09 chap 16 radiation protection 游
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Transcript of 09 chap 16 radiation protection 游
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Radiation protection
Yu Chun-Yen
Radiation OncologyChang Gung Memorial Hospital, ChiaYi, Taiw
an
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Phylosophy of radiation protection Justification
No practice shall be adopted unless its introduction produces a net positive benefit.
Optimization All exposures shall be kept as low as reasonably
achievable.
Dose limitation The dose equivalent to individuals shall not exceed the
limits recommended for the appropriate circumstances by the ICRP (International Commission on Radiological Protection).
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Objective of radiation protection To balance the risks and benefit from
radiative activities.
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Basic dosimetric quantities Dose equivalent
The absorbed dose needed to achieve a given level of biological damage is often different kinds of radiation.
The equivalent dose replaces the dose equivalent for a tissue or organ.
H=DQ H is dose equivalent.
D is the absorbed dose.
Q is the quality factor for the radiation.
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Dose equivalent Dose equivalent
The absorbed dose needed to achieve a given level of biological damage is often different kinds of radiation.
The equivalent dose replaces the dose equivalent for a tissue or organ.
Units Sivert (Sv) (西弗 )
SI unit 1 Sv = 1 J/kg Rem (侖目 ), 1 rem = 10-2 J/kg (Sv)
Dose (吸收劑量 ) 1 Gy = 1 J/kg Ray (雷得 ), 1 rad = 10-2 J/kg (Gy)
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16.1 Dose Equvalent Quality factor Q
Base on a range RBE related to the LET of the radiation
Radiation Quality Factor
X-rays, γrays, and electrons
1
Thermal neutrons 5
Neutrons, heavy particles
20
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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
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Weighting factors The proportionate risk (stochastic) of tissue when body from risk coefficients
Tissue (T) Risk Coefficient WT
Gonads 40 × 10-4 Sv-1 (40 × 10-6 rem-1) 0.25Breast 25 × 10-4 Sv-1 (25 × 10-6 rem-1) 0.15Red 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) 0.12Thyroid 5 × 10-4 Sv-1 (5 × 10-6 rem-1) 0.03Bone surface 5 × 10-4 Sv-1 (5 × 10-6 rem-1) 0.03Remainder 50 × 10-4 Sv-1 (50 × 10-6 rem-1) 0.30Total 165 × 10-4 Sv-1 (165 × 10-6 rem-1) 1.00
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Background Radiation Radiation from the natural environment Terrestrial radiation (地殼輻射 )
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 (宇宙射線 ) e.g. air travel At 30,000 feet, the dose equivalent is about 0.5 mrem/h
Radiation element in our bodies (體內輻射 ) e.g. mainly from 40K Emits β, γrays; T1/2 = 1.3 × 109 years
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Source
Dose Equivalent Rate (mSv/y)Bronchial Epitheliu
m
Other Soft
Tissues
Bone Surfaces
Bone Marrow
Cosmic 0.27 0.27 0.27 0.27
Cosmogenic 0.01 0.01 0.01 0.03
Terrestrial 0.28 0.28 0.28 0.28
Inhaled 24 - - -In the body 0.35 0.35 1.1 0.50
Rounded totals
25 0.9 1.7 1.1
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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
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Low-Level Radiation Effects
Low level radiation
< Dose required to produce acute
radiation syndrome
> Dose limits recommended by the
standards
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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
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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 notnot depend on the magnitude of the absorbed dose
All or none phenomenon
e.g. development of a cancer genetic effect
NoNo threshold dose
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The NCRP defines two general categories for harmful effects of radiationNonstochastic 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
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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”“safe” industries
ALARAALARA principles should be followed The risk are kept as low as reasonably achievable, taking into account, social and
economic factors
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OccupationNumber of Workers
× 103
Annual Fatal Accident Rate
(per 10,000 Workers)
Trade 24,000 0.5
Manufacturing 19,000 0.6
Service 28,900 0.7Government 15,900 0.9Transportation 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
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“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
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Occupational and Public Dose Limits
A. Occupation exposure (annual)
1. Effective dose equivalent limit (stochastic effects) 50 mSv 5 (rem)
2. Dose equivalent limits for tissues and organs (nonstochastic effects) a. Lens of eye 150 mSv (15 rem) b. All others (e.g. red bone marrow, breast, lung, gonads, skin and extremities) 500 mSv (50 rem)
3. Guidance: cumulative exposure 10 mSv × age (1 rem × age in years)
B. Public exposures (annual)
1. Effective dose equivalent limit, continuous or frequent exposure 1 mSv (0.1 rem)
2. Effective dose equivalent limit, infrequent exposure 5 mSv (0.5 rem)
3. Remedial action recommended when: a. Effective dose equivalent > 5 mSv (>0.5 rem) b. Exposure to radon and its decay products > 0.007 Jhm-
