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Ch 16. Radiation Protection

Ch 16. Radiation Protection. The physics of Radiation Therapy, pp. 200 - 224. 16.1 Dose Equivalent 16.2 Effective Dose Equivalent 16.3 Background Radiation 16.4 Low-Level Radiation Effects 16.5 Effective Dose Equivalent limits 16.6 Structural Shielding Design Primary Radiation Barrier

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Ch 16. Radiation Protection

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  1. Ch 16. Radiation Protection The physics of Radiation Therapy, pp. 200 - 224

  2. 16.1 Dose Equivalent • 16.2 Effective Dose Equivalent • 16.3 Background Radiation • 16.4 Low-Level Radiation Effects • 16.5 Effective Dose Equivalent limits • 16.6 Structural Shielding Design • Primary Radiation Barrier • Secondary Barrier for Scattered Radiation • Secondary Barrier for Leakage Radiation • Door Shielding • Protection Against Neutrons

  3. 16.1 Dose Equvalent • Factor affecting the biological effects of radiation • Dose • Type of radiation • Dose equivalent (H) • The dosimetric quality relevant to radiation protection H = D • Q D = absorbed dose Q = the quality factor • Units • Sivert (Sv) SI unit • 1 Sv = 1 J/kg • Rem • Unit of dose is rad • 1 rem = 10-2 J/kg (Sv)

  4. 16.1 Dose Equvalent • Quality factor Q • Base on a range RBE related to the LET of the radiation • Independent of the organ or tissue

  5. 16.1 Dose Equivalent • 16.2 Effective Dose Equivalent • 16.3 Background Radiation • 16.4 Low-Level Radiation Effects • 16.5 Effective Dose Equivalent limits • 16.6 Structural Shielding Design • Primary Radiation Barrier • Secondary Barrier for Scattered Radiation • Secondary Barrier for Leakage Radiation • Door Shielding • Protection Against Neutrons

  6. 16.2 Effective Dose Equivalent • Dose equivalent for various tissue may differ markedly • Whole body exposure are rarely uniform • Tissues vary in sensitivity • Effective dose equivalent • The sum of the weighted dose equivalents for irradiated tissues or organs • HE = WTHT • WT = weighting factor of tissue T • HT = the mean dose equivalent by tissue t

  7. Weighting factors • The proportionate risk (stochastic) of tissue when body from risk coefficients

  8. 16.1 Dose Equivalent • 16.2 Effective Dose Equivalent • 16.3 Background Radiation • 16.4 Low-Level Radiation Effects • 16.5 Effective Dose Equivalent limits • 16.6 Structural Shielding Design • Primary Radiation Barrier • Secondary Barrier for Scattered Radiation • Secondary Barrier for Leakage Radiation • Door Shielding • Protection Against Neutrons

  9. 16.3 Background Radiation • Radiation from the natural environment • Terrestrial radiation • e.g. elevation level of radon in many building • Emitted by naturally ocurring 238U in soil • Annual dose equivalent to bronchial epithelium = 24 mSv (2.4 rem) • Cosmic radiation • e.g. air travel • At 30,000 feet, the dose equivalent is about 0.5 mrem/h • Radiation element in our bodies • e.g. mainly from 40K • Emits β, γrays; T1/2 = 1.3 × 109 years

  10. 16.3 Background Radiation • Radiation from various medical procedures • The average annual genetically significant dose equivalent in 1970 = 20 mrem/year • Occupational exposure excluded exposure from • Natural background • Medical procedures

  11. 16.1 Dose Equivalent • 16.2 Effective Dose Equivalent • 16.3 Background Radiation • 16.4 Low-Level Radiation Effects • 16.5 Effective Dose Equivalent limits • 16.6 Structural Shielding Design • Primary Radiation Barrier • Secondary Barrier for Scattered Radiation • Secondary Barrier for Leakage Radiation • Door Shielding • Protection Against Neutrons

  12. 16.4 Low-Level Radiation Effects • Low level radiation < Dose required to produce acute radiation syndrome > Dose limits recommended by the standards

  13. 16.4 Low-Level Radiation Effects • Genetic effects • Radiation-induced gene mutation • Chromosome breaks and anomalies • Neoplastic disease • e.g. Leukemia, thyroid tumors, skin lesions • Effect on growth and development • Adverse effects on fetus and young children • Effect on life span • Diminishing of life span • Premature aging • Cataracts – opacification of the eye lens

  14. The NCRP defines two general categories for harmful effects of radiation Stochastic effects The probability ofoccurrence increases with increasing absorbed dose • The severity doesnot depend on the magnitude of the absorbed dose • All or none phenomenon • e.g. development of a cancer genetic effect • Nothreshold dose Nonstochastic effect • Increase in severity with increasing absorbed dose • Damage to increasing number of cells and tissues • e.g. organ atrophy, fibrosis, cataracts, blood changes, sperm counts • Possible to set threshold dose

