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 2013. 11. 14

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.

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

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)

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)

GEANT4 simulation validation test PDD(percentage depth dose)

GEANT4 simulation validation test Dose profiles of various depth

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

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

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

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

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