Lecture 4-Biological Effects of Ionizing Radiation


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Lecture 4-Biological Effects of Ionizing Radiation

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  • 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.
  • Lecture 4-Biological Effects of Ionizing Radiation

    1. 1. Biological Effects of Ionizing Radiation Prof. Hamby
    2. 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. 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
    4. 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. 5. Biological Damage Damage can occur at various biological levels Sub-cellular Cellular (cell death) Organ (disfunction) Organism (cancer, death)
    6. 6. Cellular Radiosensitivity Cells that divide more rapidly are more sensitive to the effects of radiation ... … essentially because the resulting effect is seen more rapidly.
    7. 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 LD50/30 ~ 400 rads
    8. 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. 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. 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. 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. 12. Radiation Dosimetry radiation interaction energy deposition biological response Radiation dose quantifies energy deposition Dose categories: local; whole body; extremity shallow; deep internal; external
    13. 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. 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/µm) Essentially a measure of “ionization density”
    15. 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. 16. Radiation Quality The ionization density is different among radiation types. X-ray -- not many ionizations Alpha particle -- very high density Beta particle -- high density at end
    17. 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. 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. 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 = Σ (H • wT) SI definition: still, 1 Sv = 100 rem Unit of record
    20. 20. Dosimetric Quantities Internal Dose External Dose Committed Dose Cumulative Dose Population Dose EDE CEDE TEDE
    21. 21. Dose-Response “Dose-Response Curves” Response (Cancer Fatality) Dose
    22. 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. 23. Stochastic (Probabilistic) Effects Occurs by chance Probability increases with dose Carcinogenesis Mutagenesis Teratogenesis