IMRT Shielding Symposium 2001 AAPM Annual Meeting Salt Palace Convention Center, Salt Lake City, Utah OUTLINE James E. Rodgers, Ph.D. (Approximately 25 minutes): I. Introduction a) IMRT description - Dynamic multileaf collimator (DMLC) - Segmental multileaf collimator (SMLC) - Tomotherapy b) A brief conventional therapy shielding calculation overview (NCRP 49) - Primary Barrier - Scatter Shielding - Leakage Shielding c) Conventional therapy vs. IMRT shielding assumptions - Workload - Use Factor - Field Size - Beam Energy d) IMRT shielding concerns - Primary Barrier - Scatter Shielding - Leakage Shielding II. The AAPM Task Group 57 overview Donald M. Robinson, Ph.D. (Approximately 25 minutes) III. Shielding Considerations and Calculations for Tomotherapy Delivery a) A brief MIMiC and tomotherapy machine description b) Primary barrier calculations - Workload - Use Factor - Field Size - Beam Energy
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IMRT Shielding Symposium 2001 AAPM Annual Meeting Salt … · J Rodgers AAPM 01 W*U, dpri, T, P J Rod gers AAPM 01 Primary barrier secondary isocenter gantry axis gantry or beam plane
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IMRT Shielding Symposium 2001 AAPM Annual Meeting
Salt Palace Convention Center, Salt Lake City, Utah
OUTLINE
James E. Rodgers, Ph.D. (Approximately 25 minutes): I. Introduction a) IMRT description
b) A brief conventional therapy shielding calculation overview (NCRP 49) - Primary Barrier
- Scatter Shielding - Leakage Shielding
c) Conventional therapy vs. IMRT shielding assumptions
- Workload - Use Factor - Field Size - Beam Energy
d) IMRT shielding concerns - Primary Barrier
- Scatter Shielding - Leakage Shielding
II. The AAPM Task Group 57 overview Donald M. Robinson, Ph.D. (Approximately 25 minutes) III. Shielding Considerations and Calculations for Tomotherapy Delivery
a) A brief MIMiC and tomotherapy machine description b) Primary barrier calculations
- Workload - Use Factor - Field Size - Beam Energy
c) Secondary barrier calculations - Workload - Beam Energy
IV. Shielding considerations and calculations for tomotherapy delivery
a) Primary barrier calculations - Workload - Use factor - Field size - Beam energy
b) Secondary barrier calculations - Workload - Beam energy
Sasa Mutic, M.S. (Approximately 25 minutes): V. Experimental verification of an IMRT shielding calculation formalism a) A brief formalism summary
- Tomotherapy - DMLC/SMLC
b) Measurement technique
- Tomotherapy - DMLC/SMLC
c) Data analysis
- Tomotherapy - DMLC/SMLC
VI. Conclusions
1
IMRT SHIELDING SYMPOSIUMAAPM 2001
INTRODUCTION
James E. RodgersJames E. RodgersJames E. RodgersJames E. RodgersGeorgetown University HospitalGeorgetown University HospitalGeorgetown University HospitalGeorgetown University Hospital
Dept. of Radiation MedicineDept. of Radiation MedicineDept. of Radiation MedicineDept. of Radiation Medicine
Washington, DCWashington, DCWashington, DCWashington, DC
Variations on the Technique ofVariations on the Technique ofVariations on the Technique ofVariations on the Technique ofIMRTIMRTIMRTIMRT
There are several ways tosequentially deliver dose to sub-volumes of the planning targetvolume in order to achieve abetter dose distribution throughintensity modulation.
J Rodgers AAPM 01
These include :
* DMLC = dynamic multileaf collimation
* SMLC = segmented multileaf collimation
* Combined dynamic gantry and DMLC
* Tomotherapy
J Rodgers AAPM 01
… Variations on the Technique of… Variations on the Technique of… Variations on the Technique of… Variations on the Technique ofIMRTIMRTIMRTIMRT
J Rodgers AAPM 01
PTV
Subdividing the Field
Beam’s Eye View of a Field
Abeamletirradiates asubdivision
J Rodgers AAPM 01
Static Gantry Positions for SMLCStatic Gantry Positions for SMLCStatic Gantry Positions for SMLCStatic Gantry Positions for SMLCand DMLCand DMLCand DMLCand DMLC
Tomotherapy IMRT requires more monitor unitsto deliver the “same” dose
❋❋❋❋ IMRT delivers dose to the targetvolume by sequentially deliveringdose to sub-volumes.
The finer the subdivision, the moreMU needed to treat the totalvolume.
