Evaluation of Electron Monte Carlo

Dose Calculation of RayStation

TPS in Heterogeneous Media

Sina Mossahebi, Ph.D., Mu-Han Lin, Ph.D.,

Mariana Guerrero, Ph.D.

Department of Radiation Oncology,

University of Maryland School of Medicine,

Baltimore, Maryland

Outline

• Background • RayStation Electron Monte Carlo Algorithm

• Heterogeneity Dose Evaluation • Straight incidence: dose at heterogeneous interfaces

• Oblique incidence: heterogeneity, oblique beam,

extended SSD

• Results

• Conclusions & Future Work

MC Electron Source Modeling

Use:

– Multiple Source Model

• Shape, size, position, energy of sources

Generate phase-space faster

Traditional Analytical E0

1

2 3

4 5

6

P, E’, (x,y,z), (u,v,w), w, 6

E0

1

2 3

4 5

6

P, E’, (x,y,z), (u,v,w), w, 6

Use:

– Source information

– Cross section interactions

– Position, shape, material of

each component

Generate phase-space Courtesy of Dr. Lin

RayStation EMC

Patient independent part

Analytical source modeling

• Effective energy and spatial-angular

distributions

Patient dependent part

– Component module

– Inexplicit transport

• Assumptions were made to

speed up the transport

• 1-10 % dose contribution

Phase

Space

Engine

Dose

Calculation

Engine Patient dose calculation

Courtesy of Dr. Lin

Commissioning in Homogenous Media

• RayStation MC algorithm for electron has been commissioned in homogenous media and is being used in our clinic

• PDD

• Profile

• Cone factor

• Cutout factor

• Point dose

Purpose of the Project

• Evaluate the dosimetric

performance of the electron

MC (EMC) algorithm of

RayStation (v.4.5) TPS for

clinical challenging scenarios

Heterogeneity,

Oblique angle,

Extended SSD, ...

Straight Incidence

RayStation

Polystyrene (2.5 cm)

Cork (5 cm)

Polystyrene (1.4 cm)

OSLDs

EBT

Film

Ion

Chamber

Cork (5 cm)

Experimental Setup (1)

– 6, 9, 16 MeV

– Open on a 15x15 cm2 cone , 3x6 cutout on a 10x10 cm2 cone

– 100 cm SSD

Ion Chamber vs RayStation

E

(MeV)

Field Size

(cm2)

Ion Chamber Dose

Measurement

(cGy)

RayStation Dose

Calculation (cGy) Difference (%)

6 3×6 2.9 3 2.5

9 3×6 56.9 59 3.6

16 3×6 131.0 127 3.2

6 15×15 4.1 4 2.9

9 15×15 121.0 119 1.6

16 15×15 176.9 182 2.8

Point dose comparison:

Low dose region

< 4%

difference

RayStation

Cork (5 cm)

Polystyrene (2.5 cm)

Polystyrene (1.4 cm)

Ion

Chamber

Cork (5 cm)

OSLD vs RayStation

E

(MeV)

Field Size

(cm2)

OSLD Dose

Measurement

(cGy)

RayStation Dose

Calculation (cGy) Difference (%)

6 3×6 170.0 173 1.8

9 3×6 172.8 172 0.4

16 3×6 192.1 187 2.7

6 15×15 190.3 189 0.7

9 15×15 181.5 179 1.4

16 15×15 191.0 193 1.0

RayStation

Cork (5 cm)

Polystyrene (2.5 cm)

Polystyrene (1.4 cm)

OSLDs Cork (5 cm)

Point dose comparison:

High dose region

< 3%

difference

2D Dose Evaluation at interface

Energy (MeV)

Gamma RayStation vs. EBT Film

6 >97%

9 >96%

16 >99%

100cmSSD, 10x10cone, 3x6 cutout

2D planar dose distribution

Gamma evaluation 3% / 3mm

RayStation

Cork (5 cm)

Polystyrene (2.5 cm)

Polystyrene (1.4 cm) Film

Cork (5 cm)

Passing rate

>95%

Horizontal Vertical

EBT film (reference) RayStation (target)

Oblique Incidence in Heterogeneous

Phantom Experimental Setup (2)

Axial View

Cork (5 cm)

Solid Water (6 cm)

Solid Water (2 cm)

Film

Cork (5 cm)

Solid Water (6 cm)

Solid Water (2 cm)

Sagittal View

Cork (5 cm)

Solid Water (6 cm)

Solid Water (2 cm)

