The physics of Radiation Therapy pp 200 224

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

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 A. Primary Radiation Barrier B. Secondary Barrier for Scattered Radiation C. Secondary Barrier for Leakage Radiation D. Door Shielding E. Protection Against Neutrons 2

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) 3

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 Table 16. 1 Recommended Quality Factors Radiation X-rays, γrays, and electrons Thermal neutrons Neutrons, heavy particles Quality Factor 1 5 20 Data are from NCRP. Recommendations on limits for exposure to ionizing radiation. Report No. 91 4

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 A. Primary Radiation Barrier B. Secondary Barrier for Scattered Radiation C. Secondary Barrier for Leakage Radiation D. Door Shielding E. Protection Against Neutrons 5

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 6

Weighting factors The proportionate risk (stochastic) of tissue when body from risk coefficients Table 16. 2. Recommended Values of the weighting Factors WT, for calculating Effective Dose Equivalent and the Risk Coefficients from Which They Were Derived Tissue (T) Gonads Breast Red bone marrow Lung Thyroid Bone surface Remainder Total Risk Coefficient WT 40 × 10 -4 Sv-1 (40 × 10 -6 rem-1) 25 × 10 -4 Sv-1 (25 × 10 -6 rem-1) 20 × 10 -4 Sv-1 (20 × 10 -6 rem-1) 5 × 10 -4 Sv-1 (5 × 10 -6 rem-1) 50 × 10 -4 Sv-1 (50 × 10 -6 rem-1) 165 × 10 -4 Sv-1 (165 × 10 -6 rem-1) 0. 25 0. 12 0. 03 0. 30 1. 00 From NCRP. Recommended on limits for exposure to ionizing radiation. Report No. 91. 7

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 A. Primary Radiation Barrier B. Secondary Barrier for Scattered Radiation C. Secondary Barrier for Leakage Radiation D. Door Shielding E. Protection Against Neutrons 8

16. 3 Background Radiation from the natural environment Terrestrial radiation e. g. elevation level of radon in many building Emitted by naturally ocurring 238 U in soil Annual dose equivalent to bronchial epithelium = 24 m. Sv (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 40 K 9

Table 16. 3. Estimated Total Dose Equivalent Rate for a Member of the Population in the United States and Canada from Various Sources of Natural Background Dose Equivalent Rate (m. Sv/y) Source Bronchial Epithelium Other Soft Tissues Bone Surfaces Bone Marrow Cosmic 0. 27 Cosmogenic 0. 01 0. 03 Terrestrial 0. 28 24 － － － 0. 35 1. 1 0. 50 25 0. 9 1. 7 1. 1 Inhaled In the body Rounded totals From NCRP. Exposure of the population in United States and Canada from national background radiation. 10

16. 3 Background 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 A. Primary Radiation Barrier B. Secondary Barrier for Scattered Radiation C. Secondary Barrier for Leakage Radiation D. Door Shielding E. 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 of occurrence increases with increasing absorbed dose The severity does not depend on the magnitude of the absorbed dose All or none phenomenon e. g. development of a cancer genetic effect No threshold dose Nonstochastic effect Increase in severity with increasing absorbed dose Damage to increasing number of cells and tissues 15 e. g. organ atrophy, fibrosis, cataracts, blood changes, sperm counts

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 A. Primary Radiation Barrier B. Secondary Barrier for Scattered Radiation C. Secondary Barrier for Leakage Radiation D. Door Shielding E. 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

Table 16. 4. Annual Fatality Rates from Accidents in Different Occupations Number of Workers Annual Fatal Accident Rate Occupation 3 × 10 (per 10, 000 Workers) Trade 24, 000 0. 5 Manufacturing 19, 000 0. 6 Service 28, 900 0. 7 Government 15, 900 0. 9 Transportation 5, 500 2. 7 Construction 5, 700 3. 9 Agriculture 3, 400 4. 6 Mining, quarrying 1, 000 6. 0 All industries (U. S. ) 104, 300 1. 1 From NCRP. Recommendations on limits for exposure to ionizing radiation. Report No. 91. 18

