Phylosophy of radiationprotection 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).
Objective of radiation protection To balance the risks and benefit from radiative activities.
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=D•Q H is dose equivalent. D is the absorbed dose. Q is the quality factor for the radiation.
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)
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 1 electrons Thermal neutrons 5 Neutrons, heavy 20 particles
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
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
Dose Equivalent Rate (mSv/y) Source Bronchial Other Soft Bone Bone Epithelium Tissues Surfaces MarrowCosmic 0.27 0.27 0.27 0.27Cosmogenic 0.01 0.01 0.01 0.03Terrestrial 0.28 0.28 0.28 0.28Inhaled 24 － － －In the body 0.35 0.35 1.1 0.50Rounded 25 0.9 1.7 1.1totals
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
Low-Level Radiation Effects Low level radiation < Dose required to produce acute radiation syndrome > Dose limits recommended by the standards
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
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
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
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
“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
Occupational and Public Dose LimitsA. 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, 500 mSv (50 rem) gonads, skin and extremities) (1 rem × 3. Guidance: cumulative exposure 10 mSv × age age in years)B. Public exposures (annual) 1. Effective dose equivalent limit, continuous or 1 mSv (0.1 rem) frequent exposure 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 50 mSv (5 rem) extremities
Occupational and Public Dose LimitsC. Education and training exposures (annual) 1.Effective dose equivalent 1 mSv (0.1 rem) 2.Dose equivalent limits for lens of eye, skin 50 mSv (5 rem) and extremities.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 0.01 mSv (0.001 practice rem)
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 mSv (0.001 rem) Corresponding life time risk (70 years) = 0.7 × 10-5
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)
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 b. 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
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 Primary Barrier Secondary barrier The scattered radiation The leakage radiation (from source housing)
Factors associated with the calculation of barrier thickness Workload (W) Use factor (U) Occupancy factor (T) Distance (d)
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 useFloor 1Walls ¼Ceiling ¼ - ½ , depending on equipment and techniques
Occupancy Factor (T) T = Fraction of operating time during which the area of interest is occupied by the individual. 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 Distance (d) d = distance from the radiation source to the area to be protected Applied inverse square law
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 WUT P•d2 P= 2 •B B= d WUT Determining the barrier thickness by consulting broad beam attenuation curves for the given beam energy.
Transmission through concrete of Transmission of thick-target x-raysx-rays produced by 0.1- to 0.4- through ordinary concrete, underMeV electrons, under broad beam broad-beam conditionsconditions
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
B. Secondary Barrier forScattered Radiation Factors affecting the amount of scattered radiation Beam intensity Quality of radiation The area of the beam at scatterer The scattering angle Scattered dose α= Incident dose
B. Secondary Barrier forScattered 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
Transmission factor of BS is required to reduced the scattered dose to the accepted level P α • WT F α － fractional scatter (1 cm from scatterer; P = 2 2 • • Bs Beam area 400 cm2 incident at the scatterer) d •d 400 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 P 400 2 Bs = • • d • d ′2 α WT F The required thickness of the barrier can be determined for appropriate transmission curve
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
C. Secondary Barrier forLeakage 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
Transmission factor (BL)to reduce the leakage dose to the maximum permissible level (P) For machine < 500 kVp WT P • d 2 • 60 I P= 2 • BL BL = d • 60 I WT For MV machine 0.001WT P • d 2 • 60 I P= • BL BL = d 2 WT
The required thickness of the barrier can be determined for transmission curve of the primary beam The quality of radiation: leakage ~ primary beam
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
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
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
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)
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)
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.