Dose Units and Dosimetry Radiation Units Dose Equivalent Effective Dose Equivalent Or “Dose”

Dose units and dosimetry ¾ Radiation Absorbed Dose ¾ Dose Equivalent ¾ Effective Dose Equivalent ¾ Collective Effective Dose Equivalent or Collective Dose ¾ Dose Rate Dose Equivalent Radiation Units ¾ Measures the Relative Biological Effectiveness (RBE) of different types OLD NEW of radiation - i.e. the damage caused by different types of radiation; Roentgen (R) Measure of air exposure ¾ Converts to a common unit RAD Measure of absorbed dose i.e. amount GRAY of energy absorbed from beam DOSE EQUIVALENT = RADIATION ABSORBED DOSE x Q REM Allows for different biological effects SIEVERT Q= Quality factor of different types of radiation. Provides X-rays, gamma rays Q = 1 a common unit of DOSE EQUIVALENCE Neutrons Q = 10 Alpha particles Q = 20 CONVERSION 100 RAD = 1 GRAY (Gy) 100 REM = 1 Sievert (Sv) Effective Dose Equivalent or “Dose” Example of Effective Dose Equivalent or “Dose” ¾ Measure which allows doses from investigations of different parts of the body to be compared, by converting all doses to ¾ Thyroid examination an equivalent whole body dose Measured Radiation Absorbed Dose =10mGy= 10 mGy Effective dose equivalent = Dose equivalent x W W = weighting factor, i.e. Testes, ovaries = 0.25 Dose equivalent (Q = 1) = 10 mSv Breast = 0.15 Effective dose equivalent (W = 0.03) = 10 x .03 Thyroid = 0.03 DOSE = 0.3 mSv S.I. unit - Sievert (Sv); subunit - milliSievert mSv 1 Low Dose Effects Ionizing Radiation as a Mutagen (Weak) ¾ First shown by H.J. Muller in 1927 ¾ Principle effects are non-lethal changes in cells -using fruit flies (won Nobel Prize) ¾ Usually occur at doses less than 10 cgy ( rad ) ¾ Radiation increases spontaneous mutation rate ¾ No evidence for a threshold Direct Damage: I Radiation Carcinogenesis ¾ DNA or RNA in a chromosome takes direct hit from x-ray photon disrupting bonds between nucleic acids. ¾ May cause: Cell death Abnormal replication (cancer) Failure of transfer of information i.e. protein synthesis Direct Damage: II Indirect Damage ¾ Somatic cells (non-reproductive) Æ radiation induced ¾ 70% of cells is water + - malignancy ¾ Radiation + H2O Æ H2O + e + + - H2O Æ H + OH - - ¾ Genetic cells Æ congenital abnormalities H2O + e Æ H2O - - H2O Æ H+ OH H and OH are free radicals H + H Æ H2 (Hydrogen gas) OH + OH Æ H2O2 (Hydrogen peroxide) H2 + H2O2 Æ Tissue damage 2 Indirect Damage (Summary) Somatic vs Genetic Radiation Effects ¾ Irradiation results in ionization of tissues at the atomic level ¾ Somatic: non-reproductive cells e.g. muscle, bone. Ionization ¾ Physical damage > atoms highly reactive High doses > cataracts, reduced blood supply, ¾ Chemi cal change > molecu le now has different chem ica l cancer structure ¾ ¾ Biological function > changed or impaired Genetic: Reproductive cells if damaged by radiation Biological effect may result in cell damage can produce congenital abnormalities Radiation Effects at Tissue/ Organ Level Radiation damage Additive with repeated exposures ¾ If few cells die – no clinical change Repair processes may not be complete ¾ As dose increases, cell deaths increase Æ clinical changes following initial exposure ¾ Severity of changes are proportional to cell loss SbSubsequen t exposures thus cause more ¾ Short term: hypoplasia Æ atrophy damage ¾ Long term: changes related to blood vessel necrosis, fibrosis, stenosis. O2 transport decreases Laws of Bergonie and Tribondeau Bergonie and Tribondeau Radiation response of tissues depend on: ¾ Number of cells in active proliferation ¾ Number of cells in differentiation ¾ Number of future divisions/mitotic future Other factors: ¾ Also: Oxygen tension and the capacity of cells to repopulate. 3 Cell/tissue/organ types Stochastic and non-stochastic effects ¾ Stochastic: Can occur at any dose More sensitive to ionizing radiation ex. Cancer, mutations, genetic defects. ¾ Embryonic cells - ¾ Non-stochastic effects: ¾ Rapidly multiplying cells – Effect is directly proportional to the radiation dose/dose ex. Bone marrow, some blood cells, reproductiv rate. ex. skin erythema, cataract, whole body radiation syndromes etc. Acute radiation syndromes Prodrome: subclinical ¾ Loss of apetite ¾ Prodrome: subclinical ¾ Nausea ¾ Hematopoietic ¾ Vomitting ¾ Apathy ¾ GIT ¾ Listlessness ¾ CNS ¾ Fever, sometimes ¾ Anemia ¾ Diarrhoea Hematopoietic syndrome Gastrointestinal Syndrome Main tissue affected: Bone marrow ¾ Threshold: 100 rad Main tissue affected: Small intestine ¾ Latent period: 2-3 weeks ¾ Threshold: 500 rad ¾ Death threshold: 200 rads ¾ Latent period: 3-5 days ¾ Death time: 2-8 weeks ¾ Death threshold: 1000 rads Underlying pathology: Bone marrow atrophy: ¾ Death time: 3 days - 2 weeks ¾ Infection Underlying pathology: Depletion of intestinal epithelium and neutropenia ¾ Hemorrhage from H. syndrome ¾ Anemia