improved ctimproved ct--based voxel based voxel …cmpwg.ans.org/icrs12/presentations/mike...

IMPROVED CTIMPROVED CT--BASED VOXEL BASED VOXEL PHANTOM GENERATION FORPHANTOM GENERATION FORPHANTOM GENERATION FOR PHANTOM GENERATION FOR

MCNP MONTE CARLOMCNP MONTE CARLO

Michael Speiser, Ph.D.p ,Department of Radiation OncologyUT Southwestern Medical Center

Dallas, TX

DEPT OF RADIATION ONCOLOGY

September 1st, 2012CMPWG Workshop

Medical Physics Investigations Medical Physics Investigations and Monte Carloand Monte Carlo

Patient + Source = Dose• Source Modelingg• Patient Modeling

• AccurateAccurate• Efficient• Simulation pre- and post-processing• Simulation pre- and post-processing• Radiotherapy analysis


Patient ModelingPatient ModelingPatient ModelingPatient Modeling

“RTMCNP Preprocessor” (DeMarco et al 1997)(DeMarco et al. 1997)


CT “Information”RESCALINGRESCALINGRESCALING

-707 -475 -225 -116

-787 -665 -416 -200

Hounsfield Units-805 -785 -628 -369

Hounsfield Units

-793 -804 -757 -568


MCNPMCNPMCNPMCNP

• Use “nested lattice” featureUse nested lattice feature• Bounding cell• Lattice element• Lattice element• Bounding cell origin


Creating a Lattice StructureCreating a Lattice Structure

-707 -475 -225 -116

di i /

-787 -665 -416 -200-665

y-dimension / “slice thickness”z-dimension /

“voxel height”

805 785 628 369

voxel height

-805 -785 -628 -369x-dimension / “voxel width”

-793 -804 -757 -568

“lattice element”


• Cell comprised of repeating identical shapes

Creating Creating a Lattice Structurea Lattice Structure• Hounsfield Units to Tissues

1. Air ELEMENT WEIGHT %2. Lung3. Fat HUHU

ELEMENTH (Z=1)C (Z=6)N (Z=7)

WEIGHT %10.310.53.1

HU range954 t 824

ρ (g/cm3)0 048

4. Water5. Muscle

( )O (Z=8)

Na (Z=11)Mg (Z=12)

74.90.2…

-954 to -824-824 to -674-674 to -524

0.0480.12540.2978

6. BoneP (Z=15)S (Z=16)

Cl (Z=17)K (Z 19)

0.20.30.30 2

-524 to -374-374 to -224

0.47210.6455

• Alfidi et al (Radiology 1975)

K (Z=19)Ca (Z=20)

0.2…


Alfidi et al. (Radiology, 1975)• ICRP 23 (Reference Man, 1975)• ICRU 44 (1989)

Hounsfield Units to MaterialsHounsfield Units to MaterialsLOOKUP TABLELOOKUP TABLE

DEPT OF RADIATION ONCOLOGY (DeMarco et al. 1997)

Hounsfield Units to MaterialsHounsfield Units to MaterialsLOOKUP TABLE

HU=0HU=0

HU=0 material definition is H2O ρ=1 0 g/cm3


HU 0, material definition is H2O, ρ 1.0 g/cm

RTMCNP, DeMarco RTMCNP, DeMarco et alet al..1. Read CT

G t• Geometry• Hounsfield UnitsS C2. Sub-sample CT

• Lattice of 128x128 or 64x643. HU’s into materials (%Z, ρ)4. “Fill” lattice with materials5. Add modified pulse height tally (MeV / g)


Improving CTImproving CT--Based Model Based Model GenerationGenerationGenerationGeneration

• Flexible geometry modificationsg y• Sub-sampling• Lattice trimming• Isocenter positioning

• Mesh tally definition• Pre- and post-processing

• Material definitions for physical phantoms• Normalization criteria

• Radiotherapy analysis (gamma)


Efficiency: SubEfficiency: Sub--Sampling the CTSampling the CT512x51264x64128x128256x256

Fewer surfaces, faster simulationsL ti t l ti


Less patient resolution

Efficiency: Voxel TrimmingEfficiency: Voxel Trimming

~60% reduction in time required(Speiser et al. 2007)


( p )

Original lattice:S b l d l tti

512 x 512 x 160256 256 160Sub-sampled lattice:

Trimmed lattice:256 x 256 x 160208 x 132 x 100

Trimmed lattice < 6 6% of the original scanDEPT OF RADIATION ONCOLOGY

Trimmed lattice < 6.6% of the original scan

Isocenter / Source PositioningIsocenter / Source Positioning


Energy Deposition Mesh TallyEnergy Deposition Mesh TallyEnergy Deposition Mesh TallyEnergy Deposition Mesh Tally

• Scores energy deposition from all particlesScores energy deposition from all particles• Independent of problem geometry

M V / 3 / ti l• MeV / cm3 / source_particle• Rectangular format (RMESH3)


Efficient Tally DefinitionEfficient Tally Definitiond

TALLY OUTPUT• Uncertainty matrix

“Filled” Lattice

slice thicknessUncertainty matrix• Energy matrix

MeV / cm3 / spvoxel height

1 ID the sub volume of interest

pPREPROCESSOR OUTPUT• Density matrix

voxel width

2. Define MESH tally elements as t i ll id ti l t l tti l t(M V / 3) ( / 3) (d )

1. ID the sub-volume of interestDensity matrix g / cm3

geometrically identical to lattice elements(MeV / cm3)i,j,k ÷ (g / cm3)i,j,k = (dose)i,j,k3. Further define MESH to occupy the

