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Glossary Absolute risk. An expression of excess risk based on the assumption that the excess risk from radiation exposure adds to the underlying (base- line) risk by an increment dependent on dose but independent of the underlying natural risk. Absorbed dose. The mean energy imparted by ionizing radiation to an irradiated medium per unit mass. Units: gray (Gy), red. Activity. The mean number of decays per unit time of a radioactive nuclide. Units: becquerel (Bqj, curie (Ci). Additive interaction model (AIM). The assumption that the total risk from exposures to radiation and to another risk factor is equal to the sum of the excess risks from the two taken separately. Adenosarcoma. A mixed tumor which consists of a substance like embryonic connective tissue together with glandular elements. Alpha particle. So neutrons and two protons bound as a single particle that is emitted from the nucleus of certain radioactive isotopes in the process of decay or disintegration. Aneuploid. Having numbers of chromosomes not equal to exact multiples of the haploid number. Down syndrome is an example. Ankylosing spondylitis. Arthritis of the spine. Atancia telangiectasia (ATJ. An inherited disorder associated with an in- creased risk of cancer, lymphoma in particular, and characterized by immunologic, chromosomal, and DNA defects. 391
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392 GLOSSARY Background radiation. The amount of radiation to which a member of the population is exposed from natural sources, such as terrestrial radiation due to naturally occurring radionuclides in the soil, cosmic radiation originating in outer space, and naturally occurring rad~onu- clides deposited in the human body. Baseline rate. The cancer experience observed in a population in the absence of the specific agent being studied; the baseline rate might, however, include cancers from a number of other causes, such as smoking, background radiation, etc. Becquerel (Bq). SI unit of activity. (see Units) BEIR III. Refers to the third National Research Council's Committee on Biological Effects of Ionizing Radiation, as well as to the report published by this committee in 1980. Beta particle. A charged particle emitted from the nucleus of certain unstable atomic nuclei (radioactive elements), having the charge and mass of an electron. Cancer. A malignant tumor of potentially unlimited growth, capable of invading surrounding tissue or spreading to other parts of the body by metastasis. Carcinogen. An agent that may cause cancer. Ionizing radiations are physical carcinogens; there are also chemical and biologic carcinogens and biologic carcinogens may be external (e.g., viruses) or internal (genetic defects). Carcinoma. A malignant tumor (cancer) of epithelial origin. C`lse-control study. An epidemiological study in which people with dis- ease and a similarly composed group of people without disease are compared in terms of exposures to a putative causative agent. Cell culture. The growing of cells in vitro (a glass container) in such a manner that the cells are no longer organized into tissues. Chron~osonal nondisjunction. Either a gain or a loss of chromosomes that occurs when cell division leading to either egg or sperm production goes awry. This results in aneupoidy. Cohort study. Or follow-up study; an epidemiological study in which groups of people are identified with respect to the presence or absence of exposure to a disease-causing agent and the outcomes in terms of disease rates are compared. Competing risks. Other causes of death which affect the value of the risk being studied. Persons dying from other causes are not at risk of dying from the factor in question. Confidence interval. A measure of the reliability of a risk estimate. A 90% confidence interval means that 9 times out of 10 the estimated risk would be within the specified interval.
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GLOSSARY 393 Curie. (Ci). A unit of activity equal to 3.7 x 10~° disintegrations/s. (see Units) DNA. Deoxyribonucleic acid; the genetic material of cells. Dominant mutation. The mutation is dominant if it produces its effect in the presence of an equivalent normal gene from the other parent. Dose. See absorbed dose. Dose-distribution factor. A factor which accounts for modification of the dose effectiveness in cases in which the radionuclide distribution and the resultant dose are nonuniform. Dose-e~ect (dose-response) model. A mathematical formulation of the way the effect (or biological response) depends on dose. Dose equivalent. A quantity that expresses, for the purposes of radiation protection and control, an assumed equal biological effectiveness of a given absorbed dose on a common scale for all kinds of ionizing radiation. SI unit is the Sievert. (see Units) Dose rate. The quantity of absorbed dose delivered per unit time. Dose Rate Effectiveness Factor (DREFJ. A factor by which the effect caused by a specific dose of radiation changes at low as compared to high dose rates. Doubling dose. The amount of radiation needed to double the natural incidence of a genetic or somatic anomaly. Electron volt. (eV). A unit of energy = 1.6 x 1o-~2 ergs-1.6 x 10-~9 J.; 1 eV is equivalent to the energy gained by an electron in passing through a potential difference of 1 V; 1 keV-l,OOO eV; 1 MeV-l,O0O,OOO eV. Epidemiology. The study of the determinants of the frequency of disease in man. The two main types of epidemiological studies of chronic disease are cohort (or follow-up) studies and case control (or retrospective) studies. Etiology. The science or description of causers) of disease. Euploid. Having uniform exact multiples of the haploid number of chro- mosomes. Fallout. Radioactive debris from a nuclear detonation or other source, usually deposited from air-borne particulates. Fluoroscopy. A method of visualizing internal structures by directing x rays through an object (e.g., part of the body) onto a fluorescent screen. Fractionation. The delivery of a given total dose of radiation as several smaller doses, separated by intervals of time. Gamma radiation. Also gamma rays; short wavelength electromagnetic radiation of nuclear origin, similar to x rays but usually of higher energy (100 keV to 9 MeV).