Treatment: Blood/plasma/platelet transfusion; antibiotics; isolated ¾ Malaise, GI malfunction, anorexia, fever, nausea, vomiting, dehydration, environment and in severe cases, bone marrow transplant. diarrhoea, circulatory collapse, electrolyte imbalance etc. 4 CNS Syndrome Background Radiation 3.6 mSv (360 mrem) per year Main tissue affected: Brain ¾ Threshold: 2000 rads ¾ Latent period: 1/4 - 3 hours Radon 55% ¾ Death threshold: 5000 rads ¾ Death time: < 3 days : Usually 24-48 hrs. Cosmic 8% Terrestrial 8% Underlying pathology: Vasculitis, encephalitis, meningitis, edema Internal 11% ¾ Symptoms: Lethargy, tremors, convulsions, ataxia, coma Medical/artificial 18% Is there a safe dose of x-rays? Decision to Use Radiodiagnostic Procedures ¾No. Unproven assumption of no threshold dose but a linear relationship between dose and probability of damage. Numerical Comparisons of Risks RISK ESTIMATION, PROTECTION AND SAFETY 5 Radiation Safety Basic question What is the benefit to the patient from the radiographs you prescribe? Types of Ionizing Radiation Patient benefit is directly linked to the Electromagnetic and Particulate radiation: diagnostic information provided by the ¾ Electromagnetic radiation: pure form of energy with no mass; emitted in radiograph wave form. ¾ Particulate radiation: Constituted by sub-atomic particles that have mass, energy, and sometimes, charge; Eg. Alpha, beta (negative & positive), protons, neutrons etc. as in radioactive decay. Electromagnetic radiation X and gamma radiation are penetrating radiation XX--rays:rays: Formed from energy transfers involving electrons Stopped by lead found in medical uses ϒ--rays:rays: From the nucleus when excess energy is emitted in the unstable state, interactions of nucleus with other particles in the region of the electrostatic field of the nucleus. Ex. Radioactive decay. Naturally present in soil and cosmic radiation and are EXTERNAL HAZARDS 6 Ionizing Radiation can deposit energy in neighboring atoms resulting in the removal of electrons. Non-Ionizing Radiation Does not have enough energy to remove electrons from surrounding atoms Alpha Radiation is only a hazard when inside your body Particulate radiation: (internal hazard) Alpha α++ Your skin will stop it internal - Electron ′ hazard Beta Positron ′+ stopped by Proton p+ paper 0 found in soil, radon and Neutron n other radioactive materials Beta Radiation is a Skin, Eye and Internal Hazard skin, eye and internal hazard stopped by plastic found in natural food, air and water Neutrons have no charge & can penetrate deep 7 UNSTABLE atoms emit energy ALARA As RF μwave infrared visible uv x-ray γ-ray cosmic Low lowlow energyenergy high energy As non-ionizing ionizing radiation Reasonably Achievable Background & manufactured radiation in the U.S. contributes 360 mrem per year Radon – 200 Sources of human exposure to radiation Cosmic - 28 Diet - 40 Terrestrial - 28 Manufactured sources ofradiation contribute an average of 60 mrem/year Altitude Cigarette smoking – 1300 mrem Round trip US by air Higher the altitude, the more the exposure 5 mrem per trip Medical – 53 mrem to cosmic radiation Smoke detectors - 0.0001 mrem People in Denver, CO exposed to more radiation than Floridians, for example Fallout < 1 mrem Building materials --3.63.6 mrem 8 Annual Dose Limits Occupational exposure Classified 1 Region General Public ¾ Dental Office (1988) 0.2 mSv per year (3 weeks background) Workers ¾ World data for 1990-1994 show a mean annual occupational dose Whole Body 50 mSv 1 mSv of only 0.06 mSv for dental workers (UNSCEAR, 2000) ¾ Dental personnel are not expected thus, to receive occupational exposures greater than recommended threshold for monitoring of 1 mSv per year. 50mSv per year OR 1mSv per week Ref: (with two weeks of annual vacation) 1. White & Pharoah.Oral Radiology. 2000. p 48. NCRP: Exposure of the US population from occupational radiation, NCRP Report 101, 1989. (Uses 1980 data) Dental rad studies on pregnant patients Instructions Concerning Prenatal Radiation Exposure ¾ You may expose: if the benefits outweigh the risks NCRP Regulatory Guide June 1999 at any part of the pregnancy ¾ Pregnant worker should notify dentist in writing, the approx ¾ However, you should not expose anybody to ROUTINE x- ray exams (i.e every 6, 12 months …) including pregnant date of conception women ¾ Reassure the patient that there is minimal risk to the baby – ¾ Dentist must counsel worker that because of very low use lead apron for reassurance doses it is safe to continue working with x-rays ¾ Unless the primary beam is directed at the abdomen, lead aprons are not necessary. RADIATION PROTECTION Benefits of Using Selection Criteria PATIENT PROTECTION Radiographic selection criteria – avoids unnecessary exposures ¾ 43% reduction in number

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