J Rodgers AAPM 01
3
… IMRT requires more monitorunits to deliver the “same” dose
❋❋❋❋ The IMRT ratio of MU to cGyThe IMRT ratio of MU to cGyThe IMRT ratio of MU to cGyThe IMRT ratio of MU to cGyat at at at ““““isocenterisocenterisocenterisocenter””””, , , , CCCC,,,, ranges from ranges from ranges from ranges fromabout 2 to 10 or 20.about 2 to 10 or 20.about 2 to 10 or 20.about 2 to 10 or 20.
For some proposed methods For some proposed methods For some proposed methods For some proposed methods CCCCgoes as high as 50.goes as high as 50.goes as high as 50.goes as high as 50.
P = BP = BP = BP = Bpripripripri W U T/d W U T/d W U T/d W U T/dpripripripri2 (direct radiation)2 (direct radiation)2 (direct radiation)2 (direct radiation)
Some contributions influencingmaze barrier and door design
Maze barrier
Consider a dual energy accelerator with 6 MVand 15 or 18 MV x-rays.
If W(HQ) = W(LQ) = 30,000 cGy (MU)/wk
T =1 everywhere, P= 100 mrem/y = 2 rem/wk
Primary barrier (calculated for 18 MV x-rays) :
dpri= 7m U=.25 requires n = 4.88 TVLs
or, 214 cm (84”) of normal density concrete
[ For 6 MV: 171 cm, diff. is 1 TVL ∴ 214 cm ]
Simple example of traditional designmethods for a dual energy accelerator
J Rodgers AAPM 01
4
J Rodgers AAPM 01
Simple secondary,117 cm (46”)
Primary
214 cm (84”)
Example results
Simple Secondary Barriers :
dsec= 3 m dsca = 1 m requires n = 3.22leakage TVLs. Add 1 additional HVL for the
6 MV contribution.
The thickness of normal density concrete is:
117 cm (46”)
Scatter is insignificant for this situation.
Simple example of traditional methodsfor a dual energy accelerator
J Rodgers AAPM 01
Basic Premise of Traditional (Report 49) Method
1 MU yields 1 MU yields 1 MU yields 1 MU yields ≈≈≈≈ 1 cGy to the target 1 cGy to the target 1 cGy to the target 1 cGy to the targetvolumevolumevolumevolume at 1 at 1 at 1 at 1 m from the x-ray source from the x-ray source from the x-ray source from the x-ray source
For IMRT and TBI this preFor IMRT and TBI this preFor IMRT and TBI this preFor IMRT and TBI this premise isise isise isise is
not valid.not valid.not valid.not valid.
Problem
J Rodgers AAPM 01
A SolutionA SolutionA SolutionA Solution
❋❋❋❋ Allow different workloads for thedirect, leakage and (possibly)scatter contributions.
Wdir WL Wsca
❋❋❋❋ Re-assess secondary barrierrequirements/adequacy in light ofIMRT-increased WL
J Rodgers AAPM 01
IMRT Contribution
❋❋❋❋ For a dedicated IMRT accelerator, For a dedicated IMRT accelerator, For a dedicated IMRT accelerator, For a dedicated IMRT accelerator,the Leakage workload is given by:the Leakage workload is given by:the Leakage workload is given by:the Leakage workload is given by:
WWWWLLLL = C* = C* = C* = C* WWWWdirdirdirdir
whereaswhereaswhereaswhereas
W W W Wscascascasca = = = = WWWWdirdirdirdir
J Rodgers AAPM 01
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r = fraction of patients treated with IMRTC = ratio of MU to cGy for the IMRT technique/
equipment used.
If we assume prescribed doses and # of patients treateddaily remain constant, then
WL (QX) =(1-r + r*C)* Wdir (QX)and
Wdir (QX) = Wsca (QX) ≤≤≤≤ WL (QX)
[J Rodgers, JACMP (Summer 2001)]
Potential impact of IMRT onSecondary Barrier Thickness
Additional TVLs Needed for SecondaryBarriers due to increased leakage
0.00
0.20
0.40
0.60
0.80
1.00
0 0.2 0.4 0.6 0.8 1IMRT Fraction of Patients
#of
add
itio
nal
TV
Ls
C=10
C=8
C=6
C=5
C=4C=3
C=2
C=1
J Rodgers AAPM 01
Significance of IMRT forExample Design
❋❋❋❋ Primary barrierPrimary barrierPrimary barrierPrimary barrier : no change : no change : no change : no changeprovided the prescribed total dosesprovided the prescribed total dosesprovided the prescribed total dosesprovided the prescribed total dosesremain about the same.remain about the same.remain about the same.remain about the same.
