imrt shielding symposium 2001 aapm annual meeting salt … · j rodgers aapm 01 w*u, dpri, t, p j...

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

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


c) Data analysis


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

[email protected]

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

J Rodgers AAPM 01

Segmented Field Shaping for SMLC

S2

S3S1

S4

PlanningTargetVolume

2

J Rodgers AAPM 01

Dynamic Field Shaping for DMLC

J Rodgers AAPM 01

Dynamic Field Shaping for DMLC)

J Rodgers AAPM 01

Dynamic Field Shaping for DMLC

J Rodgers AAPM 01

Dynamic Gantry Rotation andDynamic Gantry Rotation andDynamic Gantry Rotation andDynamic Gantry Rotation andField ShapingField ShapingField ShapingField Shaping

J Rodgers AAPM 01

dynamicallymodulated (mlc)

fan beam

Patienttranslated

axially

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.

J Rodgers AAPM 01

VAULT SHIELDING

Review of Traditional CalculationalMethods

NCRP Report 49 (1976)Primary barriers:Primary barriers:Primary barriers:Primary barriers:

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)

Secondary barriersSecondary barriersSecondary barriersSecondary barriers::::

P = BP = BP = BP = BLLLL *10 *10 *10 *10-3-3-3-3 *W [U] T/d *W [U] T/d *W [U] T/d *W [U] T/dLLLL2 (leakage)2 (leakage)2 (leakage)2 (leakage)

P = BP = BP = BP = Bscascascasca *a( *a( *a( *a(θθθθ)))) *W [U] T/d *W [U] T/d *W [U] T/d *W [U] T/dscascascasca2222 *F/400 *F/400 *F/400 *F/400

((((scatter radiationscatter radiationscatter radiationscatter radiation))))

J Rodgers AAPM 01

dpri

Primarybarrier

X-ray target

isocenter

J RodgersAAPM 01

W*U, dpri, T, P

J Rodgers AAPM 01

Primary

barrier

secondary

gantry axisisocenter

gantry or beam plane

Leakage from accelerator head to point S

S

X-ray target

J RodgersAAPM 01

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

& consequently:& consequently:& consequently:& consequently:

W(direct) = W(Leakage) =W(scatter)W(direct) = W(Leakage) =W(scatter)W(direct) = W(Leakage) =W(scatter)W(direct) = W(Leakage) =W(scatter)

J Rodgers AAPM 01

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

5

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.

❋❋❋❋ Secondary barrier(s)Secondary barrier(s)Secondary barrier(s)Secondary barrier(s) : : : :

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”.

J Rodgers AAPM 01

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

Whole Body DE for 70 Gy TXDose

0

200

400

600

800

1000

1200

1400

1600

0 5 10 15 20 25

TG-21 nominal accelerating pot'l ( MV)

mS

v

C=4

C=10

data from P H McGinley’s article

J Rodgers AAPM 01

Lifetime risk of fatal cancer (Q=10)

0.01.02.03.04.05.06.07.08.0

5 10 15 20 25TG-21 nominal accelerating pot'l (MV)

risk

(%)

C=4C=10

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 :

“Our anti-anti-“Our anti-anti-“Our anti-anti-“Our anti-anti-missle issle issle issle missle issle issle issle just shot itselfjust shot itselfjust shot itselfjust shot itselfdown, sir!”down, sir!”down, sir!”down, sir!”

The END

J Rodgers AAPM 01

1

IMRT Shielding IMRT Shielding considerations for considerations for

Tomotherapy Tomotherapy and MLCand MLCbeam deliverybeam delivery

Don Robinson, Don Robinson, PhPh.D..D.Dept. of Medical PhysicsDept. of Medical Physics

Cross Cancer InstituteCross Cancer Institute

Overview

• Serial and Spiral Tomotherapy

• Tomotherapy Shielding Considerations

• Shielding for IMRT with MLC

Overview




Tomotherapy(slice therapy)

Serial and Spiral Tomotherapy

• Serial (axial) Tomotherapy using the NOMOS MIMiC collimator (NOMOS Corp.)

• Spiral (helical) Tomotherapy with the Tomotherapy machine (University of Wisconsin, TomoTherapyTM Inc.)

