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Transcript of McGillllchia/HP_lectures/shielding_2_final.pdfShielding materials •Lead (Pb) backed gypsum board...
![Page 1: McGillllchia/HP_lectures/shielding_2_final.pdfShielding materials •Lead (Pb) backed gypsum board ... calculation F aWT ... The scanner operates at 125 kV and 200 mA for 1.5s per](https://reader033.fdocuments.us/reader033/viewer/2022052918/5adc5f0f7f8b9ae1408b7cd0/html5/thumbnails/1.jpg)
Radiation Oncologytreatment facility design
Simulators, CT scanners,HDR Brachytherapy
McGill MDPH 613 Fall 2004
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UNITS
For this class only:
1 R = 1 cGy = 1 cSv = 10 mSv
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Reference book
• McGinley• NCRP 49• NCRP 51• Health Physics
notes, RobertCorns
• Safety code 20A,Health Canada
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Radiation Oncology
• Linear accelerator• Brachytherapy• CT simulator• simulator
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Basic shielding concepts
• Establish a target dose-rate at a certainpoint behind a barrier
• Calculate barrier thickness necessary toachieve the target dose rate
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Shielding considerations
• Machine workload
• Type of person to protect– NEW– Public
• Type of space to protect– Public access area– Restricted access
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Shielding considerations
• Type of radiation
• Primary beam incidence
• Primary beam scatter
• Patient scatter
• Leakage radiation
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Simulator
• Operates with samegeometry as LINAC
• Radiation source isdiagnostic x-ray tube
• Capable ofradiographic andfluoroscopic functions
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Simulator
• Most exposures made in fluoroscopy mode
• X-ray beam collimated, always incident onimage intensifier (II)
• Primary beam significantly attenuated bypatient and II
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Shielding materials
• Lead (Pb) backed gypsum board (dry wall)
• Shielding provided to height of 7 feet unlessspace above is occupied
• Viewing window with lead glass is used atconsole area
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Types of barriers
• Primary barriers– Attenuate primary (direct) beam
• Secondary barriers– Leakage– Patient scatter– Wall scatter
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Simulator room
isocenter
Lead glass window
Simulator control area
Target rotational plane
shielded door
primary
secondary
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Primary beam
• Barrier thickness depends on:
– Distance to POI from source (d)– Target dose rate (P)– Workload (W)– Occupancy (T)– Usage (U)
*Patient and table attenuation not taken into account
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Basic situation
1 m
d
s
source isocenter
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Primary barrier
• At isocenter max FS is 40 x 40 cm2
• Largest dimension is diagonal (56 cm)• At barrier this will project to larger size
at barrier ~ 200 cmat iso ~ 56 cm
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Simulators: Primary beam
• Kux is transmission factor• Expressed in (R/mA min) at 1m• NCRP 49 (1976)
WUT
Pd2
Kux =
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Target dose rate P
~ 0.1
~ 2
Exposurerate(R/y)
0.0021Public
0.0420NEW
Exposurerate
(R/week)
ICRP 60 Doselimit
(mSv/y)Group
*1 year has 50 weeks of 40 hrs/week or 2000 hr/year** diagnostic X-ray installations are not licensed by CNSC but may fall underprovincial regulations
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Workload• W workload expressed in mA-min/wk:
• Radiography– 50 patient/wk x 500 mAs/patient x 1 s/60 min = 400 mA min/wk
• Fluoroscopy– 50 patient/wk x 5 mA/patient x 1 min = 250 mA min/wk
W UT
Pd2
Kux =W = 1000 mA-min/wk
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Typical workload
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Usage factor U
• U Accounts for beam orientation
• Isocentric units have same usage for floors,ceiling, and walls.
