Chapter 3.pdf

RADIATION BIOLOGY CHAPTER 3 1  Mechanisms of injury Theories of radiation injury Dose-response curve RADIATION INJURY Stochastic and nonstochastic radiation effects Sequence of radiation injury...

RADIATION BIOLOGY CHAPTER 3 1  Mechanisms of injury Theories of radiation injury Dose-response curve RADIATION INJURY Stochastic and nonstochastic radiation effects Sequence of radiation injury Determining factors for radiation injury 2 2  MECHANISMS OF INJURY Two possible mechanisms of Some x-rays do not reach the radiation injury dental x-ray film; they are absorbed by the patient’s tissue Chemical changes occur that result in Ionization biologic damage Free radical formation 3 3  IONIZATION Results when x-rays strike patient tissue  Produced through the photoelectric effect or Compton scatter  Results in formation of a positive atom and dislodged negative electron  This electron will interact with other atoms within the absorbing tissues, causing chemical changes within the cell that results in biologic damage 4 4 FREE RADICAL FORMATION Cell damage occurs primarily through formation of free radicals Free radicals are formed when an x-ray photon ionizes water  Free radical  An uncharged atom or molecule that exists with a single, unpaired electron in its outermost shell  Highly reactive and unstable 5 5 THEORIES OF RADIATION INJURY Damage to living tissue caused by exposure to ionizing radiation may result from  A direct hit and absorption of an x- ray photon within a cell  Absorption of an x-ray photon by water within a cell accompanied by free radical formation Two theories to describe how radiation damages biologic tissues  Direct theory  Indirect theory 6 6 DIRECT THEORY Cell damage results when ionizing radiation directly hits critical areas within the cell  This occurs infrequently 7 7 INDIRECT THEORY X-ray photons are absorbed within the cell and cause the formation of toxins, which in turn damage the cell  When x-ray photons are absorbed by water within a cell, free radical formation results  The free radicals combine to form toxins that damage cells 8 8 DOSE-RESPONSE CURVE Curve is used to correlate the damage of tissue with the dose of radiation received A linear, nonthreshold relationship is seen  The linear relationship indicates that the response of the tissues is directly proportional to the dose  The nonthreshold dose-response curve suggests that no matter how small the amount of radiation received, some biologic damage occurs 9 9  Stochastic effects A direct function of the dose No dose threshold; effects do not depend on the magnitude of the absorbed dose STOCHASTIC AND Examples: Cancer and genetic mutations NONSTOCHASTIC RADIATION Nonstochastic EFFECTS (deterministic) effects Somatic effects that have a threshold; effects increase in severity with increasing absorbed dose Examples: Erythema, loss of hair, cataracts, and decreased fertility 10 10 SEQUENCE OF RADIATION INJURY The time that elapses between exposure to ionizing radiation and the appearance of observable clinical Latent signs period Depends on the total dose of radiation received and the amount of time it took to receive the dose Period of A variety of cellular injuries may injury result Depending on a number of Recovery period factors, cells can repair the damage caused by radiation Effects of radiation exposure are additive Cumulative effects Unrepaired damage accumulates in tissues 11 11  Total dose DETERMINING Dose rate FACTORS Amount of tissue FOR irradiated RADIATION INJURY Cell sensitivity Age 12 12 RADIATION EFFECTS Short- term and long-term effects Somatic and genetic effects Radiation effects on cells Radiation effects on tissues and organs 13 13 SHORT- AND LONG-TERM EFFECTS Short-term effects  Associated with large doses of radiation in a short amount of time  Acute radiation syndrome (ARS)  Includes nausea, vomiting, diarrhea, hair loss, hemorrhage Long-term effects  Small doses absorbed