Serial Tomotherapy(axial delivery)

2

The NOMOS MIMiC(multileaf intensity-modulating collimator)

• special purpose collimator for delivery of serial (axial) tomotherapy

• retrofit to a standard linac• consists of 2 banks of 20 tungsten leaves

each projecting a 1 X 1 cm2 field at 100 cm• each leaf driven in and out of the field by a

pneumatic piston• IMRT by binary positioning (in - out) of

these 40 leaves as gantry arcs about patient

NOMOS MIMiC Collimator

Salient Shielding Points• Maximum field size = 2 X 20 cm2

• Continuous gantry rotation per slice• Multiple (N) gantry rotations per fraction• Each rotation treats two slices 1 cm in width


• Continuous gantry rotation per slice• Multiple (N) gantry rotations per fraction• Each rotation treats two slices 1 cm in width

2L

N field=

Tomotherapy(spiral delivery)

MegavoltageImage Detectors

Temporally ModulatedMultileaf Collimator

(MLC)

Beam Stop

Slit DefiningSecondaryCollimator

Patient

Table

In-LineLinac

Image ReconstructionTreatment Planning

Treatment VerificationComputer

Accelerator ControlTemporally Modulated

Multileaf CollimatorComputer

Tomotherapy Treatment Unit

3


• Minimum slice width SWmin = 0.5 cm• leaf width @ iso = 0.625 cm• Continuous gantry rotation• Multiple (N) gantry rotations per fraction

Salient Shielding PointsSalient Shielding Points• Maximum field size = 5 X 40 cm2

• Minimum slice width SWmin = 0.5 cm• leaf width @ iso = 0.625 cm• Continuous gantry rotation• Multiple (N) gantry rotations per fraction

SWL

N field=

Salient Shielding Points• Additional (1 TVL) head leakage shielding• Integral beam stop (1.8 TVL)• 6MV only

Overview




Tomotherapy Shielding Considerations

• primary beam barrier

• secondary (leakage) barriers


• primary beam barrier– barrier width– use factor– annual workload– barrier thickness

4

Primary Barrier Width (W)

Parameter Serial Spiral

• Max lateral 20.0 cm 40.0 cmfield size

• Max field length 2.0 cm 5.0 cm

• collimator rotation no no

• isocentric radius RT 100 cm 85 cm

Primary Barrier Width

Conventional Serial Tomo

CSerialTomo

C

SerialTomo

SerialTomo

Tomo SerialC

C

R100cmradiusisocenter Tomo serial R

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

===

==

≅=

∗=∗∗=


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

• IMRT- dose escalation?- DFT => 250 - 300 cGy/fraction, higher?

Annual Workloadfraction dose escalation factor

CFDT

FD

DFeDF

dose fractional alConventiondose fractional yTomotherap

e

∗=

=

Annual Workloadrotations (N) per patient

• Field length and slice width dependent• assume field length L = 20 to 30 cm • serial tomotherapy: N = L/2

– Nserial (L =30 cm) = 15

• spiral tomotherapy: Nspiral =L/SW– assume slice width of 0.5 cm to 1.0 cm– Nspiral (L = 20 cm, SW = 0.5 cm) => Nspiral = 40

Annual Workloaddaily throughput

• assume PT =PC

conventional treatment patient daily through put (PC)

Tomotherapy patient daily through put (PT)

Annual Workloaddays worked per year

• Assume junior therapist with minimum of 2 weeks vacation per year

=> DC = DT = 250 days/year

6

Annual WorkloadAnnual Workload

CCFDCTomo

TTTTomo

DPNeDFW

DPNDF W

∗∗∗∗=

∗∗∗=fs/2

fs/2

R

L

P

Ψ

θθ Ψ

R L

φ

( )∫ ++=

ψ

ψψ

π 0 T22

T

2TCP

T LcosRLRd

2RNW

D2

2

( )

++

=

++

+

−+=

++=

−−−

−

∫

T

11

T

1

T22

T

22T

44T

1

22T

44T

2TCP

T

0T

22T

2TCP

T

2Rfs

tan2

sin2Lfs

sin2Rfs

tan

L2RLR2tanL2R-LR

tanLR2LR2

RNW4D

LcosR2LRd

2RNW2

D

πψ

ψ

π

ψψ

πψ

( )