“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 19

Occupational and Public Dose Limits Table 16. 5. Summary of Recommendations A. Occupation exposure (annual) 1. Effective dose equivalent limit (stochastic effects) 2. Dose equivalent limits for tissues and organs (nonstochastic effects) a. Lens of eye b. All others (e. g. red bone marrow, breast, lung, gonads, skin and extremities) 50 m. Sv 5 (rem) 150 m. Sv (15 rem) 500 m. Sv (50 rem) 3. Guidance: cumulative exposure rem × age in 10 m. Sv × age (1 years) B. Public exposures (annual) 1. Effective dose equivalent limit, continuous or frequent exposure 2. Effective dose equivalent limit, infrequent exposure 3. Remedial action recommended when: a. Effective dose equivalent b. Exposure to radon and its decay products 4. Dose equivalent limits for lens of eye, skin and extremities 1 m. Sv (0. 1 rem) 5 m. Sv (0. 5 rem) > 5 m. Sv (>0. 5 rem) > 0. 007 Jhm-3 (>2 WLM) 50 m. Sv (5 rem) From NCRP. Recommendations on limits for exposure to ionizing radiation. Report. 91. 20

Occupational and Public Dose Limits Table 16. 5. Summary of Recommendations C. Education and training exposures (annual) 1. Effective dose equivalent 2. Dose equivalent limits for lens of eye, skin and extremities 1 m. Sv (0. 1 rem) 50 m. Sv (5 rem) 5 m. Sv 0. 5 m. Sv (0. 5 rem) (0. 05 rem) 0. 01 m. Sv (0. 001 rem) D. Embryo-fetus exposures 1. Total dose equivalent limit 2. Dose equivalent limit in a month E. Negligible Individual Risk Level (annnual) Effective dose equivalent per source or practice From NCRP. Recommendations on limits for exposure to ionizing radiation. Report No 91. 21

Negligible Individual Risk Level (NIRL) A level of average annual excess risk of fatal health effects attributable to irradiation, below which further effort to reduce radiation 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 m. Sv (0. 001 rem) Corresponding life time risk (70 years) = 0. 7 × 10 -5 22

Example of risk calculation Question Calculation the risk followings: a. Radiation workers b. Members of the general public c. NIRL (corresponding to respective annual effective dose equivalent limits) Risk coefficient of 10 -2 Sv-1 (10 -4 rem-1) 23

Example of risk calculation Answer a. Annual effective dose equivalent limit for radiation workers = 50 m. Sv (5 rem) Annual risk = 5 rem × (10 -4 rem-1) = 5 × 10 -4 b. Annual effective dose equivalent limit for members of general public = 1 m. Sv (0. 1 rem) Annual risk = 0. 1 rem × (10 -4 rem-1) = 10 -5 c. Annual effective dose equivalent limit for NIRL = 0. 01 m. Sv (0. 001 rem) Annual risk = 0. 001 rem × (10 -4 rem-1) = 10 -7 24

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 A. B. C. D. E. Primary Radiation Barrier Secondary Barrier for Scattered Radiation Secondary Barrier for Leakage Radiation Door Shielding Protection Against Neutrons 25

16. 6 Structural Shielding 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) Primary Barrier Secondary barrier 26

Factors associated with the calculation of barrier thickness Workload (W) Use factor (U) Occupancy factor (T) Distance (d) 27

Workload (W) For <500 k. Vp x-ray machine W = Maximum m. A × 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 1 m) Use Factor (U) U = Fraction of operation time that radiation is directed toward a particular barrier Depending on technique use Table 16. 6 Typical Use Factor for Primary Protective Barriers Location Use Factor Floor 1 Walls ¼ Ceiling ¼ - ½ , depending on equipment and techniques 28

Occupancy Factor (T) T = Fraction of operating time during which the area of interest is occupied by the individual Table 16. 7. Typical Occupancy Factors Full occupancy (T = 1) Work areas, offices, nurses’ stations Partial occupancy (T = ¼ ) Corridors, rest rooms, elevators with operators Occasional occupancy (T = 1/8 – 1/16) 1/16 Waiting rooms, toilets, stairways, unattended elevators, outside areas used only for pedestrians or vehicular traffic Distance (d) d = distance from the radiation source to the area to be protected Applied inverse square law 29

A. Primary Radiation Barrier Determine 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 30

Figure 16. 1. Figure 16. 2. Transmission through concrete of x -rays produced by 0. 1 - to 0. 4 -Me. V electrons, under broad beam conditions Transmission of thick-target x-rays through ordinary concrete, under broad-beam conditions 31