Two situations will be examined:Two situations will be examined:Two situations will be examined:Two situations will be examined:
-- all IMRT done with 18 MV -- all IMRT done with 18 MV -- all IMRT done with 18 MV -- all IMRT done with 18 MV
-- all IMRT done with 6 MV -- all IMRT done with 6 MV -- all IMRT done with 6 MV -- all IMRT done with 6 MV
J Rodgers AAPM 01
ALL IMRT with 18 MV
0
5
10
15
20
25
30
35
0.25 0.50 0.75 1.00
IMRT Fraction ( r)
Ad
dit
ion
alco
ncr
ete
(cm
)
C=10
C=4
IMRT Impact on Secondary BarrierThickness
All IMRT given with high energy x-rays
J Rodgers AAPM 01
IMRT Impact on Secondary BarrierThickness
All IMRT given with lowlowlowlow energy x-rays invault designed for 18 MV
ALL IMRT with 6 MV
-10
-5
0
5
10
15
0.25 0.50 0.75 1.00
IMRT Fraction ( r)
Ad
diti
onal
conc
rete
(cm
) C=10
C=4
J Rodgers AAPM 01
Another radiation hazards associatedwith large C and HIGH energy X-ray
production
❋❋❋❋ Whole body neutron and photondose to the patient
-- Whole body photon dose fromleakage radiation is ~ 10% of neutrondose (]15 MV).
-- For 10 MV and below, photonleakage is everything but still“acceptable”.
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6
❋❋❋❋ Followill, Geis and Boyer
“Estimates of Whole-Body Dose Equivalent Produced by BeamIntensity Modulated Conformal Therapy”IJROBP 38, 667(‘97); Eratta 38, 783 (‘97)
❋❋❋❋ P.H. McGinley
“Neutron dose to patients treated with high-energy medicalaccelerators” Paper # IAEA-CN-85-08, Proceedings of the Int’l.Conf. on Radiol. Protection of Patients, IAEA, Malaga, Spain,29 March 2001
J Rodgers AAPM 01
Two Published Investigations of theWhole body Neutron Dose Problem
Neutron risk assessment … data fromP. H. McGinley’s article
J Rodgers AAPM 01
The advantages of low energy x-raysfor IMRT
Low energy [ 10 MV & below] x-rays:
❋❋❋❋ have insignificant have insignificant have insignificant have insignificant photoneutronphotoneutronphotoneutronphotoneutronproduction.production.production.production.
Therefore, the whole body neutronTherefore, the whole body neutronTherefore, the whole body neutronTherefore, the whole body neutronproblems is problems is problems is problems is eliminated....
J Rodgers AAPM 01
(Low energy x-rays:)
❋❋❋❋ low energy bea low energy bea low energy bea low energy beam may contributeay contributeay contributeay contributeinsignificantly to the secondary barrierinsignificantly to the secondary barrierinsignificantly to the secondary barrierinsignificantly to the secondary barrierprobleprobleprobleproblem in a dual photon bea in a dual photon bea in a dual photon bea in a dual photon beam machine.achine.achine.achine.
That is, the shielding for the That is, the shielding for the That is, the shielding for the That is, the shielding for the high energyhigh energyhigh energyhigh energybeabeabeabeam may suffice may suffice may suffice may suffice....
...The advantages of lowenergy x-rays for IMRT
J Rodgers AAPM 01
Synopsis of new NCRPSynopsis of new NCRPSynopsis of new NCRPSynopsis of new NCRPCommittee Committee Committee Committee Guidleines Guidleines Guidleines Guidleines forforforforMegavoltage Megavoltage Megavoltage Megavoltage Vault DesignVault DesignVault DesignVault Design
AAPM Task Group 57, NCRP SC 46-13AAPM Task Group 57, NCRP SC 46-13AAPM Task Group 57, NCRP SC 46-13AAPM Task Group 57, NCRP SC 46-13
Peter J. Biggs, Ph.D.James A. Deye, Ph.D., Chairman
F. Marc Edwards, Ph.D., Liaison SC 91Kenneth R. Kase, Ph.D., Liaison SC 46
Eric E. Kearsley, Ph.D.Jeffrey H. Kleck, Ph.D.
Richard C. McCall, Ph.D.Patton H. McGinley, Ph.D.James E. Rodgers, Ph.D.Raymond K. Wu, Ph.D.
