Monte Carlo simulation with Geant4 for verification of rotational total
Transcript of Monte Carlo simulation with Geant4 for verification of rotational total
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4th Workshop on Geant4 bio-medical developments and Geant4 physics validation - Genova, July 2005 - Christina Jarlskog 1
Monte Carlo simulation with Geant4 for verification of rotational total skin electron
therapy (TSET)
Christina JarlskogDepartment of Radiation PhysicsLund University, Malmö University Hospital (UMAS)
Collaborators:Lena Wittgren, Sven Bäck, Joakim Medin (UMAS) andErik Traneus (Nucletron)
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4th Workshop on Geant4 bio-medical developments and Geant4 physics validation - Genova, July 2005 - Christina Jarlskog 2
Rotational total skin irradiation at UMAS
• Varian Clinac 2100 C/D• 6 MeV electrons• treatment SSD = 250 cm• 36 cm x 36 cm field• rotating platform
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4th Workshop on Geant4 bio-medical developments and Geant4 physics validation - Genova, July 2005 - Christina Jarlskog 3
Aim of this study
• What is the absorbed dose in one rotation...?• measure dose rate in a stationary geometry• use the MC to relate to the treatment geometry
R dose in a stationary geometry for 100 MUdose in one rotation
----------------------------------------------------------------------------------------------------=
RMC100270---------
DMCstationary
DMCrotation
---------------------------=
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4th Workshop on Geant4 bio-medical developments and Geant4 physics validation - Genova, July 2005 - Christina Jarlskog 4
Simulated geometry
- beam incidence angle wrt phantom x-axis: - the simulation scores energy deposition along x (‘z’)
0° ϑ 360°<≤
x
e
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4th Workshop on Geant4 bio-medical developments and Geant4 physics validation - Genova, July 2005 - Christina Jarlskog 5
The rotational depth dose distribution
,
DMCrotation D z ϑ,( ) ϑd
ϑ∫=
DMCrotation w
0°D
MC
ϑ 0= ° 2 wϑDMC0° ϑ 90°≤<
ϑ∑ 2 wϑDMC
90° ϑ 180°< <
ϑ∑+ +≅ w
180°D
MC
ϑ 180= °+
DMCrotation w DMC
ϑ 0= ° 2 DMC0° ϑ 90°≤<
ϑ∑+⎝ ⎠
⎛ ⎞≅
w Δϑ360°-----------= Δϑ 5° 10°,=
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4th Workshop on Geant4 bio-medical developments and Geant4 physics validation - Genova, July 2005 - Christina Jarlskog 6
The simulation
• geometry: homogeneous water phantom
• physics models: standard em and low-energy em
• production thresholds:
- air: 10 keV for photons/electrons- water: 10 keV photons, 100 keV/10 keV
• maximum allowed step length: not set
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4th Workshop on Geant4 bio-medical developments and Geant4 physics validation - Genova, July 2005 - Christina Jarlskog 7
SteppingAction
• score energy distributions
• grid of 0.5 mm along x-axis
• scoring volume: cylinder of radius r (not built in the geometry)
• check if step is in a water volume
• check if step is within/crossing cylinder
• find energy deposition in scoring volume
• split energy deposition in the grid
• fill energy histogram
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4th Workshop on Geant4 bio-medical developments and Geant4 physics validation - Genova, July 2005 - Christina Jarlskog 8
PrimaryGeneration: ‘OTP electrons’
• parametrization of a Varian 2100C/D accelerator with 25x25 applicator
• virtual source position: 85 cm upstream the isocenter
• exit phase space plane: 5 cm upstream the isocenter
• field size 36 cm x 36 cm (yz plane)
• y and z coordinates from flat distribution
• direction cosines corrected for scattering in accelerator
• correction independent of radial displacement
• only direct electrons simulated
• energy sampled from OTP spectrum
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4th Workshop on Geant4 bio-medical developments and Geant4 physics validation - Genova, July 2005 - Christina Jarlskog 9
OTP energy spectrum
• extracted for PDD measured at SSD = 100 cm:
• precalculated with VMC++
• is the kinetic energy of the primary electrons
• assuming an energy spectrum with unknown constants
• PDD is given by:
• the unknown constants in are calculated by fitting
• by
Dmeas z( )
Dmono E z,( )
E
F E( )
D z( ) Dγ z( ) F E( )Dmono E z,( ) Ed∫+=
F E( )
Dmeas z( ) D z( )
