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| nonmitotic, or nondivision, form of cell death that occurs when cells die without attempting division during the interphase portion of the cell life cycle is termed apoptosis, or programmed cell death. |
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| Cells vary in their radiosensitivity. This fact is particularly important in determining the types of cancer cells that will respond to radiation therapy. A classic method of displaying the sensitivity of a particular type of cell to radiation is the cell survival curve |
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| is a potential outcome when ionizing radiation interacts with a DNA macromolecule. These breaks may occur in one or both strands (sugar-phosphate chains) of the DNA ladderlike structure and are discussed previously in the discussion of direct action. |
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| When ionizing particles interact directly with vital biologic macromolecules |
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| damage to a cell primarily through an indirect action that involves the production of molecules called free radicals (solitary atoms, e.g., an unpaired hydrogen atom [H], or most often a combination of atoms that are very chemically reactive single entities as a result of the presence of unpaired electrons). |
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It does not relinquish all its energy quickly. When low-LET radiation interacts with biologic tissue, it causes damage to a cell primarily through an indirect action that involves the production of molecules called free radicals |
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Term
| law of Bergonié and Tribondeau |
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Definition
| It states that the radiosensitivity of cells is directly proportional to their reproductive activity and inversely proportional to their degree of differentiation. Thus the most pronounced radiation effects occur in cells having the least maturity and specialization or differentiation, the greatest reproductive activity, and the longest mitotic phases. |
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Term
| linear energy transfer (LET) |
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Definition
| When passing through a medium, ionizing radiation may interact with it during its passage and as a result lose energy along its path (called a track). The average energy deposited per unit length of track is called linear energy transfer (LET) |
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| In general, the interaction of high-energy radiation with a DNA molecule causes a loss of or change in a nitrogenous base on the DNA chain. |
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| oxygen enhancement ratio (OER) |
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Definition
| When irradiated in an oxygenated, or aerobic state, biologic tissue is more sensitive to radiation than when it is exposed to radiation under anoxic (without oxygen) or hypoxic (low-oxygen) conditions. This is known as the oxygen effect. |
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| If ionizing radiation interacts with a DNA macromolecule, the energy transferred could rupture one of its chemical bonds and possibly sever one of the sugar-phosphate chain side rails, or strands, of the ladderlike molecular structure (single-strand break) |
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Term
| radiation weighting factor (WR) |
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Definition
| s used to calculate the equivalent dose (EqD) to determine the ability of a dose of any kind of ionizing radiation to cause biologic damage. The WR values are similar to the values of RBE for any particular type of radiation. |
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Term
| relative biologic effectiveness (RBE) |
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Definition
| describes the relative capabilities of radiation with differing LETs to produce a particular biologic reaction. RBE of the type of radiation being used is the ratio of the dose of a reference radiation (conventionally 250-kVp x-rays) to the dose of radiation of the type in question that is necessary to produce the same biologic reaction in a given experiment. |
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| e biologic effects of exposure to radiation stem primarily from the ionizations occurring at sensitive cellular points secondary to energy transfers from that radiation. These affected locations in a cell or, more specifically, on a vital molecule within the cell are known as “targets.” Whether or not such locations are struck by radiation is a random process. From all existing evidence, it appears that producing a serious effect usually requires more than one radiation “hit” on a specific target. The damage from a single hit normally is not conclusive because of repair mechanisms. This concept of radiation damage resulting from discrete and random events is known as target theory. |
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| This means that this form of radiation can travel through space in the form of a wave but can interact with matter as a particle of energy. For this reason, x-rays may be described as both waves and particles. |
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| Ionizing radiation damages living systems by ionizing (removing electrons from) the atoms comprising the molecular structures of these systems |
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| Characteristics of ionizing radiation |
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| Radiation may be divided into two general categories according to its LET |
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| In living systems, biologic damage resulting from exposure to ionizing radiation may be observed on three levels: |
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- Molecular
- Cellular
- Organic
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