  15. 16.1 Dose Equivalent • 16.2 Effective Dose Equivalent • 16.3 Background Radiation • 16.4 Low-Level Radiation Effects • 16.5 Effective Dose Equivalent limits • 16.6 Structural Shielding Design • Primary Radiation Barrier • Secondary Barrier for Scattered Radiation • Secondary Barrier for Leakage Radiation • Door Shielding • Protection Against Neutrons

  16. 16.5 Effective Dose Equivalent limits • The criteria for recommendations on exposure limits of radiation workers • At low radiation levels, the nonstochastic effects are essentially avoided • The predicted risk for stochastic effects should not be greater then the average risk of accidental death among worker in “safe” industries • ALARA principles should be followed • The risk are kept as low as reasonably achievable, taking into account, social and economic factors

  17. “safe” industries are defined as • Annual fatality accident rate of ≦1/ 10,000 workers • An average annual risk = 1 × 10-4 • Data from studies for radiation industries • Average fatal accident rate < 0.3 × 10-4 • The radiation industries is comparatively more “safe” • The total risk coefficient of the radiation industries is assumed to be 1 × 10-2 (1 × 10-4 rem-1) • Equal fatal risk of 1 × 10-4 for the following familiar context • 40,000 miles of travel by air • 6,000 miles of travel by car • 75 cigarettes • Merely living 1.4 days for a man aged 60

  18. Occupational and Public Dose Limits

  19. Occupational and Public Dose Limits

  20. Negligible Individual Risk Level (NIRL) • A level of average annual excess risk of fatal health effects attributable to irradiation, below which further effort to reduceradiation exposure to individual is unwarranted • Trivial compare to the risk of fatality associated with ordinary, normal social activities • Dismissed from consideration • Aim: • having a reasonable negligible risk level that can be considered as a threshold • Below which efforts to reduce the risk further would not be warranted • The annual NIRL = 1 × 10-7 • Corresponding dose equivalent = 0.01 mSv (0.001 rem) • Corresponding life time risk (70 years) = 0.7 × 10-5

  21. Example of risk calculation • Question • Calculation the risk followings: • Radiation workers • Members of the general public • NIRL (corresponding to respective annual effective dose equivalent limits) • Risk coefficient of 10-2Sv-1 (10-4rem-1)

  22. Example of risk calculation • Answer • Annual effective dose equivalent limit for radiation workers = 50 mSv (5 rem) • Annual risk = 5 rem × (10-4 rem-1) • = 5 × 10-4 • Annual effective dose equivalent limit for members of general public = 1 mSv (0.1 rem) • Annual risk = 0.1 rem × (10-4 rem-1) • = 10-5 • Annual effective dose equivalent limit for NIRL = 0.01 mSv (0.001 rem) • Annual risk = 0.001 rem × (10-4 rem-1) • = 10-7

  23. 16.1 Dose Equivalent • 16.2 Effective Dose Equivalent • 16.3 Background Radiation • 16.4 Low-Level Radiation Effects • 16.5 Effective Dose Equivalent limits • 16.6 Structural Shielding Design • Primary Radiation Barrier • Secondary Barrier for Scattered Radiation • Secondary Barrier for Leakage Radiation • Door Shielding • Protection Against Neutrons

  24. Primary Barrier Secondary barrier 16.6 Structural Shielding Design • Design of protective barriers • Ensure that the dose equivalent received by any individual dose not exceed the applicable maximum permissible value • Dose equivalent limits of “controlled area” and “uncontrolled area” • Controlled area: 0.1 rem/wk (5 rem/yr) • Uncontrolled area: 0.01 rem/wk (0.5 rem/yr) • Protection against 3 type of radiation • The primary radiation • The scattered radiation • The leakage radiation (from source housing)

  25. Factors associated with the calculation of barrier thickness • Workload (W) • Use factor (U) • Occupancy factor (T) • Distance (d)

  26. Workload (W) • For <500 kVp x-ray machine • W = Maximum mA × beam “on” time • = min/week • For MV machine • W = weekly dose delivered at 1 m from the source • = no. of patient treated/wk × dose delivered/p’t at 1 m • = rad/wk (at 1m) • Use Factor (U) • U = Fraction of operation time that radiation is directed toward a • particular barrier • Depending on technique use

  27. Occupancy Factor (T) • T = Fraction of operating time during which the area of interest is occupied by the individual • Distance (d) • d = distance from the radiation source to the area to be protected • Applied inverse square law

  28. A. Primary Radiation Barrier • Determine the thickness of the primary radiation barrier • P= Maximum permissible dose equivalent for the area to be protected • Controlled area: 0.1 rad/wk • Non-controlled area: 0.01 rad/wk • B = transmission factor • Determining the barrier thickness by consulting broad beam attenuation curves for the given beam energy

