Determination of the photon mass attenuation coefficients - - PDF document

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Determination of the photon mass attenuation coefficients - - PDF document

Nov 09, 2022 266 likes •410 views

Determination of the photon mass attenuation coefficients Geometrical set-up N 0 N d primary photons generated primary photons detected Vacuum material Check on (checked) ParentID( ) surrounds Energy value the experimental

slide-1
SLIDE 1

N0 primary photons generated Nd primary photons detected

Determination of the photon mass attenuation coefficients

Check on

  • ParentID( )
  • Energy value

“Vacuum” material (checked) surrounds the experimental set-up

d

  • d

e N N

⋅ µ

= ρ         = ρ µ 1 N N ln

d

Geometrical set-up

slide-2
SLIDE 2

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

slide-3
SLIDE 3

Photon mass attenuation coefficient, Water

0.01 0.1 1 10 0.1 1 10

Geant4 EMStandard NIST

µ / ρ (cm 2 / g) in water

Photon Energy (MeV)

0.01 0.1 1 10 0.1 1 10

Geant4 LowEn NIST

µ /ρ (cm 2 /g) in water

Photon Energy (MeV)

LowE E.M. Standard

0.01 0.1 1 10

  • 16
  • 14
  • 12
  • 10
  • 8
  • 6
  • 4
  • 2

2 4 6 8 10 12 14 16

Delta = (NIST-G4EMStand) / NIST Delta = (NIST-G4LowEn) / NIST Delta (%) Photon Energy (MeV)

Differences %

slide-4
SLIDE 4

Photon mass attenuation coefficient, Iron

0.01 0.1 1 10 0.01 0.1 1 10 100 1000

Geant4 EMStandard NIST µ / ρ (cm 2 / g) in iron Photon Energy (MeV)

0.01 0.1 1 10 0.01 0.1 1 10 100 1000

Geant4 LowEn NIST µ /ρ (cm

2 /g) in iron

Photon Energy (MeV)

LowE E.M. Standard

Differences %

0.01 0.1 1 10

  • 18
  • 16
  • 14
  • 12
  • 10
  • 8
  • 6
  • 4
  • 2

2 4 6 8 10 12 14 16 18

E = (NIST-G4EMStandard)/NIST E = (NIST-G4LowEn)/NIST E (%) Photon Energy (MeV)

slide-5
SLIDE 5

Photon mass attenuation coefficient, Lead

LowE E.M. Standard

Differences %

0.01 0.1 1 0.01 0.1 1 10 100

Geant4 EMStandard NIST

µ/ρ (cm 2 / g in lead

Photon energy (MeV)

0.01 0.1 1 0.01 0.1 1 10 100

Geant4 LowEn NIST

µ/ρ (cm 2 / g in lead

Photon energy (MeV)

0.01 0.1 1

  • 10
  • 8
  • 6
  • 4
  • 2

2 4 6 8 10

E = (NIST - G4EM Standard)/NIST E = (NIST- G4LowEn)/NIST E (%) Photon Energy (MeV)

Below 100 KeV zero photons are detected, all the photons have interaction

slide-6
SLIDE 6

Determination of the isodose distribution in water around a 192Ir source

192Ir

Stainless steel

capsule and cable

Photon (gamma + x ray) spectrum

  • f Iridium is complex:

~ 24 lines in the energy range 9 -885 KeV

G4ParticleGun shoots gamma

  • random position inside the iridium core
  • random direction
  • mean energy <E> = 356 KeV

Our Primary Generator (simplified monochromatic model)

The microSelectron HDR 192Ir source

slide-7
SLIDE 7

192Ir

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) The longitudinal plane is portioned in 300 · 300, 1 mm3, voxels

Geometrical set-up Readout Geometry At every interaction, energy deposition is scored and stored in the relevant voxel (in a matrix)

slide-8
SLIDE 8

EM Processes implemented

  • G4MultipleScattering

gamma

  • G4LowEnergyRayleigh
  • G4LowEnergyPhotoElectric
  • G4LowEnergyCompton
  • G4LowEnergyGammaConversion

e-

  • G4LowEnergyIonisation
  • G4LowEnergyBremsstrahlung

e+

  • G4eIonisation
  • G4eBremsstrahlung
  • G4eplusAnnihilation

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 Output

slide-9
SLIDE 9

Isodose distribution in water around a 192Ir source

  • 20 10 6 photons generated
  • ~ 24 hours of simulation
  • 40
  • 30
  • 20
  • 10

10 20 30 40

  • 40
  • 30
  • 20
  • 10

10 20 30 40

Isodose 200% 150% 100% 75% 50% 25%

Distance along longitudinal axis (mm) 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

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SLIDE 10

Comparison with isodoses of the Plato treatment planning system

1 cm

  • 40
  • 30
  • 20
  • 10

10 20 30 40

  • 40
  • 30
  • 20
  • 10

10 20 30 40

Isodose 200% 150% 100% 75% 50% 25%

Distance along longitudinal axis (mm) Distance along transverse axis (mm)

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..

slide-11
SLIDE 11

10 20 30 40 50 0.0 0.5 1.0 1.5 2.0 2.5 3.0

Our simulation Plato PDD (%) Distance along transverse axis (mm)

Comparison with the isodoses of the Plato treatment planning system

Transverse axis Longitudinal axis

  • 50
  • 40
  • 30
  • 20
  • 10

10 20 30 40 50 0.0 0.5 1.0 1.5 2.0 2.5 3.0

Our simulation Plato PDD(%) Distance along longitudinal axis (mm)

  • Good agreement

in front of the source

  • Less good agreement

behind the source

  • Good agreement

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

slide-12
SLIDE 12

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

Water box & ROGeometry Iridium source Applicator

slide-13
SLIDE 13

Isodose distribution in water for the Leipzig applicator

  • 50
  • 40
  • 30
  • 20
  • 10
  • 40
  • 20

20 40

isodose 80% 60% 40% 20% 10% Depth from surface (mm) Distance (mm)

  • 10 8 photons

generated

  • ~ 4 days of CPU

time

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)

10 20 30 40 50 0.0 0.2 0.4 0.6 0.8

Our simulation Nucletron Data Our measurements

PDD (%) Depth from surface (mm)

Preliminary

slide-14
SLIDE 14

Conclusion

· Porting on

Windows 98+Cygwin+VisualC++ environment

· Simple test for the EM processes · Application in the medical field

Simulation for isodose distribution and comparison

  • f 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

Our activities at