Basic Radiation Protection Terminology PDF - Radiation Measurement, Cell Damage

Units of Radiation Measurement  Gray---absorbed dose  Gray (Gya)---air kerma  Gray (Gyt)---absorbed dose in tissue  Sievert (Sv)---unit of effective and equivalent dose  Becquerel (Bq)---unit of radioactivity Ionizing Radiation  Ionizing radiation is able to remove electrons from atoms;...

Units of Radiation Measurement  Gray---absorbed dose  Gray (Gya)---air kerma  Gray (Gyt)---absorbed dose in tissue  Sievert (Sv)---unit of effective and equivalent dose  Becquerel (Bq)---unit of radioactivity Ionizing Radiation  Ionizing radiation is able to remove electrons from atoms; the process is called ionization  Ionization may cause unstable atoms, free electrons, or formation of new molecules harmful to the cell  Two types of cell damage may occur: somatic (damage to the cell itself) or genetic (damage to the cell's genetic code)  Natural background radiation is present in the environment  The greatest source of natural background exposure to humans is radon  Human-produced radiation is created by human activities (e.g., medical imaging) or inventions  CT accounts for the largest increase in total dose and medical dose to the population  Total radiation dose to the U.S. population has doubled since the 1980s Photon--Tissue Interactions  Attenuation describes changes in the intensity of the x-ray beam as it traverses the patient  Two primary photon--tissue interactions are significant in diagnostic x-ray procedures---photoelectric and Compton  Photoelectric interaction results in complete absorption of an incoming x-ray photon; this interaction produces contrast in the radiographic image  Compton interaction results in scattering of the incoming x-ray photon; scatter produced by this interaction must be removed from the beam before it strikes the image receptor  Compton scatter is the source of exposure to the radiographer or radiologist during fluoroscopy Annual Dose Limits  Annual dose limits are published by the NCRP  Effective dose limit is the upper boundary dose that can be absorbed, either in a single exposure or annually, with a negligible risk for somatic or genetic damage to the individual  The ALARA (as low as reasonably achievable) principle means that radiographers do what is possible to keep doses at minimal levels  The linear-nonthreshold relationship states that no level of radiation can be considered completely safe, and the degree of response is directly proportional to the amount of radiation received  Stochastic effects are randomly occurring effects of radiation; the probability of such effects is proportional to the dose (not related to the severity of effects)  Tissue reactions (deterministic effects) become more severe at high levels of radiation exposure but do not occur below a certain threshold dose  NCRP Report \#116 contains recommendations for annual dose limits  Annual effective dose limit for occupational exposure is 50 mSv  Cumulative effective dose limit = age (in years) × 10 mSv  Annual effective dose limit for the general public is 1 mSv for frequent exposure and 5 mSv for infrequent exposure  Effective dose limit for the embryo or fetus for all of gestation is 5.0 mSv Cell  Main parts of the cell are the cell membrane, cytoplasm, and nucleus  Nucleus contains DNA  Cytoplasm contains the organelles and water  Interphase: Portion of the cellular life cycle that occurs before mitosis  Mitosis: Somatic cell division; includes four phases: prophase, metaphase, anaphase, and telophase  When mitosis is complete, each new cell contains 46 chromosomes  Meiosis: Germ (sperm or ovum) cell division; halves the number of chromosomes in each cell so that the union of two germ cells produces a new cell with 46 chromosomes  LET: Amount of energy deposited per unit length of travel of radiation passing through matter  An increase in LET results in an increase in the potential for biological damage  RBE: Ability of radiation to produce biological damage; it varies with LET  Direct effect: Occurs when radiation transfers its energy directly to the DNA (the master molecule) or RNA  Mutation: Erroneous information passed to subsequent generations via cell division  Indirect effect: Occurs when radiation transfers its energy to the water in the cytoplasm; may cause radiolysis and produce free radicals or H2O2  Law of Bergonié and Tribondeau: Cells are most sensitive to radiation when they are immature, undifferentiated, and