+

++

+

−++=

=

+=

=

=

−

2C

2CCC

T22

T

22T

44T

1

22T

44T

2TCDF

10CT

LPC

PT

10CT

L

PC

10CL

PT

10T

LR

RUW

L2RLR2

?tanL2R-LRtan

L2RLR2p

RNW4e

logNN

limit dose designD DD

logNN

DD

logN DD

logN

Primary Barrier ThicknessSerial Tomo vs Conventional

( )DFprimC

primT

C

TC

elog0.83NN

15N 0.25U

4mL 1m,R 1m,RFor

++=

==

===

Primary Barrier ThicknessSpiral Tomo vs Conventional

( )DFprimC

primT

C

TC

elog1.04NN

40N 0.25U

4mL 0.85m,R 1m,RFor

++=

==

===

7


• primary beam barrier

• secondary (leakage) barriers


• secondary beam barriers– Modulation Factor– Leakage Factor– barrier thickness

Modulation Factor

1 rotation)(per factor efficiency delivery IMRTE

1 )conditions on(calibratifactor efficiency delivery alconventionE

cGy 1deliver to

rotationper required MU MF:where

EE

MF

IMRT

C

Tomo

IMRT

CTomo

<=

==

=

=

Leakage Factor

• Serial Tomo (NOMOS MIMiC)=> LFT = LF C

• Spiral Tomo (Tomotherapy Machine)=> LFT = 0.1 LFC

TDFCTomoLT LFeWNMFD ∗∗∗∗∝

Secondary Barrier ThicknessSecondary Barrier Thickness(leakage)(leakage)

R

L

X

Z

Gan

try

R

x

y

z

P(x,z)

8

( )

( )

load work primary alconventionW:where

R4xRzx

rLFNMFeWD

?cos2xR-Rzxd?

2prLFNMFeW2

D

LFeWNMFD

PC

2T

22T

22

2TTomoDF

PCL

T

p

0 T2T

22

2TTomoDF

PCL

T

TDFCTomoLT

=

−++

∗∗∗∗∗=

++∗∗∗∗∗∗

=

∗∗∗∗∝

∫

Secondary Barrier ThicknessSecondary Barrier Thickness(leakage)(leakage)

Secondary Barrier ThicknessTomo vs Conventional

C

TTomoDFL

C

LT

LFLF

NMFeDD

∗∗∗≅

Secondary Barrier ThicknessSecondary Barrier ThicknessSerialSerial TomoTomo vs Conventionalvs Conventional

( )

( )DFLeakC

LeakT

TomoDFLCL

C

LTL

CLT

CTTomo

elogTVL 1.88NN

NMFelogNDD

logN N

LFLF 15, N 5, MF

++=∴

∗∗+=

+=

===

Secondary Barrier ThicknessSecondary Barrier ThicknessSpiral Spiral TomoTomo vs Conventionalvs Conventional

( )

( )DFLeakC

LeakT

TomoDFLCL

C

LTL

CLT

CTTomo

elogTVL 1.3NN

NMFelogNDD

logN N

LF*0.1LF 40, N 5, MF

++=∴

∗∗+=

+=

===

Tomotherapy ShieldingConclusions

Tomotherapy requires:

• Narrower primary barrier – ~ 1/10 (spiral) to 1/28 (serial) that of conventional

• Thicker primary barrier– ~ 1 TVL thicker

• Thicker secondary barriers– up to 2 TVL’s thicker

Overview



• Shielding for IMRT with MLC (discrete gantry angles)

9

IMRT with MLCShielding Considerations

• primary beam barrier– barrier width– use factor– annual workload– barrier thickness

Primary Barrier Width (W)Parameter Conventional MLC-IMRT

• Max lateral 40.0 cm 40.0 cmfield size

• Max field length 40.0 cm 40.0 cm

• collimator rotation yes yes

• isocentric radius R 100 cm 100 cm


• Primary barrier width for IMRT with MLC remains unchanged from that for conventional linac use.

Use FactorsConventional = MLC-IMRT?

0o

180oU = 1/4

U=1

90o U=1/4U = 1/4

270o

Primary Beam Barriers

Annual Workload • Annual dose delivered by the primary beam

– dose per fraction (DF)– dose escalation factor ( )– daily through put (P) [patients per day]– days worked per year (D)

e

Annual Workloaddaily throughput

• assume PIMRT =PC

conventional treatment patient daily through put (PC)

MLC-IMRT patient daily through put (PIMRT)

10

Annual Workloaddays worked per year

• Assume junior therapist with minimum of 2 weeks vacation per year

=> DC = DIMRT = 250 days/year

Annual Workload

CCFDCIMRT

IMRTIMRTIMRTIMRT

DPeDF W

DPDFW

∗∗∗=

∗∗=


• primary beam barrier– barrier width– use factor– annual workload– barrier thickness ( )

limit dose designD

elogND

DlogNN

DDPDFD

logN

DDPeDFD

logN

L

FD10CPC

PIMRT

10CIMRT

L

CCCPC

10C

L

IMRTIMRTFDCPIMRT

10IMRT

=

+=

+=

∗∗==

∗∗∗==

Primary Barrier ThicknessIMRT with MLC vs

Conventional

( )DFprimC

primIMRT elogNN +=

• Small increase in primary beam shield thickness


• secondary beam barriers– Modulation Factor– Leakage Factor– barrier thickness

11

Modulation Factor(CCI experience)

• Nasopharynx: sparing of parotid glands and esophagus

• field length ranging from 17 cm to 26 cm• mean field length ~ 22 cm

• MF’s ranging from 3.6 to 10.3• mean MF => 6.6

Modulation Factor

• for shielding calculations: MFIMRT = 10

Leakage Factor

• Performed with a conventional accelerator outfitted with an MLC

• LFIMRT = LCCCDFCIMRT

LIMRT LFeWMFD ∗∗∗∝

Secondary Barrier Thickness(leakage)

Secondary Barrier ThicknessMLC- IMRT vs Conventional

FDIMRTLC

LIMRT eMFD

D∗=


( ) ( )

( )FDLeakC

LeakIMRT

FDIMRTLCL

C

LIMRTL

CLT

IMRT

elogTVL 1.0NN

elogMFlogND

DlogN N

10 MF

++=∴

++=

+=

=

12


( )DFleakC

leakIMRT elogTVL 1.0NN ++=

• increase in secondary shielding thickness~ 1TVL

IMRT with MLC Conclusions

IMRT with MLC requires:

• Same width primary barrier • Small increase in primary barrier thickness• Thicker secondary barriers

– ~ 1 TVL thicker

WashingtonWashingtonWASHINGTONWASHINGTON••UNIVERSITYUNIVERSITY••ININ••STST••LOUISLOUIS

Sasa Mutic , AAPM 2001

Room Shielding for Intensity Modulated Radiation Therapy Treatment Facilities

Sasa Mutic, Daniel A. Low, Eric E. Sasa Mutic, Daniel A. Low, Eric E. Klein, James F. Dempsey, and James A. Purdyand James A. Purdy

Mallinckrodt Institute of RadiologyMallinckrodt Institute of RadiologyWashington University School of MedicineWashington University School of Medicine

St. Louis, Missouri 63110 St. Louis, Missouri 63110



ReferenceReference

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).



Conventional Radiation TherapyConventional Radiation TherapyRoom Shielding Formalism Room Shielding Formalism

NCRP Reports No. 49, 51, and 79Primary and secondary (leakage and scatter) barriersBarrier thickness based on:

Where: B = barrier transmissionP = weekly design exposure rateW = workloadU = use factorT = occupancyd = distance from radiation sourcem = correction factors for secondary barriers

mWUTPd

B •=2



Conventional Radiation TherapyConventional Radiation TherapyRoom Shielding Formalism Room Shielding Formalism

The workload is assumed to be equal to the Total Weekly Dose Delivered (TD) and to the Total Weekly Monitor Units Delivered (MU)

Subscript “c” stands for “conventional”

Ec = conventional radiation therapy delivery efficiency factor, assumed to be equal to 1

The workload value is the same for primary and secondary barriercalculations

MUTDEcMUcMUcETDcW

≡≅•==



The Proposed IMRT The Proposed IMRT Room Shielding Formalism Room Shielding Formalism

Depending on the complexity of the target volume and the intensity modulation device

EEcc = conventional radiation therapy delivery efficiency factor= conventional radiation therapy delivery efficiency factorEEIMRTIMRT = IMRT delivery efficiency factor= IMRT delivery efficiency factor

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

IMRTEEc >> IMRTMUMUc <<



The Proposed IMRTThe Proposed IMRTRoom Shielding Formalism Room Shielding Formalism

1) For workload estimates, tthe total delivered weekly monitor units and the total delivered weekly dose are are decoupleddecoupled

2) The DMLC and serial tomotherapy delivery shielding calculations are separated

3) Occupancy factor (T) remains unchanged4) Weekly design exposure rate (P) remains unchanged5) Primary, leakage, and scatter components still separated6) We did not address neutron shielding requirements



Primary Barrier FormalismPrimary Barrier Formalism--DMLCDMLC

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,

TDDpW =,

( ) cDp UfsU η=,( )

( )4040)(

xDfsD

fs =η

Outside the barrier



Primary Barrier FormalismPrimary Barrier Formalism--Serial TomotherapySerial Tomotherapy

Workload is based on the total weekly tumor dose

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

ti θθκ =

TDW tp =,

( ) λκη IxfsU tp 208.3, ==



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

Secondary Barrier Formalism Secondary Barrier Formalism -- LeakageLeakage

MUW Dl =, MUW tl =,



Secondary Barrier Formalism Secondary Barrier Formalism -- ScatterScatter

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

6 MVMaterial TVL (in) HVL (in)Concrete 13.5 4.1Iron/Steel 3.9 1.2Lead 2.2 0.7

TDW Ds =, TDW ts =,



Formalism SummaryFormalism Summary--DMLCDMLC

- 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

TUWPd

BDPDP

PDP

,,

2

, =

Primary Barrier

( ) cDp UfsU η=,



Formalism SummaryFormalism Summary--DMLCDMLC

- 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

Leakage Barrier

TWPd

BDl

Dl,

2sec

,1000

=



Formalism SummaryFormalism Summary--Serial TomotherapySerial Tomotherapy

- 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

Primary Barrier

TUWPd

BtPtP

PtP

,,

2

, =

( ) λκη IxfsU tp 208.3, ==



Formalism SummaryFormalism Summary--Serial TomotherapySerial Tomotherapy

- 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

Leakage Barrier

TWPd

Btl

tl,

2sec

,1000

=



Experimental VerificationExperimental Verification

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

Dose Fraction = 0.000

Center of the Field


Center of the Field


Center of the Field


Center of the Field



Experimental VerificationExperimental Verification

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

30cm from the wall

Varian Clinac 2300C/D



Normalized Output Factors Inside and Outside the RoomNormalized Output Factors Inside and Outside the Room

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 5 10 15 20 25 30 35 40

Outside RoomInside RoomIsocenter

Nor

mal

ized

Dos

e R

ate

Static Field Size (cm)WashingtonWashington

WASHINGTONWASHINGTON••UNIVERSITYUNIVERSITY••ININ••STST••LOUISLOUISSasa Mutic , AAPM 2001

4 cm Window Example4 cm Window Example

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.0 0.2 0.4 0.6 0.8 1.0

Dos

e R

ate

Rat

io

Fraction of Delivered MUs



Dose Delivery Efficiency Dose Delivery Efficiency -- DMLCDMLC

WindowWidth14x40 cm

2

EIMRT

BeyondShield

EIMRT

IsocenterRatio±0.04

2 0.194 0.169 1.154 0.264 0.259 1.026 0.343 0.332 1.038 0.394 0.393 1.0010 0.448 0.444 1.0112 0.492 0.486 1.0114 0.521 0.524 0.994, 10x10 cm2 0.326 0.323 1.01



Results Results -- TomotherapyTomotherapy

0.0

0.2

0.4

0.6

0.8

1.0

1.2

-15 -10 -5 0 5 10 15

Rel

ativ

e D

ose

Rat

e

Incidence Gantry Angle (deg)



Transmission Correction Transmission Correction -- TomotherapyTomotherapyLambdaLambda

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

0 50 100 150 200 250 300 350 400

λ

Arc Length (degrees)



Results Results -- TomotherapyTomotherapy

0.0

0.2

0.4

0.6

0.8

1.0

0 50 100 150 200 250 300 350 400Rotation Angle (degrees)

κ



ConclusionsConclusions

Primary ShieldingPrimary Shielding––Significant Field Size Factor (0.35 Significant Field Size Factor (0.35 -- 0.80 relative to 0.80 relative to

40x40)40x40)––DMLCDMLC

»» 40x40 overestimates IMRT conformal therapy 40x40 overestimates IMRT conformal therapy shielding requirementsshielding requirements

»» Using TD for sliding window W Using TD for sliding window W -- accurate within accurate within 10% to 2 cm window10% to 2 cm window

–– Primary Barriers Primary Barriers -- Comparable to conventional Comparable to conventional shieldingshielding

–– Secondary Barriers Secondary Barriers -- Possible increase due to leakagePossible increase due to leakage



ConclusionsConclusions

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

ηκλITDUW tt =

imrt shielding symposium 2001 aapm annual meeting salt … · j rodgers aapm 01 w*u, dpri, t, p j...

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