09 chap 16 radiation protection 游

Radiation protection

Yu Chun-Yen

Radiation OncologyChang Gung Memorial Hospital, ChiaYi, Taiw

an

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).

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=DQ 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 electrons

1

Thermal neutrons 5

Neutrons, heavy particles

20

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

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

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

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

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 notnot depend on the magnitude of the absorbed dose

All or none phenomenon

e.g. development of a cancer genetic effect

NoNo 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”“safe” industries

ALARAALARA principles should be followed The risk are kept as low as reasonably achievable, taking into account, social and

economic factors

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

“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 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)

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)

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

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)

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

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 use

Floor 1Walls ¼

Ceiling 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.

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

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

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

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 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

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%

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

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

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 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)

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

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 treatm

ent 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 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)

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

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.

Thanks for your attention!