  29. Figure 16.1. Transmission through concrete of x-rays produced by 0.1- to 0.4-MeV electrons, under broad beam conditions Figure 16.2. Transmission of thick-target x-rays through ordinary concrete, under broad-beam conditions

  30. A. Primary Radiation Barrier • The choice of barrier material • e.g. concrete, lead, steel • Depends on structural spatial considerations • Calculation of equivalent thickness of various material • Comparing tenth value layers (TVL) for the given beam energy

  31. B. Secondary Barrier for Scattered Radiation • Factors affecting the amount of scattered radiation • Beam intensity • Quality of radiation • The area of the beam at scatterer • The scattering angle

  32. For MV beams, αusually be assumed at 90° scatter = 0.1%

  33. B. Secondary Barrier for Scattered Radiation • Energy of the scatter • For orthovoltage radiation • Beam energy: Scatter = incident (assumed) • For MV beams • Beam energy at 90° scattered photon = 500 keV • Transmission of 500 kVp useful beam • Relatively lower energy in compare with the incident energy • Beam softening by Compton effect

  34. α－ fractional scatte (1 cm from scatterer; Beam area 400 cm2 incident at the scatterer) d － source to scatterer distance d’ － scatterer to the area of interest area F － area of the beam incident at the scatterer • Transmission factor of BS is required to reduced the scattered dose to the accepted level P • The barrier transmission of the scattering beam • The required thickness of the barrier can be determined for appropriate transmission curve

  35. C. Secondary Barrier for Leakage Radiation • Described in the NCRP Report No. 102 • The recommended leakage exposure rate for different energy of the beams (< 500 kVp) 5-50 kVp • <0.1 R (in any h at any point 5 cm from the source) > 50 kVp, < 500 kVp • < 1 R (in 1 h, at 1 m from the source) • < 30 R/h at 5 cm

  36. C. Secondary Barrier for Leakage Radiation • The recommended absorbed dose rate for different energy of the beam (> 500 kVp) > 500 kVp • < 0.2% of the useful beam dose rate • (any point outside the max field size, within a circular plane of radius 2 m) Cobalt teletherapy • Beam “off” position • < 2mrad/h (on average direction, 1m from the source) • < 10 mrad/h (in any direction, 1m from the source) • Beam “on” position • < 0.1% of the useful beam dose rate (1 m from the source)

  37. Transmission factor (BL)to reduce the leakage dose to the maximum permissible level (P) For machine < 500 kVp For MV machine I = maximum tube current Reason for “× 60”: conversion from h to min (R/h to R/min), ∵unit of W is mA-min/week Reason for “×0.001”: 0.1% leakage limit through the source housing

  38. The required thickness of the barrier can be determined for transmission curve of the primary beam • The quality of radiation: leakage ~ primary beam

  39. For MV machine • Leakage radiation > Scattered radiation (∵penetrating power of leakage radiation is greater) • For lower energy x-ray beam: • Leakage radiation ~ scattered radiation • For primary radiation barrier • Adequate protect against leakage & scattered radiation • For secondary radiation barrier • Calculate the difference between HVL required for scattering and leakage • > 3 HVL • Choose the thicker one • < 3 HVL • Choose the thicker one + 1 HVL

  40. D. Door Shielding • Advantages of the maze arrangement in treatment room • Reduces the shielding requirement of the door • Expose mainly to multiply scattered radiation Radiation experience scatter at least twice

  41. D. Door Shielding • The required door shielding • Repeat calculation of the barrier transmission factor BS by tracing different path of the scattered radiation • The attenuation curve of 500 kVp is used ∵Compton scatter of MV radiation at 90° < 500 kVp • In most cases, the required thickness of door shielding is < 6 mm lead

  42. E. Protection against Neutrons • Neutron contamination • High energy photon (> 10 MV) or electrons incident on the various materials of target, flattening filter, collimators and other shielding components • Increase rapidly in the range of 10 – 20 MV beam energy • The energy spectrum of emitted neutrons • Within the beam :range 1 MeV • Inside of the maze:few fast neutrons (> 0.1 MeV)

  43. E. Protection against Neutrons • Protection against neutrons should be considered in door shielding only • 1° and 2° barriers for x-ray shielding are adequate • Solution • Increase reflection from the walls by accelerator configuration • Longer maze (> 5 m) • Add a hydrogenous material (e.g. polyethylene, few inches) • Add steel or lead sheet

  44. E. Protection against Neutrons • Neutron capture γrays • Generated by thermal neutrons absorbed by the shielding door • Spectrum energies up to 8 MeV (mostly 1 MeV) • Solution • Thick lead sheet (high enerjgy γray) • Longer maze (reduce neutron fluence) • Practically, treatment room with long maze, the intensity of neutron capture γrays is low

  45. Thanks for your attention!

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