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 32

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 33

Table 16. 8 Ratio α of Scattered to Incident Exposure γRays X-Rays Scattering Angle 60 Co (From Central Ray) 4 MV 6 MV 15 9 × 10 -3 30 6. 0 × 10 -3 7 × 10 -3 45 3. 6 × 10 -3 9 × 10 -3 1. 8 × 10 -3 60 2. 3 × 10 -3 1. 1 × 10 -3 90 0. 9 × 10 -3 0. 6 × 10 -3 135 0. 6 × 10 -3 0. 4 × 10 -3 From NCRP. Medical x-ray and gamma-ray protection for energies up to 10 Me. V. Structural shielding design and evaluation. Report No. 34. For MV beams, αusually be assumed at 90° scatter = 0. 1% 34

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 ke. V Transmission of 500 k. Vp useful beam Relatively lower energy in compare with the incident energy Beam softening by Compton effect 35

Transmission factor of BS is required to reduced the scattered dose to the accepted level P α－ fractional scatte (1 cm from scatterer; Beam area 400 cm 2 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 The barrier transmission of the scattering beam The required thickness of the barrier can be determined for appropriate transmission curve 36

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 k. Vp) 5 -50 k. Vp <0. 1 R (in any h at any point 5 cm from the source) > 50 k. Vp, < 500 k. Vp < 1 R (in 1 h, at 1 m from the source) < 30 R/h at 5 cm 37

C. Secondary Barrier for Leakage Radiation The recommended absorbed dose rate for different energy of the beam (> 500 k. Vp) > 500 k. Vp < 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 < 2 mrad/h (on average direction, 1 m from the source) < 10 mrad/h (in any direction, 1 m from the source) Beam “on” position < 0. 1% of the useful beam dose rate (1 m from the source) 38

Transmission factor (BL)to reduce the leakage dose to the maximum permissible level (P) For machine < 500 k. Vp I = maximum tube current Reason for “× 60”: conversion from h to min (R/h to R/min), ∵unit of W is m. A-min/week For MV machine Reason for “× 0. 001”: 0. 1% leakage limit through the source housing 39

The required thickness of the barrier can be determined for transmission curve of the primary beam The quality of radiation: leakage ~ primary beam 40

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 thicker one < 3 HVL Choose thicker one + 1 HVL 41

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 42

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 k. Vp is used ∵Compton scatter of MV radiation at 90° < 500 k. Vp In most cases, the required thickness of door shielding is < 6 mm lead 43

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 Me. V Inside of the maze: few fast neutrons (> 0. 1 Me. V) 44

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 45

E. Protection against Neutrons Neutron capture γrays Generated by thermal neutrons absorbed by the shielding door Spectrum energies up to 8 Me. V (mostly 1 Me. V) 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 46

Thanks for your attention! 47

Table 16. 1 Recommended Quality Factors Radiation Quality Factor X-rays, γrays, and electrons 1 Thermal neutrons 5 Neutrons, heavy particles 20 Data are from NCRP. Recommendations on limits for exposure to ionizing radiation. Report No. 91 48

Table 16. 2. Recommended Values of the weighting Factors WT, for calculating Effective Dose Equivalent and the Risk Coefficients from Which They Were Derived Tissue (T) Risk Coefficient WT Gonads 40 × 10 -4 Sv-1 (40 × 10 -6 rem-1) 0. 25 Breast 25 × 10 -4 Sv-1 (25 × 10 -6 rem-1) 0. 15 Red bone marrow 20 × 10 -4 Sv-1 (20 × 10 -6 rem-1) 0. 12 Lung 20 × 10 -4 Sv-1 (20 × 10 -6 rem-1) 0. 12 Thyroid 5 × 10 -4 Sv-1 (5 × 10 -6 rem-1) 0. 03 Bone surface 5 × 10 -4 Sv-1 (5 × 10 -6 rem-1) 0. 03 50 × 10 -4 Sv-1 (50 × 10 -6 rem-1) 0. 30 165 × 10 -4 Sv-1 (165 × 10 -6 rem-1) 1. 00 Remainder Total From NCRP. Recommended on limits for exposure to ionizing radiation. Report No. 91. 49

