0. Photon beam modeling of medical linear accelerator using GEANT4 virtual source model
Hyung Dong Kim*, Byung Yong Kim*,
Jeong Ku Kang†, Sung Kyu Kim††
*Department of Physics, College of Science, Yeungnam University,
†Department of Radiation Oncology, Prebysterian Medical Center,
††Department of Radiology&Oncology, College of Medicine, Yeungnam University

1. MCS overview in radiation therapy
Monte Carlo method has the most accurate dose calculations.
Improved dose distributions will affect clinical benefit
(i.e., tumor recurrence, local control, normal tissue complications)
Gradual replacement of analytic dose calculation algorithms
Several Monte Carlo codes in medical LINAC
Most widely used EGS/BEAM code
Other systems
- MCNP, PENELOPE, GEANT4, PEREGRINE, VMC, XVMC, VMC++, etc.
Commercial MCTP
- Monaco(CMS), PrecisePlan(Eleckta), iPlan(BrainLab), etc.
Use of GEANT4
Initially designed for use in high-energy physics
- application : nuclear, accelerator physics, medical and space science
It has been validated for use in medical physics
It is gaining popularity in the medical physics community.

2. Modeling of medical linear accelerator
Accurate beam modeling is an important
for dose calculation within the patient.
[Ref] Indrin J. Chetty et al, Med.Phys.34 (2007)
Possible approaches
direct use of phase-space information from accelerator head simulation
virtual, multiple-source models reconstructed from accelerator head simulation with or without enhancement from measurements
measurement driven models
- advantage: they may be developed without dependence on the details of the accelerator treatment head

3. VSM Motivation A. Limitation of direct phase-space information
Accurate phase-space simulation is dependent upon accurate input parameters
- incident electron energy, detailed geometric and material specifications
Large storage requirements
- tens or hundreds of gigabytes
Reading the phase-space data can be bottleneck in the calculation
- due to hardware performance
B. Virtual(multiple) source model
Source particles are grouped by the location of their last interaction
VSM could be “adjusted” without redoing the simulation
- to produce agreement with measured data
Improve the calculation efficiency
[Ref] Matthias Fippel et al, Med. Phys. 30 (2003)

4. 1st Ver. Beam modeling ▪ Measured Data
- PDD, Profile from Varian Clinac iX 6 MV
▪ Create Photon energy spectrum[Ref]
- 𝑝 𝐸 𝑑𝐸=𝑁 𝐸 𝑙 𝑒 −𝑏𝐸 𝑑𝐸 𝐸 𝑚𝑖𝑛 ≤𝐸≤ 𝐸 𝑚𝑎𝑥
▪ Create Scatter beam histogram
- 𝑁(𝑚, 𝜎 2 )
▪ Create virtual structure
- Target, Collimator, Flattening filter
▪ Create primary beam
- position, direction, energy
▪ Create scatter beam
- σ(standard deviation), R(scatter rate)
Compare
▪ Calculate dose distribution
- 6 MV, SSD 100 cm,
- Voxel size: 0.5×0.5×0.5 cm3
- Particle histories: 3×109~1.2×1010
[Ref] Fippel et al. Phys Med Biol 44 (1999)

5. GEANT4 simulation validation test
PDD(percentage depth dose)

6. GEANT4 simulation validation test
Dose profiles of various depth

7. CT DICOM file application
Conversion of Hounsfield numbers to materials
In the CT2Density.dat file
Convert CT number to physical density
In the Data.dat file
Assignment of material densities to materials

8. CT DICOM file application
Dose calculation for patient
In the DicomPrimaryGeneratorAction.cc file
Beam direction
Particle gun position
In the macro file
Beam On
Field size

9. 2nd Ver. Beam modeling 1st Ver. Limitation
In specific field size, dose distributions are good agreement with experimental data. But not all field size
Need to get some more exact information of primary & scatter beam
Construct the accelerator treatment head (Varian Clinac iX)
Validate the accuracy of 6 MV photon beam modeling
- PDD, dose profile
Evaluate of phase-space data from head simulation
- energy spectrum, particle fluence, energy fluence
Geometry of the accelerator head implemented in the MC code to generate the phase-space data
Electron beam
Primary collimator
Flattening filter
Ion chamber
Mirror
Upper Jaw
Be window
Phase space
Target
Lower Jaw

10. GEANT4 simulation validation test
PDD(percentage depth dose)
Dose profiles of various depth

11. Evaluation of the phase-space
[Ref] L Grevillot et al., Phys. Med. Biol. 56 (2001)