3 (>2 WLM)
4. Dose equivalent limits for lens of eye, skin and extremities 50 mSv (5 rem)
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Occupational and Public Dose Limits
C. Education and training exposures (annual) 1.Effective dose equivalent 1 mSv (0.1 rem)
2.Dose equivalent limits for lens of eye, skin and extremities. 50 mSv (5 rem)
D. Embryo-fetus exposures
1.Total dose equivalent limit 5 mSv (0.5 rem)
2.Dose equivalent limit in a month 0.5 mSv (0.05 rem)
E. Negligible Individual Risk Level (annnual) 1.Effective dose equivalent per source or
practice 0.01 mSv (0.001 rem)
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Negligible Individual Risk Level (NIRL) A level of average annual excess risk of fatal health effects attributable to irradiatio
n, below which further effort to reduce radiation exposure to individual is unwarraunwarrantednted
TrivialTrivial 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 thresholdthreshold 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
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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 10-2Sv-1 (10-4rem-1)
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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)
Primary Barrier Secondary barrier
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Factors associated with the calculation of barrier thickness
Workload (W)
Use factor (U)
Occupancy factor (T)
Distance (d)
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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
Floor 1Walls ¼
Ceiling ¼ - ½ , depending on equipment and techniques
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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
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
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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.
Bd
WUTP
2 WUT
dPB
2
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Transmission of thick-target x-rays through ordinary concrete, under broad-beam conditions
Transmission through concrete of x-rays produced by 0.1- to 0.4-MeV electrons, under broad beam conditions
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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
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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
doseIncident
dose Scattered
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Scattering Angle(From Central Ray)
γRays X-Rays
60Co 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
For MV beams, αusually be assumed at 90° scatter = 0.1%
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B. Secondary Barrier for Scattered Radiation Energy of the scatter
For orthovoltage radiationFor orthovoltage radiation Beam energy: Scatter = incident (assumed)
For MV beamsFor 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
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Transmission factor of B S 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
sBF
dd
WTP
400
2'2
α - fractional scatter (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
22400
dd
FWT
PBs
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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
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C. Secondary Barrier for Leakage Radiation The recommended absorbed dose rate for different en
ergy 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” positionBeam “off” position
< 2mrad/h (on average direction, 1m from the source) < 10 mrad/h (in any direction, 1m from the source)
Beam “on” positionBeam “on” position < 0.1% of the useful beam dose rate (1 m from the source)
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Transmission factor (BL)to reduce the leakage dose to the maximum permissible level (P)
For machine < 500 kVp
For MV machine
LBId
WTP
602 WT
IdPBL
602
LBd
WTP
2
001.0WT
IdPBL
602
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The required thickness of the barrier
can be determined for transmission
curve of the primary beam The quality of radiation: leakage ~
primary beam
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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
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D. Door Shielding Advantages of the maze arrangement in treatm
ent room. Reduces the shielding requirement of the door Expose mainly to multiply scattered radiation
Radiation experience scatter at least twice
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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
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E. Protection against Neutrons
Neutron contamination High energy photon (> 10 MV) or electrons incident on th
e 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 MeV1 MeV Inside of the maze: few fast neutrons (> 0.1 MeV)(> 0.1 MeV)
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E. Protection against Neutrons
Protection against neutrons should be considered in door shielding only 1° and 2° barriers for x-ray shielding are adequate
SolutionSolution 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
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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 energy γray). Longer maze (reduce neutron fluence).
Practically, treatment room with long maze, the intensity of neutron capture γrays is low.
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Thanks for your attention!