Table 16. 3. Estimated Total Dose Equivalent Rate for a Member of the Population in the United States and Canada from Various Sources of Natural Background Dose Equivalent Rate (m. Sv/y) Source Bronchial Epithelium Other Soft Tissues Cosmic 0. 27 Cosmogenic 0. 01 0. 03 Terrestrial 0. 28 24 － － － 0. 35 1. 1 0. 50 25 0. 9 1. 7 1. 1 Inhaled In the body Rounded totals Bone Surfaces Bone Marrow From NCRP. Exposure of the population in United States and Canada from national background radiation. 50

Table 16. 4. Annual Fatality Rates from Accidents in Different Occupations Number of Workers Annual Fatal Accident Rate Occupation × 103 (per 10, 000 Workers) Trade 24, 000 0. 5 Manufacturing 19, 000 0. 6 Service 28, 900 0. 7 Government 15, 900 0. 9 Transportation 5, 500 2. 7 Construction 5, 700 3. 9 Agriculture 3, 400 4. 6 Mining, quarrying 1, 000 6. 0 All industries (U. S. ) 104, 300 1. 1 From NCRP. Recommendations on limits for exposure to ionizing radiation. Report No. 91. 51

Table 16. 5. Summary of Recommendations A. Occupation exposure (annual) 1. Effective dose equivalent limit (stochastic effects) 2. Dose equivalent limits for tissues and organs (nonstochastic effects) a. Lens of eye b. All others (e. g. red bone marrow, breast, lung, gonads, skin and extremities) 3. Guidance: cumulative exposure C. Public exposures (annual) 1. Effective dose equivalent limit, continuous or frequent exposure 2. Effective dose equivalent limit, infrequent exposure 3. Remedial action recommended when: a. Effective dose equivalent b. Exposure to radon and its decay products 4. Dose equivalent limits for lens of eye, skin and extremities 50 m. Sv 5 (rem) 150 m. Sv (15 rem) 500 m. Sv (50 rem) 10 m. Sv × age (1 rem × age in years) 1 m. Sv (0. 1 rem) 5 m. Sv (0. 5 rem) > 5 m. Sv (>0. 5 rem) > 3 0. 007 Jhm (>2 WLM) 50 m. Sv (5 rem) From NCRP. Recommendations on limits for exposure to ionizing radiation. Report. 91. 52

Table 16. 5. Summary of Recommendations D. Education and training exposures (annual) 1. Effective dose equivalent 2. Dose equivalent limits for lens of eye, skin and extremities E. Embryo-fetus exposures 1. Total dose equivalent limit 2. Dose equivalent limit in a month F. Negligible Individual Risk Level (annnual) Effective dose equivalent per source or practice 1 m. Sv (0. 1 rem) 50 m. Sv (5 rem) 5 m. Sv 0. 5 m. Sv (0. 5 rem) (0. 05 rem) 0. 01 m. Sv (0. 001 rem) From NCRP. Recommendations on limits for exposure to ionizing radiation. Report No 91. 53

Table 16. 6 Typical Use Factor for Primary Protective Barriers Location Use Factor Floor Walls Ceiling 1 ¼ ¼ - ½ , depending on equipment and techniques Table 16. 7. Typical Occupancy Factors Full occupancy (T = 1) Work areas, offices, nurses’ stations Partial occupancy (T = ¼ ) Corridors, rest rooms, elevators with operators Occasional occupancy (T = 1/8 – 1/16) Waiting rooms, toilets, stairways, unattended elevators, outside areas used only for pedestrians or vehicular traffic 54

Table 16. 8 Ratio α of Scattered to Incident Exposure γRays X-Rays Scattering Angle 60 Co (From Central Ray) 4 MV 6 MV 15 9 × 10 -3 30 6. 0 × 10 -3 7 × 10 -3 45 3. 6 × 10 -3 9 × 10 -3 1. 8 × 10 -3 60 2. 3 × 10 -3 1. 1 × 10 -3 90 0. 9 × 10 -3 0. 6 × 10 -3 135 0. 6 × 10 -3 0. 4 × 10 -3 From NCRP. Medical x-ray and gamma-ray protection for energies up to 10 Me. V. Structural shielding design and evaluation. Report No. 34. 55