10˚

20˚

45˚

Measurement Conditions

– 6, 9, 16 MeV

– 3x6 cutout on a 10x10 cm2 cone

– 0˚, 10˚ SSD = 100 cm

– 20˚, 45˚ SSD = 110 cm

(to avoid collision)

Axial View

Cork (5 cm)

Solid Water (6 cm)

Solid Water (2 cm)

Film

Cork (5 cm)

Solid Water (6 cm)

Solid Water (2 cm)

Sagittal View

Cork (5 cm)

Solid Water (6 cm)

Solid Water (2 cm)

10˚ 20˚

45˚

2D dose comparison between RayStation and EBT film measurement

Oblique Incidence 2D Depth Dose Distribution

6 MeV

EBT film (reference) RayStation (target)

0˚

10˚

20˚

45˚

16 MeV

EBT film (reference) RayStation (target)

0˚

10˚

20˚

45˚

Oblique Incidence

Axial View

Cork (5 cm)

Solid Water (6 cm)

Solid Water (2 cm)

Film

Cork (5 cm)

Solid Water (6 cm)

Solid Water (2 cm)

Sagittal View

Cork (5 cm)

Solid Water (6 cm)

Solid Water (2 cm)

10˚ 20˚

45˚

Gamma Passing Rate

SSD 100 cm 110 cm

Energy (MeV)

0˚ 10˚ 20˚ 45˚

6 98% 96% 66% 64%

9 91% 91% 81% 79%

16 83% 82% 69% 79%

10x10cone, 3x6 cutout

Gamma evaluation 3% / 3mm

Passing rate

>90%

Low passing rate with increase of

beam angle and SSD

Low passing rate at high energy

cork

Failing Regions of Low Energy Electron

(6MeV)

For low energy electron as beam angle and SSD increase:

• Acceptable at target region

• Failing at low dose region

• RayStation overestimates the dose about 10%

10˚ 20˚ 45˚

Vertical Dose Profile (PDD)

0˚

Gamma Pass\Fail

cork cork cork

Failing Regions of High Energy Electron

(16MeV)

For high energy electron:

• Acceptable at target region

• Failing at heterogeneity (low density)

• RayStation overestimates the dose about 10%

Vertical Dose Profile (PDD)

0˚ 10˚ 20˚ 45˚

Gamma Pass\Fail

cork cork cork

More energy deposition

Less energy

deposition

cork

Conclusions

Straight incidence

– The RayStation dose calculation had a good agreement with ion chamber

and OSLD measurements for different electron energies.

– Gamma analysis showed a strong agreement at the interface of high dose

region.

– Poor agreement at heterogeneity region for high energy electron beam:

RayStation overestimates the dose (dose to lung)

Oblique incidence

Gamma analysis showed

– Good agreement at target area.

– Agreement ↓ as oblique angle ↑ and SSD ↑

– Poor agreement at heterogeneity region for high energy electron beam.

Future Work

Further investigation is in progress for

– Extended SSD

– Oblique angle

– Higher energy (20 MeV)

– Various fields and cut-outs

– Different setups

Acknowledgements

• Mu-Han Lin, Ph.D.

• Mariana Guerrero, Ph.D.

Thank you!

Oblique Incidence 2D Depth Dose Distribution

9 MeV

EBT film (reference) RayStation (target)

0˚

10˚

20˚

45˚

16 MeV

EBT film (reference) RayStation (target)

0˚

10˚

20˚

45˚

Low Energy Electron (6MeV) Dose profile

Horizontal Dose Profile

Vertical Dose Profile

0˚ 10˚ 20˚ 45˚

Calibration Curves

6 MeV 9 MeV 16 MeV

Straight Incidence

Dose profile at polystyrene-cork interface

EBT film (reference) RayStation (target)

Straight Incidence

Horizontal dose profiles

EBT film vs RayStation dose profile

Vertical dose profiles

MC - Linac Simulation

History: a single particle sampled by the random number

Phase space file: a binary file storing the particle

information of the scoring plane.

- Particle type, energy, position, direction, weight, last

interaction position

E0

1

2

3 4

5

6

P, E’ , (x,y,z), (u,v,w), w, 6

More history, lower statistical uncertainty

100

101

102

103

104

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Mean Value

Statistical Errors

Courtesy of Dr. Lin

RayStation Electron Algorithm

• With the intention of significantly improving accuracy

of dose and MU calculation

phase space

engine

• Simulate what

comes out of

treatment head

2 separate “engines”

dose calculation

engine

• Simulate energy

transport and

scoring in patient

RayStation Monte Carlo