• U = 0.25
W U T
Pd2
Kux =
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Occupancy factor T
OccasionalWaiting room, stairway, janitor closet
1/16
PartialCorridors, restrooms, parking
1/4
FullOffices, shops, labs, living area
1
Type of areaT
W U T
Pd2
Kux =
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Transmission factor Kux
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Transmission - lead
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Transmission - concrete
(cm)
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Simulators: Leakage
• Assumption leakage is 0.1 R/hr at 1m
• Shielded to a factor of 600 per minute
WT
600 I Pds2
B =
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Simulators: Leakage
WT
600 I Pds2
B =
• B is the factor by which the intensity ofradiation (Po) must be reduced to achieve thetarget dose rate P
Po
PB =
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Simulators: Leakage
• I is the tube current (mA)
• ds is the distance from source to POI
WT
600 I Pds2
B =
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TVL - Tenth Value Layer
B
1n = log ( )
HVL - Half Value Layer
1 TVL = 3.32 HVL
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TVL and HVL
2x
1
10
1
2x 10
x log 2 log 10
=
=
=
x = 3.32
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TVL-HVL
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Simulators: Scatter
• Scattered x-rays have same barrierpenetration as primary beam
• NCRP 49 (1976)
F aWT
400 P D2 d2
K =
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Simulators: Scatter
• D is the distance from the source to scatterer• d is the distance from scatterer to POI• F is the field area on patient• a is the scatter fraction
F aWT
400 P D2 d2
K =
![Page 33: McGillllchia/HP_lectures/shielding_2_final.pdfShielding materials •Lead (Pb) backed gypsum board ... calculation F aWT ... The scanner operates at 125 kV and 200 mA for 1.5s per](https://reader033.fdocuments.us/reader033/viewer/2022052918/5adc5f0f7f8b9ae1408b7cd0/html5/thumbnails/33.jpg)
Scatter fraction
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Lead Glass
• Leaded glass may be used for patientobservation window
220190145Cost/m2
200150150X-ray kVp
14118Glass
3.1 (1/8”)2.6 (3/32”)1.9 (1/16”)Lead
thickness(mm)
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Doors
• Doors contain the lead equivalent thicknessrequired for secondary barrier shielding
• 1 - 2 mm Pb in wood
• Make sure door is not in primary beam
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Simulator (125 kVp) room
isocenter
Lead glass window
Simulator control area
Target rotational plane
shielded door
primary
secondary
A
B
Determine wall thickness(concrete and Pb) required forPOI A and B. What would be thethickness of the lead glassrequired for the console area?
d iso to POI is 4m
A is an office T =1
B is a waiting room T = 1/16
U = 0.25 for simulators
W = 1000 mA min/week
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Simulator room
• Determine target P
– At A , office with NEW (+ALARA ?)
– 20 mSv/year (ICRP 60)
– Target dose rate is 20 mSv/yr = 2 R/yr = 0.04 R/wk
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Simulator room
• P = 0.04 R/week• d = 4m• W = 1000 mA min/week• U = 0.25, T = 1
WUT
Pd2
Kux = = 0.00256
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Simulator room
1000 mA min/wk x 0.25 x 1
0.04 R/wk x 42
Kux =
Kux = 0.00256 R/mA min at 1m
~ 12 cm concrete or 1 - 2 mm Pb
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Simulator example
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Simulator example
cm
![Page 42: McGillllchia/HP_lectures/shielding_2_final.pdfShielding materials •Lead (Pb) backed gypsum board ... calculation F aWT ... The scanner operates at 125 kV and 200 mA for 1.5s per](https://reader033.fdocuments.us/reader033/viewer/2022052918/5adc5f0f7f8b9ae1408b7cd0/html5/thumbnails/42.jpg)
Leakage barrier
• Leakage barrier (at B )
– I = 5 mA, T = 1/16, ds = 4m
WT
600 I Pds2
B =
![Page 43: McGillllchia/HP_lectures/shielding_2_final.pdfShielding materials •Lead (Pb) backed gypsum board ... calculation F aWT ... The scanner operates at 125 kV and 200 mA for 1.5s per](https://reader033.fdocuments.us/reader033/viewer/2022052918/5adc5f0f7f8b9ae1408b7cd0/html5/thumbnails/43.jpg)
Simulator room• Leakage barrier (at B)
WT
600 I Pds2
B =
1000 x 1/16
600 x (5) x 0.002 R/wk x 42
= 1.536=
= 0.186 TVLs or 0.618 HVLs
![Page 44: McGillllchia/HP_lectures/shielding_2_final.pdfShielding materials •Lead (Pb) backed gypsum board ... calculation F aWT ... The scanner operates at 125 kV and 200 mA for 1.5s per](https://reader033.fdocuments.us/reader033/viewer/2022052918/5adc5f0f7f8b9ae1408b7cd0/html5/thumbnails/44.jpg)
Simulator room– Concrete @ 125 kVp = 0.186 x 6.6 cm = 1.2 cm– Lead @ 125 kVp = 0.186 x 0.93 = 1.7 mm– Lead glass equivalent = 8 mm