J Rodgers AAPM 01
7
… Synopsis of new NCRP… Synopsis of new NCRP… Synopsis of new NCRP… Synopsis of new NCRPCommittee Committee Committee Committee Guidleines Guidleines Guidleines Guidleines forforforforMegavoltage Megavoltage Megavoltage Megavoltage Vault DesignVault DesignVault DesignVault Design
1. X-rays from 4 MV to 50 MV (including 60Co)
2. All shielding data has been reviewed and updated.
3. Calculational scheme--follow Report 49 methodswhere possible and apply more general methodswhen IMRT and/or TBI are involved.
J Rodgers AAPM 01
… Synopsis of new NCRP… Synopsis of new NCRP… Synopsis of new NCRP… Synopsis of new NCRPCommittee Committee Committee Committee Guidleines Guidleines Guidleines Guidleines forforforforMegavoltage Megavoltage Megavoltage Megavoltage Vault DesignVault DesignVault DesignVault Design
4. Addresses new techniques for maze doorrequirements:
* Low energy ([[[[ 10 MV)
old and new semi-empirical formulas
* High energy
new empirical formulas for neutrons
& capture gamma-rays
5. Laminated barriers for high energy x-raysJ Rodgers AAPM 01
SummarySummarySummarySummary
❋❋❋❋ IMRT procedures cansignificantly increase secondarybarrier thickness requirements.(…can be as much as 1 TVL)
❋❋❋❋ IMRT with IMRT with IMRT with IMRT with high-energyhigh-energyhigh-energyhigh-energy x-rays x-rays x-rays x-raysalso produces a significant wholealso produces a significant wholealso produces a significant wholealso produces a significant wholebody neutron effective dosebody neutron effective dosebody neutron effective dosebody neutron effective dose
J Rodgers AAPM 01
SummarySummarySummarySummary
❋❋❋❋ IMRT with IMRT with IMRT with IMRT with low-energylow-energylow-energylow-energy x-rays x-rays x-rays x-raysavoids the neutron problem andavoids the neutron problem andavoids the neutron problem andavoids the neutron problem andmay require no additionalmay require no additionalmay require no additionalmay require no additionalsecondary shielding if a dualsecondary shielding if a dualsecondary shielding if a dualsecondary shielding if a dualenergy machine.energy machine.energy machine.energy machine.
J Rodgers AAPM 01
AiAiAiAirman to officer :an to officer :an to officer :an to officer :
radiusisocenter linac alconvention R barrier of side proximal toisocenter from distance L :where
28.3
1R
CR
240.02.0
CW
SerialTomo
R
L2.0 W
RL
40.02 W
W
===
==
≅=
∗=∗∗=
Primary Barrier Width
Conventional Spiral Tomo
CSpiralTomo
C
SpiralTomo
SpiralTomo
Tomo SpiralC
C
R cm 85 radiusisocenter Tomo spiralR
radiusisocenter linac alconvention R barrier of side proximal toisocenter from distance L :where
9.61
RCR
240.05.0
CW
SpiralTomo
RL
5.0 W RL
40.02 W
W
≠==
==
≅=
∗=∗∗=
Conventional Use Factors0o
180oU = 1/4
U=1
90o U=1/4U = 1/4
270o
fs/2
fs/2
R
L
P
Ψ
θθ Ψ
R L
φ
++
= −−−
T
11
T
1T 2R
fstan2
sin2Lfssin
2Rfstan1U π
π
0.09U 4m L and 40cm fs 0.85m,RFor
0.04U 4m L and 20cm fs 1.0m,RFor
2Rfs
2Lfs
2Rfs
U
TSpiralTomo
TSerialTomo
TTT
====
====
++
= −−− 111 tan
2sinsintan
1 ππ
5
Primary Beam Barriers
Annual Workload • Annual dose delivered by the primary beam
– dose per fraction (DF) [DF per rotation]– rotations per patient (N)– dose escalation factor ( )– daily through put (P) [patients per day]– days worked per year (D)
e
Annual Workloaddose per fraction
• Conventional therapy- DFC => 180 to 225 cGy per fraction
S. Mutic, D. A. Low, E. E. Klein, J. F. Dempsey, and J. S. Mutic, D. A. Low, E. E. Klein, J. F. Dempsey, and J. A. Purdy, Room Shielding for Intensity Modulated A. Purdy, Room Shielding for Intensity Modulated Radiation Therapy Treatment Facilities, Int. J.Radiation Therapy Treatment Facilities, Int. J. RadiatRadiat..OncolOncol. Biol. Phys., 50, 239. Biol. Phys., 50, 239--246, (2001).246, (2001).