rapidly dividing  OER: If cells are more oxygenated, they are more susceptible to radiation damage  As cells mature and become specialized, they are less sensitive to radiation  Lymphocytes are the most radiosensitive blood cells in the body  Stem cells in bone marrow are especially radiosensitive  Epithelial tissue: Highly radiosensitive  Muscle: Relatively insensitive to radiation  Adult nerve tissue: Requires very high doses of radiation (beyond medical levels) to cause damage; is relatively insensitive to radiation  Immature sperm cells: Very radiosensitive  Ova in female fetus and child: Very radiosensitive  Ova radiosensitivity decreases until near middle age, then increases again  Somatic effects manifest in the individual being exposed  Early tissue effects: Hematopoietic syndrome, GI syndrome, central nervous system syndrome, erythema, epilation, decreased blood count  Late tissue effects: Cataractogenesis, embryologic effects  Stochastic effects: Carcinogenesis, genetic effects  Genetic effects: Manifest in the next generation because of damage to the DNA; follow a linear-nonthreshold curve  Doubling dose: Amount of radiation that causes the number of mutations in a population to double; estimated to be 1.56 Sv for humans Patient Exposure and Protection  Always observe the ALARA principle  Beam limiters: Collimator, cylinder cones, aperture diaphragms  PBL: Positive beam limitation, or automatic collimation  Beam filtration: Removes long-wavelength rays; total filtration must be at least 2.5-mm aluminum equivalent  Exposure factors: Use optimum kVp for the part; use lowest practical mAs  Grids remove scatter radiation from exit beam; increase total dose to the patient because of the increased mAs needed  Maintain a minimum of 12 inches source-to-skin distance for portable radiography  Use intermittent fluoroscopy  Maintain a minimum of 15 inches source-to-tabletop distance for fixed fluoroscopes  Maintain a minimum of 12 inches source-to-tabletop distance for portable fluoroscopes (15 inches preferred)  Monitor fluoroscopy timer that must sound alarm after 5 minutes (300 seconds) of beam-on time  Fluoroscopy foot switch must be dead-man type  Fluoroscopy dose at the tabletop is limited to no more than 100 mGya per minute  MMD: Average dose to active bone marrow  Cardinal principles of radiation protection: Time, distance, shielding; distance is the best protection  Dose is governed by the inverse square law  Lead apron: Must be at least 0.25-mm lead equivalent; should be at least 0.5-mm lead equivalent  The radiographer must never be exposed to the primary beam  Source of radiation exposure to radiographer is scatter radiation produced by Compton interactions in the patient during fluoroscopy, portable radiography, and surgical radiography  Scattered beam intensity is about the intensity of the primary beam at a 90-degree angle at a distance of 1 m from the patient  Beam collimation helps reduce the incidence of Compton interactions, resulting in decreased scatter from the patient  Primary protective barriers: Must be at least -inch lead equivalent and extend from the floor to a height of 7 feet  Secondary protective barriers: Must be at least -inch lead equivalent and extend from the primary protective barrier to the ceiling with a ½-inch overlap  Determinants of barrier thickness: Distance, occupancy, workload, use  Uncontrolled area: General public areas such as waiting rooms and stairways  Controlled area: Occupied by persons trained in radiation safety and wearing personnel monitoring devices  X-ray tube leakage: May not exceed 1 mGya per hour at a distance of 1 m from the housing  Fluoroscopic protective curtain: Minimum 0.25-mm lead equivalent  Bucky slot shield: Minimum 0.25-mm lead equivalent  Portable x-ray machine exposure switch must be on a cord at least 6 feet long  OSL dosimeters: Aluminum oxide layer stores energy that is released when exposed to a laser; correlate to dose; provide readings as low as 10 μGya; insensitive to environmental factors  TLDs: Store energy in lithium fluoride crystals that is released when heated; correlate to dose; provide readings as low as 50 μGya; insensitive to environmental factors  Digital ionization dosimeter: Measure exposures as low as 50 μGya  Handheld ionization chamber: Used to measure radiation in an area; measures doses of 10 μGya per hour  Geiger-Mueller detector: Used to detect radioactive particles; meter reads in counts per minute