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4th Workshop on Geant4 bio-medical developments and Geant4 physics validation - Genova, July 2005 - Christina Jarlskog 10
Fitted PDD
the distribution is fitted for z > 4 mm to neglect low-energy electrons that deposit energy close to the surface of the phantom
0
20
40
60
80
100
0 1 2 3 4
depth (cm)
%D
measured
VMC++
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4th Workshop on Geant4 bio-medical developments and Geant4 physics validation - Genova, July 2005 - Christina Jarlskog 11
Kinetic energy of primary electrons
0
0.2
0.4
0.6
0.8
1
0 2 4 6 8 10
VMC++
Geant4
E (MeV)
F(E
)
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Overview of the simulations (uniform thresholds)
Stationary geometries (r = 2 cm, cubic phantom 50 cm side):• SSD = 100 cm, standard em physics• SSD = 100 cm, low-energy em physics• SSD = 250 cm, low-energy em physics
Rotational geometries (SSD = 250 cm, low-energy em physics):
• test runs: r = ?, ?, N = ? if
ellipsoid phantom 35 cm in x, 50 cm in z, 25 cm in y - run 1: r = 2 cm, (10 M events/angle) - run 2: r = 0.5 cm, (100 M events/angle) - run 3: r = 0.5 cm, (100 M events/angle)
• ellipsoid phantom 20 cm in x, 40 cm in z, 25 cm in y• cylindrical phantom 30 cm diameter, 25 cm height
Δϑ = σRMC2%≈
Δϑ 10°=Δϑ 10°=Δϑ 5°=
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4th Workshop on Geant4 bio-medical developments and Geant4 physics validation - Genova, July 2005 - Christina Jarlskog 13
Comparison MC/measurement at SSD=100 cm
0
0.01
0.02
0.03
0 10 20 30 40
measured
low energystandard
depth (mm)
D (
Gy/
MU
)
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4th Workshop on Geant4 bio-medical developments and Geant4 physics validation - Genova, July 2005 - Christina Jarlskog 14
Comparison MC/measurement at SSD=250 cm
0
0.001
0.002
0.003
0.004
0.005
0 10 20 30 40
depth (mm)
D (
Gy/
MU
)
measured
low-energy
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4th Workshop on Geant4 bio-medical developments and Geant4 physics validation - Genova, July 2005 - Christina Jarlskog 15
Results of test run 1 (r = 2 cm, )Δϑ 10°=
0
0.2
0.4
0.6
x 10-13
0 10 20 30
depth (mm)
D (
Gy/
even
t)
stationary
10 degrees
20 degrees
30 degrees
0
0.2
0.4
0.6
x 10-13
0 10 20 30
depth (mm)D
(G
y/ev
ent)
40 degrees
50 degrees
60 degrees
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4th Workshop on Geant4 bio-medical developments and Geant4 physics validation - Genova, July 2005 - Christina Jarlskog 16
Results of test run 1 (r = 2 cm, )
is the surface dose too high at large beam angles?
Δϑ 10°=
0
0.2
0.4
0.6
x 10-13
0 10 20 30
depth (mm)
D (
Gy/
even
t)
70 degrees
80 degrees
90 degrees
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4th Workshop on Geant4 bio-medical developments and Geant4 physics validation - Genova, July 2005 - Christina Jarlskog 17
Results of test run 1 (r = 2 cm, )Δϑ 10°=
0
0.1
0.2
0.3
0.4
0.5
x 10-13
0 10 20 30
depth (mm)
D (
Gy/
even
t)
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4th Workshop on Geant4 bio-medical developments and Geant4 physics validation - Genova, July 2005 - Christina Jarlskog 18
Results of test run 1 (r = 2 cm, )Δϑ 10°=
0
1
2
3
0 10 20
depth (mm)
R
0
2
4
6
0 10 20
depth (mm)σ st
at (
%)
Rstationary depth doserotational depth dose
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4th Workshop on Geant4 bio-medical developments and Geant4 physics validation - Genova, July 2005 - Christina Jarlskog 19
Test runs: difference in stationary depth dose
0
0.1
0.2
0.3
0.4
0.5
x 10-13
0 10 20 30
depth (mm)
Dst
at (
Gy/
even
t)
2 cm radius
0.5 cm radius
-0.2
0
0.2
x 10-14
0 10 20 30 40 50
depth (mm)ΔD
stat
(G
y/ev
ent)
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4th Workshop on Geant4 bio-medical developments and Geant4 physics validation - Genova, July 2005 - Christina Jarlskog 20
Test runs: difference in rotational depth dose
0
0.05
0.1
0.15
0.2
x 10-13
0 10 20 30
depth (mm)
D (
Gy/
even
t)
r = 2 cm, Δθ = 10o
r = 0.5 cm, Δθ = 10o
r = 0.5 cm, Δθ = 5o
-0.05
0
0.05
0.1
x 10-14
0 10 20 30 40 50
run 1 - run 2
run 2 - run 3
depth (mm)ΔD
rot (
Gy/
even
t)
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4th Workshop on Geant4 bio-medical developments and Geant4 physics validation - Genova, July 2005 - Christina Jarlskog 21
Test runs: difference in R
0
0.5
1
1.5
2
2.5
0 10 20