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394 GLOSSARY Geometric mean. The geometric mean of a set of positive numbers is the exponential of the arithmetic mean of their logarithms. The geometric mean of a lognormal distribution is the exponential of the mean of the associated normal distribution. Geometric standard deviation (GSD). The geometric standard deviation of a lognormal distribution is the exponential of the standard deviation of the associated normal distribution. Gray (Gy). SI unit of absorbed dose. (see Units) Half-life, biologic. Time required for the body to eliminate half of an administered dose of any substance by regular processes of elimination; it is approximately the same for both stable and radioactive isotopes of a particular element. Half-life, radioactive. Time required for a radioactive substance to lose 505 of its activity by decay. ICES. The International Classification of Diseases Adapted for use in the U.S. The ICD is periodically revised by the World Health Organization; the 8th ICDA is adapted from the 8th ICD and was issued in 1972. Incidence. Or incidence rate; the rate of occurrence of a disease within a specified period of time, often expressed as number of cases per 100,000 individuals per year. In utero. In the womb, i.e., before birth. In vitro. (Literally, in glass), in culture or in the test-tube (as onnosed to in viva, in the living individual). In vivo. In the living organism. Ionizing radiation. Radiation sufficiently energetic to dislodge electrons from an atom. Ionizing radiation includes x and gamma radiation, electrons (beta radiation), alpha particles (helium nuclei), and heavier charged atomic nuclei. Neutrons ionize indirectly by colliding with atomic nuclei. Isotopes. Nuclides that have the same number of protons in their nuclei, and hence the same atomic number, but that differ in the number of neutrons, and therefore in the mass number; chemical properties of isotopes of a particular element are almost identical. Kenya. Kenetic Energy Released in Material. A unit of exposure, expressed in red, that represents the kinetic energy transferred to charged parti- cles per unit mass of irradiated medium when indirectly ionizing (un- charged) particles, such as photons or neutrons, traverse the medium. If all of the kinetic energy is absorbed "locally," the kerma is equal to the absorbed dose. id- -or
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GLOSSARY 395 Latent period. The period of time between exposure and expression of the disease. After exposure to a dose of radiation, there is a delay in several years (the minimum latent period) before any cancers are seen. Life-span study (LSS). Life-span study of the Japanese atomic-bomb sur- vivors; the sample consists of 120,000 persons, of whom 82,000 were exposed to the bombs, mostly at low doses. Life table. A table showing the number of persons who, of a given number born or living at a specified age, live to attain successive higher ages, together with the numbers who die in each age interval. Linear energy transfer (LETJ. Average amount of energy lost per unit track length. Low LET Radiation characteristic of light charged particles such as electrons produced by x rays and gamma rays where the distance between ionizing events is large on the scale of a cellular nucleus. High LET: Radiation characteristic of heavy charged particles such as protons and alpha particles where the distance between ionizing events is small on the scale of a cellular nucleus. Linear (LJ model. Also, linear dose-effect relationship; expresses the effect (e.g., mutation or cancer) as a direct (linear) function of dose. Linear-quadratic (LQ) mode! Also, linear~uadratic dose-effect relation- ship; expresses the effect (e.g., mutation or cancer) as partly directly proportional to the dose (linear term) and partly proportional to the square of the dose (quadratic term). The linear term will predominate at lower doses, the quadratic term at higher doses. Lymphosarcoma. A sarcoma of the lymphoid tissue. This does not include Hodgkin's disease. Monosomy. The absence of one chromosome from the complement of an otherwise diploid cell. Monte Carlo Calculation. The evaluation of a probability distribution by means of random sampling. Mortality (rateJ. The rate to which people die from a disease, e.g., a specific type of cancer, often expressed as number of deaths per 100,000 per year. Multiplicative interaction model (MIM). The assumption that the relative risk (the relative excess risk plus one) resulting from the exposure to two risk factors is the product of the relative risks from the two factors taken separately. Neoplasms. Any new and abnormal growth, such as a tumor; neoplastic disease refers to any disease that forms tumors, whether malignant or benign.