repeatedly over a long period of time  Effects seen after years, decades, or generations  Cancer, birth abnormalities, genetic defects 14 14 SOMATIC AND GENETIC EFFECTS Somatic cells Genetic cells  All cells in the body except  The reproductive cells the reproductive cells Biologic effects of radiation can be classified as somatic or genetic Somatic effects Genetic effects  Not seen in the person  Seen in the person irradiated irradiated  Passed on to future  Not seen in future generations generations RADIATION EFFECTS ON CELLS A cell that is sensitive to radiation is termed radiosensitive; one that is resistant is termed radioresistant The response is determined by:  Mitotic activity  Cell differentiation  Cell metabolism 16 16 RADIATION EFFECTS ON TISSUES AND ORGANS Radiosensitive organs  Lymphoid tissue  Bone marrow  Testes  Intestines Radioresistant tissues  Salivary glands  Kidney  Liver 17 17 RADIATION EFFECTS ON TISSUES AND ORGANS Critical organ  An organ that, if damaged, diminishes the quality of a person’s life Critical organs exposed during dental radiographic procedures include  Skin  Thyroid gland  Lens of the eye  Bone marrow 18 18 RADIATION MEASUREMENTS Units of measurement Exposure measurement Dose measurement Dose equivalent measurement 19 19 UNITS OF MEASUREMENT Traditional (older) units of radiation measurement  Roentgen (R)  Radiation absorbed dose (rad)  Roentgen equivalent (in) man (rem) SI (newer) units of radiation measurement  Coulombs/kilogram (C/kg)  Gray (Gy)  Sievert (Sv) 20 20 EXPOSURE MEASUREMENT Roentgen No SI equivalent Roentgen measures Exposure is stated radiation by in coulombs per determining kilogram amount of ionization that occurs in air It does not describe amount of radiation absorbed 21 21 DOSE MEASUREMENT The amount of energy absorbed by tissue Traditional unit is the rad (radiation absorbed dose) SI equivalent is the gray 1 Gy = 100 rads 22 22 DOSE EQUIVALENT MEASUREMENT Dose equivalent measurement is used to compare biologic effects of different kinds of radiation  Traditional unit is the rem (roentgen equivalent man) SI equivalent is the sievert  1 Sv = 100 rems 23 23 MEASUREMENTS USED IN DENTAL IMAGING Milli means 1/1000  Used to express the small doses used in dental imaging 24 24  Sources of radiation exposure Risk and risk estimates RADIATION Dental radiation and RISKS exposure risks Patient exposure and dose Risk versus benefit of dental images 25 25 SOURCES OF RADIATION EXPOSURE Natural background Artificial or humanmade radiation radiation A form of ionizing radiation Resulting from modern that is ubiquitous in the technology environment  Includes consumer products,  Cosmic radiation fallout from atomic weapons, Stars and sun weapons production, and the nuclear fuel cycle  Terrestrial radiation Radioactive materials in the  Medical radiation including earth and air medical radiographic procedures, dental imaging, In the United States the fluoroscopy, nuclear medicine, average dose of background and radiation therapy radiation received by an individual ranges from 150 to 300 mrads per year 26 26  RISK AND RISK ESTIMATES ▪Potential risk of dental 1 in a million risks of a fatal imaging inducing a fatal outcome cancer in an individual has  10 miles on a bicycle been estimated to be 3 in 1  300 miles in an auto million  1000 miles in an airplane  Smoking 1.4 cigarettes a day ▪Risk of a person developing a cancer spontaneously is much higher, or 3300 in 1 million DENTAL RADIATION AND EXPOSURE RISKS Risk estimates  Thyroid gland  Bone marrow  Skin  Eyes 28 28 PATIENT EXPOSURE AND DOSE Film speed Collimation Technique Exposure factors 29 29 RISK VERSUS BENEFIT OF DENTAL IMAGES ▪Dental images should be prescribed for a patient only when the benefit of disease detection outweighs the risk of biologic damage ▪When dental images are properly prescribed and exposed, the benefit of disease detection far outweighs the risk of damage ▪ALARA 30 30