depth (mm)
R
r = 2 cm, Δθ = 10o
r = 0.5 cm, Δθ = 10o
r = 0.5 cm, Δθ = 5o
-0.2
-0.1
0
0.1
0 10 20
depth (mm)R
(run
1)
- R
(run
3)
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4th Workshop on Geant4 bio-medical developments and Geant4 physics validation - Genova, July 2005 - Christina Jarlskog 22
Simulation parameters
Subsequent simulations were run with the following parameters:
- a scoring volume radius of 2 cm
- a step in angle of beam incidence
- 10 million events per angle for
- 40 million events for
Δϑ 10°=
ϑ 0°>
ϑ 0°=
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4th Workshop on Geant4 bio-medical developments and Geant4 physics validation - Genova, July 2005 - Christina Jarlskog 23
Dependence on phantom cross-section
0
0.05
0.1
0.15
0.2
x 10-13
0 10 20 30 40
depth (mm)
Dro
t (G
y/ev
ent)
cylinder 30 cm
ellipse 20 cm x 40 cm
ellipse 35 cm x 50 cm
0.5
1
1.5
2
2.5
0 10 20
depth (mm)R
ellipse 20 cm x 40 cm
ellipse 35 cm x 50 cm
cylinder 30 cm
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4th Workshop on Geant4 bio-medical developments and Geant4 physics validation - Genova, July 2005 - Christina Jarlskog 24
Thresholds by region for the cylindrical geometry
• inner phantom with 10 keV threshold for electrons:- outer radius: 15 cm, inner radius: 11 cm
- Δφ 30°=
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Threshold dependence at SSD = 250 cm
0
0.1
0.2
0.3
0.4
x 10-13
0 10 20 30
depth (mm)
Dst
at (
Gy/
even
t)
10 keV
100 keV
0
0.05
0.1
0.15
0.2
x 10-13
0 10 20 30
10 keV
100 keV
depth (mm)D
rot (
Gy/
even
t)
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4th Workshop on Geant4 bio-medical developments and Geant4 physics validation - Genova, July 2005 - Christina Jarlskog 26
Threshold dependence at SSD = 250 cm
0
0.1
0.2
0.3
0.4
0.5
x 10-13
0 10 20 30
depth (mm)
D (
Gy/
even
t)
θ = 10o
10 keV
100 keV
0
0.1
0.2
0.3
0.4
0.5
x 10-13
0 10 20 30
depth (mm)D
(G
y/ev
ent)
θ = 20o
10 keV
100 keV
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4th Workshop on Geant4 bio-medical developments and Geant4 physics validation - Genova, July 2005 - Christina Jarlskog 27
Threshold dependence at SSD = 250 cm
0
0.1
0.2
0.3
0.4
0.5
x 10-13
0 10 20 30
depth (mm)
D (
Gy/
even
t)
θ = 30o
10 keV
100 keV
0
0.1
0.2
0.3
0.4
0.5
x 10-13
0 10 20 30
depth (mm)D
(G
y/ev
ent)
θ = 40o
10 keV
100 keV
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Threshold dependence at SSD = 250 cm
0
0.1
0.2
0.3
0.4
0.5
x 10-13
0 10 20 30
depth (mm)
D (
Gy/
even
t)
θ = 50o
10 keV
100 keV
0
0.1
0.2
0.3
0.4
0.5
x 10-13
0 10 20
depth (mm)D
(G
y/ev
ent)
θ = 60o
10 keV
100 keV
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4th Workshop on Geant4 bio-medical developments and Geant4 physics validation - Genova, July 2005 - Christina Jarlskog 29
Threshold dependence at SSD = 250 cm
0
0.2
0.4
0.6
x 10-13
0 5 10 15 20
depth (mm)
D (
Gy/
even
t)
θ = 70o
10 keV100 keV
0
0.1
0.2
0.3
0.4
0.5
x 10-13
0 2.5 5 7.5 10
depth (mm)D
(G
y/ev
ent)
θ = 80o
10 keV
100 keV
depth (mm)D
(G
y/ev
ent)
θ = 90o
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4th Workshop on Geant4 bio-medical developments and Geant4 physics validation - Genova, July 2005 - Christina Jarlskog 30
Ratio Dstat/Drot at SSD = 250 cm
Absorbed dose in one rotation at
the prescription depth (5 mm):
fraction dose = 2 Gy
- without degrader:
,
- with degrader:
, 0.5
1
1.5
2
2.5
0 10 20
depth (mm)
R
10 keV
100 keV
R 0.88=
Drot 0.42 Gy= Nrot 5=
Drot 0.23 Gy= Nrot 9=
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4th Workshop on Geant4 bio-medical developments and Geant4 physics validation - Genova, July 2005 - Christina Jarlskog 31
Threshold dependence at SSD = 100 cm
0
0.01
0.02
0.03
0 10 20 30 40
measured100 keV10 keV
depth (mm)
D (
Gy/
MU
)
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Threshold dependence at SSD = 100 cm
0
25
50
75
100
0 10 20 30 40
VMC++
100 keV
depth (mm)
%D
0
25
50
75
100
0 10 20 30 40
VMC++
10 keV
depth (mm)%
D
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Conclusions
• the low-energy em physics model gives a good agreement both with measurements and with VMC++• the simulation has a dependence on the electron pro-duction threshold (when no limit on step length is set)• R can be calculated for 100 keV electron threshold• step length distribution? • dependence on beam angle?• set maximum step length?• ...