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396 GLOSSARY Neutron. Uncharged subatomic particle capable of producing ionization in matter by collision with charged particles. Nonstochastic. Describes effects whose severity is a function of dose; for these, a threshold may occur; some nonstochastic somatic effects are cataract induction, nonmalignant damage to skin, hematological deficiencies, and impairment of fertility. Nuclide. A species of atom characterized by the constitution of its nucleus, which is specified by its atomic mass and atomic number (Z), or by its number of protons (Z), number of neutrons (N), and energy content. On cogen es. Genes which carry the potential for cancer. Person-gray. Unit of population exposure obtained by summing individual dose-equipment values for all people in the exposed population. Thus, the number of person-grays contributed by 1 person exposed to 1 Gy is equal to that contributed by 100,000 people each exposed to 10 logy. Person-years-at-r~sk (PYAR). The number of persons exposed times the number of years after exposure minus some lag period during which the dose is assumed to be unexpressed (minimum latent period). Prevalence. The number of cases of a disease in existence at a given time per unit population, usually 100,000 persons. Probability of causation. A number that expresses the probability that a given cancer, in a specific tissue, has been caused by a previous exposure to a carcinogenic agent, such as radiation. Progeny. The decay products resulting after a series of radioactive decay. Progeny can also be radioactive, and the chain continues until a stable nuclide is formed. Projection model. A mathematical model that simultaneously described the excess cancer risk at different levels of some factor such as dose, time after exposure, or baseline level of risk, in terms of a parametric function of that factor. It becomes a projection model when data in a particular range of observations is used to assign values to the parameters in order to estimate (or project) excess risk for factor values outside that range. Promoter An agent which is not by itself carcinogenic, but which can amplify the effect of a true carcinogen by increasing the probability of late-stage cellular changes needed to complete the carcinogenic process. Protraction. The spreading out of a radiation dose over time by continuous delivery at a lower dose rate. Quadratic-dose model. A model which assumes that the excess risk is proportional to the square of the dose. Quality factor: (Q). An LET dependent factor by which absorbed doses are multiplied to obtain (for radiation-protection purposes) a quantity
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GLOSSARY 397 which corresponds more closely to the degree of biological effect produced by x or low-energy gamma rays. Dose in Gy x Q = Dose equivalent in Sv. Rad. A unit of absorbed dose. Replaced by the gray in SI units. (see Units) Radioactiviy. The property of some nuclides of spontaneously emitting particles or gamma radiation, emitting x radiation after orbital electron capture, or undergoing spontaneous fission. Artificial radioactivity. Man-made radioactivity produced by fission, fusion, particle bombardment, or electromagnetic irradiation. Natural radioactivity. The property of radioactivity exhibited by more than 50 naturally occurring radionuclides. Radiogenic. Caused by radiation. Radioisotopes. A radioactive atomic species of an element with the same atomic number and usually identical chemical properties. Radionuclide. A radioactive species of an atom characterized by the con- stitution of its nucleus. Radiosensitivity. Relative susceptibility of cells, tissues, organs, and or- ganisms to the injurious action of radiation; radiosensitivity and its antonym, radioresistance, are used in a comparative sense rather than an absolute one. Recessive gene disorder. This requires that a pair of genes, one from each parent, be present in order for the disease to be manifest. An example is cystic fibrosis. Relative biological effectiveness (RBEJ. Biological potency of one radiation as compared with another to produce the same biological endpoint. It is numerically equal to the inverse of the ratio of absorbed doses of the two radiations required to produce equal biological effect. The reference radiation is often 200-kV x rays. Relative risk An expression of excess risk relative to the underlying (base- line) risk; if the excess equals the baseline risk the relative risk is 2. Rem. (red equivalent, man); unit of dose equivalent. The dose equivalent in "rem" is numerically equal to the absorbed dose in "red" multiplied by the "quality factor" (see Quality factor), the distribution factor and any other necessary modifying factor. RERE Radiation Effects Research Foundation; a binationally funded Japa- nese foundation chartered by the Japanese Welfare Ministry under an agreement between the the U.S.N and Japan. The RERF is the successor to the ABCC (Atomic Bomb Casualty Commission). Risk coefficient. The increase in the annual incidence or mortality rate per unit dose: (1) absolute risk coefficient is the observed minus the