Because of the relatively inefficient delivery of IMRT with respect to use of monitor units (EIMRT<< 1), we propose a model that decouples workloads for primary, leakage, and scatter shielding calculations
The total weekly tumor dose (TD) is used for primary barrier calculations
The use factor for DMLC (Up, D) is based on conventional conformal therapy use factor (Uc) and is modified by the ratio of exposure rates outside the primary barrier for the average field size fs to that of the 40 x 40 cm2 field size,
A primary barrier workload calculation includes the average number of indexes (I) used to treat patients
In addition, the use factor is the fraction of gantry angle (κ) that the beam is incident on the primary barrier. λ accounts for leakage radiation contribution to primary barrier and scatter within the primary barrier
The workload is based on the total weekly monitor unitsThe workload is based on the total weekly monitor units
The ratio EThe ratio EDD/ E/ EC C or Eor Et t / E/ EC C will determine the potential will determine the potential increase in leakage barrier shielding requirements increase in leakage barrier shielding requirements Values for the ratios EValues for the ratios EDD/ E/ EC C or Eor Et t / E/ ECC are likely greater are likely greater than 2 for DMLC and as high as 10 for serial tomotherapythan 2 for DMLC and as high as 10 for serial tomotherapyThere may be a significant increase in secondary barrier There may be a significant increase in secondary barrier thickness due to the increased leakage componentthickness due to the increased leakage component
Scattered radiation intensity is proportional to the energy absorbed in the patientIgnore scatter radiation shielding due to the increased shielding provided for the higher energy leakage radiation
- BP,D - the primary barrier transmission factor- dp – the distance from the x-ray target to the point of interest- P - the permissible weekly exposure rate [Sv wk -1]- Wp,D - the weekly workload and is equal to the total weekly dose- Up,D - the primary barrier use factor, T is the occupancy factor
- Bl,D - the secondary barrier transmission factor- dsec - the distance from the isocenter to the point of interest- P - the weekly permissible exposure rate [Sv wk -1]- Wl,D - the workload and is equal to the total weekly MUs- T - the occupancy factor
- BP,t - the primary barrier transmission factor- dp – the distance from the x-ray target to the point of interest- P - the weekly permissible exposure rate [Sv wk -1]- Wp,t - the weekly workload and is equal to the total weekly dose- Up,t - the primary barrier use factor, T is the occupancy factor
- Bl,t - the secondary barrier transmission factor- dsec - the distance from the isocenter to the point of interest- P - the weekly permissible exposure rate [Sv wk -1]- Wl,t - the workload and is equal to the total weekly MUs- T - the occupancy factor
For DMLC delivery 14 cm wide, 40 cm long portal was used, with 2, 4, 6, 8, 10, 12, and 14 cm wide sliding windowsFor tomotherapy a 3.7 x 20 cm 2 open field (at isocenter) was used300 cm 3, 6 atmosphere air-filled pressurized ionization chamber located at 30 cm beyond the primary barrier30 cm beyond the primary barrier
Measurements should, ideally, have been made in integrate modeDue to low signal in integrate mode - measurements were made in dose rate modeDose rate was measured for static subportalsThe integral dose rate was determined using the MLC positions as functions of fractional monitor units obtained from the sliding window ports Measurement Point
Sequential TomotherapySequential Tomotherapy»» MU component (MU component (λλ) from 15% to 60% compared to geometric estimate ) from 15% to 60% compared to geometric estimate TD * TD * κκ of of
single index workload. single index workload. »» 7 indexes, I 7 indexes, I κκ λλ ranges from 0.61 for 180ranges from 0.61 for 180°° to 0.35 for 360to 0.35 for 360°° arcarc»» ηη ((fsfs=3.8 x 20 cm=3.8 x 20 cm22) = 0.24) = 0.24»» 7 indexes, 7 indexes, ηη ((fsfs=3.8 x 20 cm=3.8 x 20 cm22)I )I κκ λλ ranges from 0.15 for 180ranges from 0.15 for 180°° to 0.08 for 360to 0.08 for 360 °° arcarc
–– Primary BarrierPrimary Barrier»» Comparable to conventional shielding due to reduced number of trComparable to conventional shielding due to reduced number of treated eated
patients and patients and ηη I I κκ λλ productproduct–– Secondary Barriers Secondary Barriers
»» Increased due to leakageIncreased due to leakage»» MIR experience MIR experience ≅≅ 3,000 MU/fraction3,000 MU/fraction»» 300,000 MU/week for 20 patients300,000 MU/week for 20 patients