L MESH ffi i llDEPT OF RADIATION ONCOLOGY

same spaceLess MESH = more efficient tally

Simulation Results ProcessingSimulation Results Processing• Post-processing software linked with

voxelized phantom creationvoxelized phantom creation• Preprocessor generates:

• MCNP formatted input file• MCNP-formatted input file• Density matrix

CT d l i• CT underlay images• MCNP simulation creates:

Dose matrix

• Energy deposition matrix• Uncertainty matrix


Post ProcessingPost Processing• Compare simulation results with

somethingsomething– Film Measurement– Treatment Planning System Calculation– Treatment Planning System Calculation– Water Phantom profileGamma Analysis (Low et al )• Gamma Analysis (Low, et al.)–Usually 2D, can be 3D


10x10 cm10x10 cm22

DoseDose

DEPT OF RADIATION ONCOLOGYUncertainty

NormalizationNormalization

This voxel has relatively• low energy deposited?• low density• high energy deposited• equivalent density• low energy deposited• equivalent density

y

y• med-high dose• high uncertainty

q y• high dose• low uncertainty

q y• low dose• high uncertainty


Normalization Normalization MethodMethod

(energy)x y zMESH tally (density)x y z( gy)x,y,zy ( y)x,y,z

(dose)x,y,z(uncertainty)x,y,z

Max dose with sufficient uncertaintyDEPT OF RADIATION ONCOLOGY

Max dose with sufficient uncertainty

Source Model DevelopmentSource Model Development• Develop a clinically accurate Monte Carlo

source model of the Novalis®source model of the Novalis®

• Compare Monte Carlo simulation results against• Measurements• TPS calculations


Gamma AnalysisGamma Analysis• Low et al., Med Phys, 1998• Quantitative comparison of two dose distributions

Measured dose point (M)

Calculated dose point (C)Calculated Dose Distribution (C)Overlay Data Sets

Measured Dose Distribution (M)

z z’

IF the minimum Γ(rm,rc) > 1, THEN GREEN overlaySubset of both

data setsx x’


Comparative AnalysisComparative Analysis

• Gamma analysis• 2D or 3D capability

• CT image import and registration for g p gisodose “underlay”

• MCNP simulation compared with:MCNP simulation compared with:• Film measurements• TPS calculations• TPS calculations


Voxelized Models of Real PhantomsVoxelized Models of Real Phantoms

Source benchmarkingg• Monte Carlo• Measurement


Lookup Lookup TablesTablesHU RANGE

MATERIAL &DENSITY (g/cm3)HU RANGE MATERIAL g/cm3

≥ 796 2.1

200

CIRS AVG BONE1.600

616

436

256

76

BONE1.891.68

1.48

1.26200

CIRS PLASTIC WATER1 039

76

29

19

-29

MUSCLE

WATER

1.14

1.06

1.0

0 98

-224

1.039-94

-159

-224

-374

FAT

0.98

0.925

0.85

0.6455

CIRS LUNG0.210

374

-524

-674

-824

LUNG

0.4721

0.2978

0.1254

0.048


-954

-1024 AIR 0.0012-954

-1024 AIR

0.048

0.0012

Validate Validate Source and Phantom Source and Phantom Models Against MeasurementModels Against MeasurementModels Against MeasurementModels Against Measurement

“S lid t ” l b• “Solid water” slabs• Kodak EDR-2 Film• AP fields


Source Model vs. MeasurementsSource Model vs. Measurements

24x24 mm2 field24x24 mm field


Red line: filmBlack line: MC simulation


IMRT segment

R d li fil




Composite IMRT field




• “Solid water” slabs• “Lung” slabs• Lung slabs



Composite IMRT fieldComposite IMRT field



Monte Carlo Patient Model

Monte Carlo Virtual Source

ModelAnalysisAnalysisTPS Simulation Film

yy

Calculation ResultsMeasurement

Γ analysis(Low et al.)


Monte Carlo Patient Model

Monte Carlo Virtual Source

ModelAnalysisAnalysisTPS Simulation Film

yy

Calculation ResultsMeasurement


Lung “Target”Lung “Target”

lung

wat

er

wat

er

lung

wat

erDEPT OF RADIATION ONCOLOGY

Patient SimulationsPatient Simulations


ConclusionsConclusions• Improvements in CT-based voxelized

phantom generationp g• Flexible geometry modifications

• Sub-sampling• Sub-sampling• Lattice trimming• Isocenter positioning• Isocenter positioning

• Mesh tally definition• Pre- and post-processing• Radiotherapy analysis (gamma)


py y (g )

ConclusionsConclusions“All models are wrong – some are useful.”

George Box-George Box• CT-based phantoms can be used for

lid ti di th d l ivalidating radiotherapy source models in preparation for and in addition to i ti ti

It’s easy to make a model

investigations.

It s easy to make a model……but the model needs to be accurate.


Ongoing / FutureOngoing / Future• Modeling techniques

used to supplement smallused to supplement small animal SBRT research

• Validate brachytherapytreatment planning p gsystem for heterogeneous dose gcalculations


Thank YouThank You

• John DeMarco, Ph.D.• Adam Kesner, Ph.D.


improved ctimproved ct--based voxel based voxel …cmpwg.ans.org/icrs12/presentations/mike...

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