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Biological Effects of Ionizing Radiation

Objectives. Describe how ionizing radiation interacts with biological materialDiscuss the major factors that influence the severity or type of biological effectDefine terms describing biological effectDefine radiation dose quantitiesDescribe meaning of

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Biological Effects of Ionizing Radiation

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    1. Biological Effects of Ionizing Radiation Prof. Hamby

    2. Objectives Describe how ionizing radiation interacts with biological material Discuss the major factors that influence the severity or type of biological effect Define terms describing biological effect Define radiation dose quantities Describe meaning of “dose-response” Define stochastic and non-stochastic processes

    3. Ionizing Radiation Radiation having adequate energy to ionize atoms, dissociate molecules, or alter nuclear structures Particles, alpha, beta, electrons, neutrons, protons Electromagnetic waves, x-rays, gamma rays Direct or indirect ionization of atoms Canberra; “Gas-Filled Detectors A gas-filled detector is basically a metal chamber filled with gas and containing a positively biased anode wire. A photon passing through the gas produces free electrons and positive ions. The electrons are attracted to the anode wire and collected to produce an electric pulse. At low anode voltages, the electrons may recombine with the ions. Recombination may also occur for a high density of ions. At a sufficiently high voltage nearly all electrons are collected, and the detector is known as an ionization chamber. At higher voltages the electrons are accelerated toward the anode at energies high enough to ionize other atoms, thus creating a larger number of electrons. This detector is known as a proportional counter. At higher voltages the electron multiplication is even greater, and the number of electrons collected is independent of the initial ionization. This detector is the Geiger-Müller counter, in which the large output pulse is the same for all photons. At still higher voltages continuous discharge occurs. The different voltage regions are indicated schematically in Figure 1.3. The actual voltages can vary widely from one detector to the next, depending upon the detector geometry and the gas type and pressure.Canberra; “Gas-Filled Detectors A gas-filled detector is basically a metal chamber filled with gas and containing a positively biased anode wire. A photon passing through the gas produces free electrons and positive ions. The electrons are attracted to the anode wire and collected to produce an electric pulse. At low anode voltages, the electrons may recombine with the ions. Recombination may also occur for a high density of ions. At a sufficiently high voltage nearly all electrons are collected, and the detector is known as an ionization chamber. At higher voltages the electrons are accelerated toward the anode at energies high enough to ionize other atoms, thus creating a larger number of electrons. This detector is known as a proportional counter. At higher voltages the electron multiplication is even greater, and the number of electrons collected is independent of the initial ionization. This detector is the Geiger-Müller counter, in which the large output pulse is the same for all photons. At still higher voltages continuous discharge occurs. The different voltage regions are indicated schematically in Figure 1.3. The actual voltages can vary widely from one detector to the next, depending upon the detector geometry and the gas type and pressure.

    4. Energy Deposition Radiation interacts by either ionizing or exciting the atoms or molecules in the body (water) Energy is deposited and absorbed as a result of these interactions Absorbed Dose is defined as the energy absorbed per unit mass of material (tissue in this case)

    5. Biological Damage Damage can occur at various biological levels Sub-cellular Cellular (cell death) Organ (disfunction) Organism (cancer, death)

    6. Cellular Radiosensitivity

    7. Acute Radiation Syndrome Sub-clinical 25 - 200 rads; no symptoms, but signs Hematopoietic 200 - 600 rads; changes in blood Gastrointestinal 600 - 1000 rads; intestinal lining failure Cerebral > 1000 rads; nervous system failure

    8. Factors Influencing Biological Effect Total absorbed energy (dose) Dose rate Acute (seconds, minutes) Chronic (days, years) Type of radiation Source of radiation External Internal Age at exposure

    9. Factors Influencing Biological Effect Time since exposure Area or location being irradiated Localized (cells, organ) Extremities (hands, forearms, feet, lower legs) Entire body (trunk including head) Superficial dose (skin only - shallow) Deep tissue (“deep dose”)

    10. Terms Acute exposure - dose received in a short time (seconds, minutes) Acute effects - symptoms occur shortly after exposure Chronic exposure - dose received over longer time periods (hrs, days) Delayed effects - symptoms occur after a latent (dormant) period

    11. Terms Somatic effects - those which occur in the person exposed Genetic effects - those which occur in the offspring of exposed persons Stochastic effects - likelihood of effect is random, but increases with increasing dose Non-stochastic effects - likelihood of effect is based solely on dose exceeding some threshold

    12. Radiation Dosimetry Radiation dose quantifies energy deposition Dose categories: local; whole body; extremity shallow; deep internal; external

    13. Dosimetric Quantities Erythema; Photographic fog Exposure (1 R = 1 SC/cm3) Defined for photons in air SI definition: 1 X unit = 1 C/kg Absorbed Dose, D (1 rad = 100 ergs/gm) Defined for all radiations/all media SI definition: 1 Gy = 1 J/kg = 100 rads 1 rad (tissue) ~ 1 R (air)

    14. Radiation Quality Not all radiations are created equal What is the “quality” of radiation? Linear Energy Transfer (LET) Energy absorbed per unit length (keV/mm) Essentially a measure of “ionization density”

    15. Relative Biological Effectiveness RBE is an empirically determined measure of radiation quality Expresses the different absorbed dose required by two radiations in order to cause the same endpoint Biological endpoint is undefined Standard radiations are either 250 kVp x-rays or 60Co gamma rays

    16. Radiation Quality

    17. Dosimetric Quantities Dose Equivalent, H (rem) Used to “normalize” over different radiation types Quality factor, QF, describes ionization density (wR) QF related to both LET and RBE H = D • QF SI definition: 1 Sv = 100 rem

    18. Dosimetric Quantities Fatal cancer is the biological endpoint of importance Estimates have been made of organ-specific risks of cancer fatality Some cancers can be treated successfully Therefore, need to consider individual organ risks

    19. Dosimetric Quantities Effective Dose Equivalent, E (rem) Used to “normalize” over different organ radio-sensitivities Tissue weighting factor, wT, describes relative cancer risk E = S (H • wT) SI definition: still, 1 Sv = 100 rem Unit of record

    20. Dosimetric Quantities Internal Dose External Dose Committed Dose Cumulative Dose Population Dose EDE CEDE TEDE

    21. Dose-Response

    22. Non-Stochastic (Deterministic) Effects Occurs above threshold dose Severity increases with dose Alopecia (hair loss) Cataracts Erythema (skin reddening) Radiation Sickness Temporary Sterility

    23. Stochastic (Probabilistic) Effects Occurs by chance Probability increases with dose

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