and are referred to as nonionizing radiation.
From page 45...
... Types of Ionizing Radiations Radioactive nuclei can emit several kinds of particles, but there are three primary types: alpha particles () , beta particles ()
From page 46...
... Absorbed dose is the energy absorbed in a volume of material divided by the mass of the material. It is the result of the physical interactions of the ionizing radiation within the volume of material.
From page 47...
... To a first approximation, RBE increases with LET. Rules for and regulation of radiation protection of humans must be related to the risks associated with exposure to ionizing radiation.
From page 48...
... Some people might develop cancer as a result of the absorbed dose received. The types of cancer associated with radiation would be distributed according to the fraction represented by wT in Table 3.1.
From page 49...
... . The primary contributions to external radiation from natural background are cosmic rays and penetrating gamma rays emitted by radioactive materials in rocks and soil, in particular 40K, 232Th and 238U.
From page 50...
... The striped sections are from man-made sources of ionizing radiation. The other sections are from natural background radiation.
From page 51...
... Risk to Miners In general, the most hazardous exposure pathway for underground miners is not related to the ore dust itself or external gamma rays. About halfway through the uranium decay process, 226Ra decays into 222Rn.
From page 52...
... Exposure to external radiation and radon decreases rapidly with distance from the tailings. Nuclear Weapons and Fallout Yellowcake is an oxide of natural uranium.
From page 53...
... Dosimetry Dosimetry is the process of determining the effective dose received by persons exposed to ionizing radiation. The most accurate way to determine dose to an individual is to make measurements with a dosimeter assigned to each person.
From page 54...
... As described earlier in this chapter, some ionizing radiation can travel through a few or several layers of cells (beta-particle radiation) or through many cell layers into and through tissues deep within the body (x and gamma radiation)
From page 55...
... External radiation dose is deposited independent of differential uptake in cells and sub-cellular regions due to ongoing local metabolic processes. Inhomogeneous dose distribution is more characteristic of internal emitters than external radiation sources.
From page 56...
... Acute or Early Deterministic Effects Acute or early deterministic effects become clinically evident within minutes up to about 2 months after an acute radiation exposure of the whole body or partial body of sufficient magnitude to cause a critical number of cells in individual tissue systems, such as the blood-forming tissues, to die prematurely or to lose their ability to divide. The higher the acute radiation dose, the earlier the deterministic effects occur after the exposure and the more severe they are.
From page 57...
... . On the basis of the committee's review of information about reconstructed radiation dose estimates of downwinders and onsite participants, it is considered highly unlikely that people in the RECA populations received acute whole- or partial-body doses of gamma radiation of sufficient magnitude to cause deterministic effects, including the ARS (Lloyd et al., 1990; Henderson and Smale, 1990; Till et al., 1995; Caldwell et al., 1983)
From page 58...
... Exposure of a pregnant woman to radiation may cause nonspecific deterministic effects in the embryo or fetus. Such in utero effects may be expressed clinically in the embryo or fetus or after the child's birth.
From page 59...
... Age-at-exposure and the magnitude of the radiation dose have been shown in epidemiologic studies to influence the latent periods of some specific tumor types that have been causally associated with radiation exposure. The relative risk (RR)
From page 60...
... Salivary glands Thyroid Moderate susceptibility Urinary bladder Colon Stomach Liver Lung Ovary Skin Low susceptibility Bone Brain Connective tissue Kidney Larynx Nasal sinuses Very low or absent susceptibility Cervix of the uterus Body of the uterus (endometrium) Chronic lymphocytic leukemia Oral cavity Esophagus Melanoma Prostate Pancreas Rectum Gallbladder Hodgkin's disease Lymphatic system and myeloma Testes Muscle Small intestine a SOURCE: Adapted from Mettler and Upton, 1995.
From page 61...
... . RADIATION EPIDEMIOLOGY Much of what is known about the long-term effects of ionizing radiation in humans has been learned from epidemiologic studies of exposed populations at risk of exposure.
From page 62...
... The atomicbomb survivors study and the Colorado Plateau Uranium Miners study are examples of prospective cohort studies. The historical cohort has the advantage that a cohort is identified many years in the past and is followed forward from that time with existing records.
From page 63...
... These studies are often highly biased, so the committee has chosen to avoid citing them in discussions of radiation risk estimates. Measures of Risk Relative Risk The term relative risk (RR)
From page 64...
... Although the term relative risk is sometimes used in a general sense, in this report the committee attempts to specify the type of RR estimate for any study cited. Absolute Risk Absolute risk (AR)
From page 65...
... Excess Relative Risk vs Excess Absolute Risk In discussing the results of epidemiologic studies, the terms excess relative risk (ERR) or excess absolute risk (EAR)
From page 66...
... 1 600 Exposed population 0.9 500 0.8 Deaths/100,000 0.7 EAR 400 U.S population 0.6 ERR 0.5 300 ERR 0.4 200 0.3 0.2 100 0.1 0 0 42 47 52 57 62 67 72 77 82 Age (years) FIGURE 3.3 Lung-cancer constant ERR vs increasing EAR.
From page 67...
... In short, when the mortality rate in the unexposed population rises with age, as is the case with most health outcomes, a constant ERR results in an increasing EAR, while a constant EAR implies a decreasing ERR. Mortality and Incidence Studies Many of the studies discussed in this report use death from a particular cancer as the end point.
From page 68...
... For example, in an epidemiologic study of the relationship between radiation dose from an identified source and disease frequency, little information might be available on the doses that people received. In addition, the number of exposed persons in a study might be too small to produce precise risk estimates.
From page 69...
... Derivation of Radiation Risk Estimates General approaches for estimating health risks (most specifically cancer risks) posed by exposure to ionizing radiation have been developed by ICRP and in the United States by the National Council on Radiation Protection and Measurements (NCRP)
From page 70...
... . For a population of working age composed of both sexes, the lifetime risk of death from cancer is 8 × 10­2 per sievert for high doses and high dose rates of external radiation and 4 × 10­2 per sievert for low doses and low dose rates (ICRP, 1991)
From page 71...
... In addition to the risk estimates described above for external radiation exposure, scientists have developed separate estimates for specific exposure conditions. For example, the risk of lung cancer from radon exposure (for example, from uranium mining)
From page 72...
... 72 RADIATION EXPOSURE SCREENING AND EDUCATION PROGRAM patients in the clinical arena. It differs, however, in that most clinical rules are applied prospectively to predict the likelihood of an outcome or event whereas the PC is applied retrospectively to analyze the likelihood that a known cancer was caused by radiation.


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