Determination of the
photon mass attenuation coefficients
Geometrical set-up
N0
primary photons generated
“Vacuum” material
(checked)
surrounds
the experimental set-up
Nd = N0 e
-µ⋅d
Nd
primary photons detected
Check on
• ParentID( )
• Energy value
 Nd  1
µ

= ln 
ρ
 N0  ρ
E.M processes implemented
• E.M. Standard
•G4PhotoElectricEffect
•G4ComptonScattering
•G4GammaConversion
• Low Energy
•G4LowEnergyRayleigh
•G4LowEnergyPhotoElectric
•G4LowEnergyCompton
•G4LowEnergyGammaConversion
Simulation in water, iron, lead
with
E.M.Standard and LowE extension
and comparison with the
National Institute for Standard and Technology
(NIST) data
Photon mass attenuation coefficient,
Water
LowE
E.M. Standard
Geant4 LowEn
NIST
0.1
0.01
0.1
1
Geant4 EMStandard
NIST
10
µ / ρ (cm 2 / g) in water
1
1
0.1
10
0.01
Photon Energy (MeV)
0.1
Photon Energy (MeV)
Differences %
Delta = (NIST-G4EMStand) / NIST
Delta = (NIST-G4LowEn) / NIST
16
14
12
10
8
6
Delta (%)
µ /ρ (cm 2 /g) in water
10
4
2
0
-2
-4
-6
-8
-10
-12
-14
-16
0.01
0.1
Photon Energy (MeV)
1
10
1
10
Photon mass attenuation coefficient,
Iron
LowE
E.M. Standard
Geant4 LowEn
NIST
Geant4 EMStandard
NIST
µ / ρ (cm 2 / g) in iron
1000
100
10
1
100
10
1
0.1
0.1
0.01
0.01
0.01
0.1
1
0.01
10
0.1
Photon Energy (MeV)
Photon Energy (MeV)
Differences %
E (%)
µ /ρ (cm 2 /g) in iron
1000
E = (NIST-G4EMStandard)/NIST
E = (NIST-G4LowEn)/NIST
18
16
14
12
10
8
6
4
2
0
-2
-4
-6
-8
-10
-12
-14
-16
-18
0.01
0.1
Photon Energy (MeV)
1
10
1
10
Photon mass attenuation coefficient,
Lead
LowE
E.M. Standard
Geant4 LowEn
NIST
Geant4 EMStandard
NIST
100
µ /ρ (cm 2 / g in lead
10
1
0.1
0.01
10
1
0.1
0.01
0.01
0.1
1
0.01
Photon energy (MeV)
0.1
1
Photon energy (MeV)
Below 100 KeV zero photons are detected,
all the photons have interaction
Differences %
E = (NIST - G4EM Standard)/NIST
E = (NIST- G4LowEn)/NIST
10
8
6
4
2
E (%)
µ/ρ (cm 2 / g in lead
100
0
-2
-4
-6
-8
-10
0.01
0.1
Photon Energy (MeV)
1
Determination of the isodose distribution
in water around a 192Ir source
The microSelectron HDR 192Ir source
192 Ir
Photon (gamma + x ray) spectrum
of Iridium is complex:
~ 24 lines in the energy range 9 -885 KeV
Our Primary Generator
(simplified monochromatic model)
G4ParticleGun shoots gamma
• random position inside the iridium core
• random direction
• mean energy <E> = 356 KeV
Stainless steel
capsule and cable
Geometrical set-up
The source is placed inside a 30 cm water box
which is associated with a Sensitive Detector object
We wish to determine the energy deposition
in water around the source
in a longitudinal plane
(cylindrical symmetry)
Readout Geometry
The longitudinal plane
is portioned in
300 · 300, 1 mm3, voxels
192 Ir
At every interaction, energy deposition is scored and
stored in the relevant voxel (in a matrix)
EM Processes implemented
• G4MultipleScattering
gamma
• G4LowEnergyRayleigh
• G4LowEnergyPhotoElectric
• G4LowEnergyCompton
• G4LowEnergyGammaConversion
e• G4LowEnergyIonisation
• G4LowEnergyBremsstrahlung
e+
• G4eIonisation
• G4eBremsstrahlung
• G4eplusAnnihilation
Output
As in the advanced
Brachytherapy example,
at the end of the RUN
an ASCII file is produced
for further processing
with X,Z energy deposition
Isodose distribution in water
around a 192Ir source
• 20 10 6 photons generated
• ~ 24 hours of simulation
Distance along longitudinal axis (mm)
40
Isodose
200%
150%
100%
75%
50%
25%
30
20
10
0
-10
-20
-30
-40
-40
-30
-20
-10
0
10
20
30
40
Distance along transverse axis (mm)
The dose deposition is not isotropic due to
•source geometry
•auto-absorption,
•encapsulation and shielding effects
At the bottom centre of the source we can see
the effect of the stainless steel cable
Comparison with isodoses of the
Plato treatment planning system
Distance along longitudinal axis (mm)
40
Isodose
200%
150%
100%
75%
50%
25%
30
20
10
0
-10
-20
-30
-40
-40
-30
-20
-10
0
10
20
30
40
Distance along transverse axis (mm)
1 cm
Plato uses an analytical method to calculate the
dose distribution:
• Point approximation of the source to compute the dose
• Anisotropy function F(ϕ), instead of F(r,ϕ) as it should
be, to account for the shielding effect of the capsule, etc..
Comparison with the isodoses of the
Plato treatment planning system
Transverse axis
3.0
Our simulation
Plato
2.5
PDD (%)
2.0
• Good agreement
1.5
1.0
0.5
0.0
0
10
20
30
40
50
Distance along transverse axis (mm)
Longitudinal axis
Our simulation
Plato
3.0
2.5
PDD(%)
2.0
• Good agreement
in front of the source
• Less good agreement
behind the source
1.5
1.0
0.5
0.0
-50
-40
-30
-20
-10
0
10
20
30
40
50
Distance along longitudinal axis (mm)
The disagreement in the region behind the source is due to the
approximation introduced by the anisotropy function in this direction
Geant4 yields a more realistic behaviour
Initial results for the
Leipzig applicator
The Leipzig applicators
were designed to give
the optimum isodoses and treat
to the correct depth
This applicator has the shape of a
hollow cylinder of tungsten
containing the source
Geometrical set-up
Iridium source
Applicator
Water box &
ROGeometry
Isodose distribution in water
for the Leipzig applicator
Distance (mm)
-40
-20
0
20
40
0
isodose
80%
60%
40%
20%
10%
Depth from surface (mm)
-10
-20
-30
-40
-50
• 10 8 photons
generated
• ~ 4 days of CPU
time
Our simulation
Nucletron Data
Our measurements
0.8
0.6
PDD (%)
Preliminary
0.4
0.2
0.0
0
10
20
30
Depth from surface (mm)
These are the initial results with this kind of applicator
• good agreement with manufacturer’s data (Nucletron)
• satisfactory agreement with our measurements
(we have just started making measurements and the
systematic errors for the response of the detector
are not well known yet)
40
50
Conclusion
Our activities at
· Porting on
Windows 98+Cygwin+VisualC++ environment
· Simple test for the EM processes
· Application in the medical field
Simulation for isodose distribution and comparison
of the result with those of our TPS and measure
· Promotion of Geant4 with poster and oral
presentation in several medical physics congress
(AIFM, ESTRO, AIRO, …)
We think that Geant4 is an useful instrument in
medical radiation physics
Its use out of the HEP community could be
advantageous both for users (as we are) and for the
code developers