![Page 45: McGillllchia/HP_lectures/shielding_2_final.pdfShielding materials •Lead (Pb) backed gypsum board ... calculation F aWT ... The scanner operates at 125 kV and 200 mA for 1.5s per](https://reader033.fdocuments.us/reader033/viewer/2022052918/5adc5f0f7f8b9ae1408b7cd0/html5/thumbnails/45.jpg)
Scatter barrier• Scatter barrier (at B )
– F = 202 cm2, T = 1/16, D = 1m, d = 4m– a = 0.002
– Equivalent to about 1 cm concrete so use leakagecalculation
F aWT
400 P D2 d2
K = = 0.256
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Simulator example
cm
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CT simulator room
• Dedicated CT scanner forradiotherapy
• Flat table, lasers, big bore
• X-ray tube operating at 125 kVpand 250 mAs
• Primary beam is inherentlyshielded and U = 1
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CT simulator roomcontrol area
waiting room
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CT simulator
• Workload
– W = 50 pt/wk x 100 slices/pt = 5000 slices/wk
• Isodose plots are provided from themanufacturer to estimate the dose rate indifferent parts of the room
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CT scanner dose
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CT scanner dose
• The workload at any unprotected point in theroom:
D = W Do T
• Do is the isodose value, T is the occupancy
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CT simulator room• The required transmission is:
TR = P / D = P / W Do T
and TR = Xs/Xo
Xs = Xo TR
Xs = Xo P / W Do T
• Xs is the shielded intensity• Xo is conversion R per mA min at 1m
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CT simulator room
• R per mA min at 1 m from the x-ray target
0.5050
0.7370
0.86100
0.90125
0.95150
XokV
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CT simulator roomCalculate the barrierthickness required at point X.
10 patients are scanned aday, 100 slices each patient.
The area to be protected is apublic access area withoccupancy T=1.
The scanner operates at 125kV and 200 mA for 1.5s perslice.
X
a) Calculate the Workloadb) Calculate the Pc) Determine the thickness of lead required
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Xs = Xo P / W Do T
• Workload W– 10 pt/day x 100 slices/pt x 5 day/wk = 5000 slices/wk
• Target dose rate P– Public limit 1 mSv/year is 0.02 mSv/wk
• Dose from isodoses Do– Do = 0.03
• R per mA min conversion Xo– Xo = 0.95
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CT simulator room
Xs = Xo P / W Do T
= (0.95 x 0.02 mSv/wk) 5000 slices/wk x 0.02 x 10-3 mGy
Xs = 0.19
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Xs = Xo P / W Do T
Xs = Xo P / W Do T
= (0.95 x 0.02 mSv/wk) 5000 slices/wk x 0.03 x 10-3 mGy
=
cm
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HDR brachytherapy
• Ir-192• 10 Ci, welded to flexible steel cable• Remotely controlled• Source driven out of safe through
a catheter to patient• Typical room ~ 60 cm concrete
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HDR brachytherapy
• Shielding calculations based on transmissionfactor B, where:
• d is distance from source to POI
WT
Pd2
B =
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HDR brachytherapy
• Workload based on the total dose deliveredto all patients to be treated per week
• W is also function of source activity andtreatment time
• W = Γ f A t
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HDR brachytherapyW = Γ f A t
• Γ exposure rate constant– Relates exposure rate to activity at 1m– For Ir-192 Γ = 0.48 m2 R/hr Ci
• F factor– relates cGy to R and is 0.96 cGy/R for Ir-192
• t treatment time per week– T = dose x #patients / doserate @ 1cm
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HDR brachytherapy
• Calculate thickness of concrete required for a 10 CiIr-192 installation that treats 25 pt/wk to a dose of 10Gy per patient. The dose is delivered at 1 Gy perminute. d = 2 m, and the POI is a control area T = 1,we want to protect NEWs.
• Workload• Target dose rate
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HDR brachytherapy
• Workload– Time = dose / doserate = 25 patients x 10 Gy / 1 Gy/min = 250 min = 4.16 hr/wk
W = 0.48 R/hr Ci x 0.96 cGy/R x 10 Ci x 4.16 hr/wk = 19.2 cGy/wk @1m
• Target dose rate– NEW 0.04 cGy/wk (or cSv)
W = Γ f A t
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HDR brachytherapy
• From graph ~ 48 cm concrete
WT
Pd2
B = = 0.008
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Brachytherapy - concrete
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Brachytherapy - lead
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Lead underwear
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MGH brachytherapy/SIM
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MGH CT simulators 1
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MGH CT simulators 2