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398 GLOSSARY expected number of cases per person year at risk for a unit dose; (2) the relative-risk coefficient is the fractional increase in the baseline incidence or mortality rate for a unit dose. Risk estimate. The number of cases (or deaths) that are projected to occur in a specified exposed population per unit dose for a specified exposure regime and expression period: number of cases per person-gray or, for radon, the number of cases per person cumulative working-level month. Rem. A unit of dose equivalent. Replaced by the sievert. (see Units) Sarcoma A malignant growth arising in tissue of mesodermal origin (con- nective tissue, bone, cartilage or striated muscle). Senc-linked mutation (or X-linked). A mutation associated with the X chro- mosome. It will usually only manifest its effect in males (who have only a single X chromosome). SI units. The International System of Units as defined by the General Conference of Weights and Measures in 1960. These units are generally based on the meter/kilogram/second units, with special quantities for radiation including the becquerel, gray, and sievert. Sievert. The SI unit of radiation dose equivalent. It is equal to dose in grays times a quality factor times other modifying factors, for example, a distribution factor; 1 sievert (Sv) equals 100 rem. Specific activity. Total activity of a given nuclide per gram of a compound, element, or radioactive nuclide. Specific energy. The actual energy per unit mass deposited per unit volume in a given event. This is a stochastic quantity as opposed to the average value over a large number of instances (i.e., the absorbed dose). Spline. A curve of predetermined shape; a spline with 1 knot has a single inflection point and thus two different segments. Squamous cell carcinoma A cancer composed of cells that are scaly or platelike. Standard mortality ratio (SMR). Standard mortality ratio is the ratio of the disease or accident mortality rate in a certain specific population compared with that in a standard population. The ratio is based on 100 for the standard so that an SMR of 200 means that the test population has twice the mortality from that particular cause of death. Stochastic. Random events leading to effects whose probability of occur- rence in an exposed population (rather than severity in an affected individual) is a direct function of dose; these effects are commonly regarded as having no threshold; hereditary effects are regarded as being stochastic; some somatic effects, especially carcinogenesis, are regarded as being stochastic.
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GLOSSARY 399 Target theory (hit theory). A theory explaining some biological effects of radiation on the basis that ionization, which occurs in a discrete volume (the target) within the cell, directly causes a lesion that later results in a physiological response to the damage at that location; one, two, or more hits (ionizing events within the target) may be necessary to elicit the response. Threshold hypothesis. The assumption that no radiation injury occurs below a specified dose. Time-since-encposure (TSE) model A model in which the risk is not constant but varies with the time after exposure. Transformed cells. Tissue culture cells changed in vitro from growing in an orderly pattern and exhibiting contact inhibition to growing in a pattern more like that of cancer cells, due to the loss of contact inhibition. Transolocai?on. A chromosome aberration resulting from chromosome breakage and subsequent structural rearrangement of the parts be- tween the same or different chromosomes. 7fisomy. The presence of an additional (third) chromosome of one type In an otherwise diploid cell. 7hmorigenici~y. Ability of cells to proliferate into tumors when inoculated into a specified host organism under specified conditions. unitsa Becquerel (SI) Curie Gray (SI) Rad Rem Sievert (SI) aInternational Units are designated SI. Conversion Factors 1 disintegration/s = 2.7 x 10-~ Ci 3.7 x 10~° disintegrations/s = 3.7 x 10~° Bq 1 J/kg - 100 red 100 erg/g - 0.01 Gy 0.01 Sievert 100 rem UNSCEAR. United Nations Scientific Committee on the Effects of Atomic Radiation publishes periodic reports on sources and effects of ionizing radiation. x radiation. Also x rays; penetrating electromagnetic radiation, usually produced by bombarding a metallic target with fast electrons in a high vacuum. Xeroderma pigmentosum (my. An inherited disease in which skin cells are highly susceptible to sun-induced cancer; XP cells have a defect in DNA repair after ultraviolet irradiation which apparently accounts for the propensity for this neoplasm.
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Representative terms from entire chapter: