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The Ottawa L’HopitalHospital d’OttawaRegional Cancer Centre

Neelam Tyagi, Ph.D.

Joanna E. Cygler, Ph.D.2The Ottawa Hospital Cancer Centre, Ottawa, Canada3Carleton University Dept. of Physics, Ottawa, Canada

4University of Ottawa, Dept. of Radiology. Ottawa, Canada

Clinical Implementation and Application of Monte Carlo Methods in Photon & Electron

Dose Calculation – New Issues to Consider in Clinical Practice

Part II: Electron beamsPart II: Electron beams

Joanna E. Cygler, Ph.D., FCCPM, FAAPM

The Ottawa Hospital Cancer Centre, Ottawa, CanadaCarleton University, Dept. of Physics, Ottawa, Canada

University of Ottawa, Dept. of Radiology, Ottawa, Canada

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Outline

• Rationale for MC dose calculations for electron beams

• Commercially available Monte Carlo based electron treatment planning systems

• Clinical implementation of MC-based TPS• Issues to pay attention to when using MC based

system • Timing comparisons of major vendor MC codes in

the clinical setting.

Rationale for Monte Carlo dose calculation for electron beams

• Difficulties of commercial pencil beam based algorithms– Monitor unit calculations for arbitrary

SSD values – large errors*– Dose distribution in inhomogeneous media

has large errors for complex geometries

* can be circumvented by entering separate virtual machines for each SSD – labour consuming

3

-10 -5 0 5 100

5

10

15

6.2 cm

9 MeV

depth = 7 cm

depth = 6.2 cm

98-10-21/tex/E TP /abs/X TS K 09S .OR G

Measured Pencil beam Monte Carlo

Rela

tive

Dose

Horizontal Position /cm

Rationale for Monte Carlo dose calculation for electron beams

Ding, G. X., et al, Int. J. Rad. Onc. Biol Phys. (2005) 63:622-633

Commercial implementations• MDS Nordion (now Nucletron) 2001

- First commercial Monte Carlo treatment planning for electron beams

– Kawrakow’s VMC++ Monte Carlo dose calculation algorithm (2000)– Handles electron beams from all clinical linacs

• Varian Eclipse eMC 2004– Neuenschwander’s MMC dose calculation algorithm (1992)

– Handles electron beams from Varian linacs only (23EX)

– work in progress to include linacs from other vendors

• CMS XiO eMC for electron beams 2010– Based on VMC (Kawrakow, Fippel, Friedrich, 1996)

– Handles electron beams from all clinical linacs

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Nucletron Electron Monte Carlo Dose Calculation Module

•Originally released as part of Theraplan Plus

•Currently sold as part of Oncentra Master Plan

•Fixed applicators with optional, arbitrary inserts, or variable size fields defined by the applicator like DEVA

•Calculates absolute dose per monitor unit (Gy/MU)

•User can change the number of particle histories used in calculation (in terms of particle #/cm2)

•Data base of 22 materials

•Dose-to-water is calculated in Oncentra

•Dose-to-water or dose-to-medium can be calculatedin Theraplan Plus MC DCM

•Nucletron performs beam modeling

510(k) clearance (June 2002)

Varian Macro Monte CarloVarian Macro Monte Carlotransport model in Eclipsetransport model in Eclipse

• An implementation of Local-to-Global (LTG) Monte Carlo:– Local: Conventional MC simulations of electron transport performed

in well defined local geometries (“kugels” or spheres). • Monte Carlo with EGSnrc Code System - PDF for “kugels”• 5 sphere sizes (0.5-3.0 mm)• 5 materials (air, lung, water, Lucite and solid bone)• 30 incident energy values (0.2-25 MeV)• PDF table look-up for “kugels”

This step is performed off-line.

– Global: Particle transport through patient modeled as a series of macroscopic steps, each consisting of one local geometry (“kugel”)

C. Zankowski et al “Fast Electron Monte Carlo for Eclipse”

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Varian Macro Monte CarloVarian Macro Monte Carlotransport model in Eclipsetransport model in Eclipse

• Global geometry calculations– CT images are pre-processed to

user defined calculation grid

– HU in CT image are converted to mass density

– The maximum sphere radius and material at the center of each voxel is determined

• Homogenous areas → large spheres

• In/near heterogeneous areas →small spheres

C. Zankowski et al “Fast Electron Monte Carlo for Eclipse”

Varian Eclipse Monte CarloVarian Eclipse Monte Carlo

• User can control– Total number of particles per simulation

– Required statistical uncertainty

– Random number generator seed

– Calculation voxel size

– Isodose smoothing on / off• Methods: 2-D Median, 3-D Gaussian

• Levels: Low, Medium, Strong

• Dose-to-medium is calculated

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CMS XiO Monte Carlo systemCMS XiO Monte Carlo system• XiO eMC module is based on VMC*

– simulates electron (or photon) transport through voxelizedmedia

• The beam model and electron air scatter functions were developed by CMS

• The user can specify– voxel size – dose-to-medium or dose-to-water – random seed– total number of particle histories per simulation – or the goal Mean Relative Statistical Uncertainty (MRSU)

• CMS performs the beam modeling

*Kawrakow, Fippel, Friedrich, Med. Phys. 23 (1996) 445-457*Fippel, Med. Phys. 26 (1999) 1466–1475

Software commissioning tests• Criteria for acceptability

– Van Dyk et al, Int. J. Rad. Oncol. Biol. Phys., 26, 261-273,1993; – Fraass, et al, AAPM TG 53: Quality assurance for clinical radiotherapy

treatment planning,” Med. Phys. 25, 1773–1829 1998

• Homogeneous water phantom• Inhomogeneous phantoms (1D, 2D, 3D, complex)

– Cygler et al, Phys. Med. Biol., 32, 1073, 1987 – Ding G.X.et al, Med. Phys., 26, 2571-2580, 1999– Shiu et al, Med.Phys. 19, 623—36, 1992; – Boyd et al, Med. Phys., 28, 950-8, 2001

• Measurements, especially in heterogeneous phantoms, should done with a high (1 mm) resolution

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User input data for MC based TPS

• Position and thickness of jaw collimators and MLC

• For each applicator scraper layer:ThicknessPositionShape (perimeter and edge)Composition

• For inserts:ThicknessShapeComposition

Treatment unit specifications:

No head geometry details required for Eclipse, since at this time it only works for Varian linac configuration

User input data for MC TPS contDosimetric data for beam characterization, as

specified in User Manual

• Beam profiles without applicators:-in-air profiles for various field sizes–in-water profiles

–central axis depth dose for various field sizes–some lateral profiles

• Beam profiles with applicators:– Central axis depth dose and profiles in water – Absolute dose at the calibration point

Dosimetric data for verification

– Central axis depth doses and profiles for various field sizes

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Clinical implementation of MC treatment planning software

• Beam data acquisition and fitting• Software commissioning tests*

• Clinical implementation– procedures for clinical use– possible restrictions– staff training

*should include tests specific to Monte Carlo

A physicist responsible for TPS implementation should have a thorough understanding of how the system works.

Software commissioning tests: goals

• Setting user control parameters in the TPS to achieve optimum results (acceptable statistical noise, accuracy vs. speed of calculations)– Number of particle histories

– Required statistical uncertainty

– Voxel size

– Smoothing

• Understand differences between water tank and real patient anatomy based monitor unit values

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XiO: 9 MeV XiO: 9 MeV -- Trachea and spineTrachea and spine

Bone

Air

Bone Bone Bone Film Film

Air

Vandervoort and Cygler, COMP 56th Annual Scientific Meeting, Ottawa, June 2010

Lateral profiles at various depths, SSD=100cm, Nucletron TPS

9 MeV, 10x10cm2 applicator, SSD=100cm. Homogeneous water phantom,cross-plane profiles at various depths. MC

with 10k/cm2.

0

10

20

30

40

50

60

70

80

90

100

110

-10 -5 0 5 10

Off - axis / cm

Dos

e / c

Gy

meas.@2cm

calc.@2cm

meas.@3.0cm

calc.@3.0cm

meas.@4.0cm

calc.@4.0cm

20 MeV, 10x10cm2 applicator, SSD=100cm. Homogeneous water phantom. Cross-plane profiles at

various depths. MC with 10k and 50k/cm2.

0

10

20

30

40

50

60

70

80

90

100

110

-10 -5 0 5 10

Off - axis / cm

Dos

e / c

Gy

meas.@d=3cmcalc.@d=3cmcalc.@d=3cm,50kmeas.@d=7.8cmcalc.@d=7.8cmcalc.@d=7.8cm,50kmeas.@d=9cmcalc.@d=9cmcalc.@d=9cm,50k

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Monte Carlo SettingsMonte Carlo Settings: : Noise in the Noise in the distributions distributions

MRSU=10%MRSU=10% MRSU=5%MRSU=5% MRSU=2%MRSU=2%

Histories=1.2x10Histories=1.2x1066 Histories=2.8x10Histories=2.8x1066 Histories=1.6x10Histories=1.6x1077

Varying MRSU, voxel size=2.5×2.5×2.5 mm3, dose-to-medium, 6 MeV beam, 10x10 cm2 applicator

CMS CMS eMCeMC: Cut: Cut--out factorsout factors

Vandervoort and Cygler, COMP 56th Annual Scientific Meeting, Ottawa ,June 2010

Cutout Output Factors: 9 MeV

0.350

0.450

0.550

0.650

0.750

0.850

0.950

1.050

1 2 3 4 5 6 7 8 9

Square Cutout Length (cm)

Out

put F

acto

r (cG

y/M

U)

ExperimentalXiO Calculated

Cutout Output Factors: 17 MeV

0.600

0.650

0.700

0.750

0.800

0.850

0.900

0.950

1.000

1.050

1 2 3 4 5 6 7 8 9

Square Cutout Length (cm)

Out

put F

acto

r (cG

y/M

U)

ExperimentalXiO Calculated

SSD=100 cm

SSD=115 cm

SSD=100 cm

SSD=115 cm

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-6 -4 -2 0 2 4 60

10

20

30

40

50

60

70

80

90

100

110

120

4.7 cm

depth = 4.7 cm

Bone

Off-axis Y position /cm

Rel

ativ

e D

ose

18 MeV

Measured eMC

Off-axis X position /cm-6 -4 -2 0 2 4 6

0

10

20

30

40

50

60

70

80

90

100

110

120

depth = 6.7 cm

depth = 7.7 cm

depth = 4.7 cm

Air Air

Bone

Relat

ive D

ose

Measured eMC

18 MeV

Eclipse Eclipse eMCeMC no smoothingno smoothingVoxel size = 2 mm

Ding, G X., et al (2006). Phys. Med. Biol. 51 (2006) 2781-2799.

Eclipse Eclipse eMCeMCEffect of voxel size and smoothing

-6 -4 -2 0 2 4 60

10

20

30

40

50

60

70

80

90

100

110

120

2 mm and with 3D smoothing

2 mm and no smoothing

4.7 cm

5 mm and with 3D smoothing

depth = 4.9 cm

Bone

Off-axis Y position /cm

Relat

ive

Dose

18 MeV

Off-axis X position /cm-6 -4 -2 0 2 4 6

0

10

20

30

40

50

60

70

80

90

100

110

120

2 mm and with 3D smoothing

5 mm and with 3D smoothing

2 mm and no smoothing

depth = 4.9 cm

Air Air

Bone

Relat

ive

Dose

18 MeV

Ding, G X., et al (2006). Phys. Med. Biol. 51 (2006) 2781-2799.

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DoseDose--toto--water vs. dosewater vs. dose--toto--mediummedium

Ding, G X., et al Phys. Med. Biol. 51 (2006) 2781-2799.

0 1 2 3 4 50

10

20

30

40

50

60

70

80

90

100

110

Bone cylinderlocation

BEAM/dosxyz simulation

Bone cylinder is replaced by water-like medium but with bone density

depth in water /cm

Dose

Central Axis Depth /cm

0 1 2 3 4 51.10

1.11

1.12

1.13

1.14

9 MeV

Water/Bone stopping-power ratiosSP

R

01 cm diameter and 1 cm length

Hard bone cylinder 2 cm

Dm - energy absorbed in a medium voxel divided by the mass of the medium element.

Dw - energy absorbed in a small cavity of water divided by the mass of that cavity.

Voxel of medium

w

mmw

SDD ⎟⎠

⎞⎜⎝

⎛=

ρ

Small volume of water

Voxel of medium

DoseDose--toto--water vs. Dosewater vs. Dose--toto--mediummedium

DTM DTM DTWDTW

DTWDTW--DTMDTM

Dose-to-water vs. dose-to-medium, MRSU=2%, voxel size=4×4×4 mm3, 6 MeV beam, 15x15 cm2 applicator, both 602 MU

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Good clinical practice

• Murphy’s Law of computer software (including

Monte Carlo)

“All software contains at least one bug”• Independent checks

MU MC vs. hand calculations

Monte CarloMonte Carlo Hand CalculationsHand Calculations

Real physical dose Real physical dose calculated on a patient calculated on a patient anatomy anatomy

Rectangular water Rectangular water tanktank

Inhomogeneity Inhomogeneity correction includedcorrection included

No inhomogeneity No inhomogeneity correctioncorrection

Arbitrary beam angleArbitrary beam anglePerpendicular beam Perpendicular beam incidence onlyincidence only

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9 MeV, full scatter phantom(water tank)

RDR=1 cGy/MU

Lateral scatter missing

Real contour / Water tank =

=234MU / 200MU=1.17

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MU real patient vs.water tank

MC / Water tank= 292 / 256=1.14

MUMU--real patient vs. water tank:real patient vs. water tank:Impact on DVHImpact on DVH

0

20

40

60

80

100

120

0.0 10.0 20.0 30.0 40.0 50.0 60.0dose / Gy

% v

olum

e

PTV-MU-MC

PTV-MU-WT

LT eye-MU-MC

LT eye-MU-WT

RT eye-MU-MC

RT eye-MU-WT

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Posterior cervical lymph node Posterior cervical lymph node irradiation irradiation -- impact on DVHimpact on DVH

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

45.0

0.0 5.0 10.0 15.0 20.0 25.0 30.0

dose / Gy

PTV

/ cm

3

conventional

customized

Jankowska et al, Radiotherapy & Oncology, 2007

Internal mammary nodes

MC / Water tank= 210 / 206=1.019

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How long does it take?How long does it take?• MC gives entire distribution, not just a few points• time for N beams is the same as for 1 beam

• timing is a complex question since it depends on– statistical uncertainty and how defined– voxel size– field size– beam energy and whether photons or electron– accuracy sought– speed of CPU and optimization of compiler- complexity of patient specific beam modifiers

MonteMonte--Carlo Settings: Effect on Carlo Settings: Effect on computation timecomputation time

Timing Results XiO:

For 9 and 17 MeV beams, 10x10 cm2 applicator and the trachea and spine phantom, timing tests were performed for a clinical XiO Linux workstation, which employs 8 processors, 3 GHz each, with 8.29 GB of RAM.

y = 3.4x-2.0

y = 6.4x-2.1

y = 0.7x-2.0

y = 0.4x-2.0

0

5

10

15

20

25

30

0 0.5 1 1.5 2 2.5MRSU %

time

/ min

9 MeV 2.5 mm voxel

17 MeV 2.5 mm voxel

17 MeV 5 mm voxel

9MeV 5 mm voxel

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Timing – Pinnacle3

dual processor 1.6 GHz Sun workstation, 16 GB RAM.

Fragoso et al.: Med. Phys. 35, 1028-1038, 2008

24.55.2x108134.54.7x10832.11.1x1074 (face)

5.41.5x10829.91.4x1077.13.3x1063 (breast)

1.57.1x1078.1 6.5x1062.11.7x1062 (ear)

3.91.6x108201.4x1074.83.4x1061(cheek)

CPU time(h)

# historiesCPU time(min)

# histories

CPU time(min)

# histories

Patient

2% 1% 0.5%

Overall uncertainty

Timing Timing –– Nucletron TPSNucletron TPSOncentra 4.0Oncentra 4.0

4 MeV Timer Results:Init = 0.321443 secondsCalc = 42.188 secondsFini = 0.00158201 secondsSum = 42.5111 seconds

20 MeV Timer Results:Init = 0.311014 secondsCalc = 110.492 secondsFini = 0.00122603 secondsSum = 110.805 seconds

Anatomy - 201 CT slicesVoxels 3 mm3

10x10 cm2 applicator50k histories/cm2

Faster than pencil beam!

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Timing – Varian Eclipse

Eclipse MMC, Varian single CPU Pentium IV

XEON, 2.4 GHz

10x10 cm2, applicator, water phantom,

cubic voxels of 5.0 mm sides

6, 12, 18 MeV electrons,

3, 4, 4 minutes, respectively

Chetty et al.: AAPM Task Group Report No. 105: Monte Carlo-based treatment planning, Med. Phys. 34, 4818-4853, 2007

Summary - electron beams• Commercial MC based TP systems are available

– fairly easy to implement and use– MC specific testing required

• Fast and accurate 3-D dose calculations• Single virtual machine for all SSDs• Large impact on clinical practice

– Accuracy of dose calculation improved– More attention to technical issues needed– Dose-to-medium is calculated, although some systems calculate

dose-to-water as well– MU based on real patient anatomy (including contour

irregularities and tissue heterogeneities)

• Requirement for well educated physics staff

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New clinical dilemmaNew clinical dilemma

• Accurate dose calculation systems– at last we know what dose we are giving to

tumors and OAR• How are we to incorporate this knowledge

into clinical practice?– Will we change the dose prescription?– We have done it for photon beams and

inhomogeneity corrections in lungs…

• MC-calculated doses can in some instances be significantly different (10-20%) from conventional algorithms, such as radiological pathlength, and convolution-based methods or pencil beam in case of electron beams

• In light of these differences:

- should dose prescriptions change with MC-based

calculations ?

- How?

Discussion: dose prescription issuesDiscussion: dose prescription issues

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Dose prescription Dose prescription -- AAPM TGAAPM TG--105 105 perspectiveperspective

• MC method is just a more accurate dose algorithm

• Dose prescription issues are not specific to MC-based

dose calculation

• As with other changes to the therapy treatment

process, users should correlate doses and prescriptions

with respect to previous clinical experience

ConclusionsConclusions

• Clinical implementation of MC-based systems must be performed thoughtfully and users, especially physicists, must understand the differences between MC-based and conventional dose algorithms

• Successful implementation of clinical MC algorithms requires strong support from the clinical team and an understanding of the paradigm shift with MC algorithms

• A properly commissioned MC-based dose algorithm will improve dose calculation accuracy for electron and photon beams

• More accurate dose calculations may improve dose-biological effect correlations

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AcknowledgementsAcknowledgementsGeorge X. Ding Indrin ChettyGeorge Daskalov Margarida FragosoGordon Chan Charlie MaRobert Zohr Eric Vandervoort Ekaterina Tchistiakova David W.O. Rogers

In the past I have received research support from Nucletron and Varian.TOHCC has a research agreement with Elekta.I hold a research grant from Elekta

Thank YouThank You