Monte Carlo Simulations in Proton Dosimetry with Geant4 Zdenek Moravek, Ludwig Bogner Klinik und Poliklinik für Strahlentherapie Universität Regensburg
Objectives of the Study what particles and how much they contribute to the depth dose and lateral profiles what processes and how they constitute the depositions how often a process is called in the simulation an amount of energy that escapes the volume and by how means All these objectives are investigated for a possible dependence on the initial proton energy. Application of the Results insight in the physics of the proton mass interactions justification for approximations adopted in faster simulation engines verification of already available simulation engines Objectives 2 / 11
Methods MC simulation with step splitting Step: position incident particle process released energy direction change { secondaries } } Primary Secondary Nuclear Primary_escaping Secondary_escaping Nuclear_escaping ( particle, process ) Methods 3 / 11
Methods Geant4 Implementation Physics all common particles and physics for proton water interactions standard Geant4 models (EEDL parametrizaion, PreCompound model for nuclear interaction) Step information each deposition is classified by process and particle that produced it Grouping secondaries and all their offsprings are marked by a flag nuclear depositions and offsprings are marked by another flag Data Tables for Evaluation Matrices in which a contribution is catalogued against a particle (row) and process (column) that created it. We define the following tables primary depositions, secondary depositions, nuclear depositions variants for escaping energy (double filling) significant deflection table histograms for calls to a process and a process with significant deflection Methods 4 / 11
Parameters & Verification: Material: water Beam : width = 7mm dp/p = 1% profile = gaussian, symmetric in x, y E = 97, 160, 214 MeV ( range [20,214] MeV ) [1] Pedroni E et al 2005. Phys Med Biol 50, 541-561. [2] Scheib S 1993. Doctoral thesis. Verification 5 / 11
Results energy per particle type: Total deposited energy Energy deposited due to nuclear processes and their offsprings Results 6 / 11
Results energy per process, etc.: Total energy due to a process Deflection angle due to a processes Results 7 / 11
Results process calls / escaping energy: Number of routine calls for a processes Escaping energy from nuclear and related processes Results 8 / 11
Results neutron properties 0.13 neutrons per 160MeV proton (E 2 dependence) with ¼ of energy taken away is a small effect, but may become important at the peak tail Neutrons and offsprings Prodiction of secondary protons Neutron dose beyond the peak as tail-to-peak ratio for depths of 2cm (red) and 5cm Results 9 / 11
Results engine test PETRA is a Monte Carlo simulation engine for proton interactions in water with transport of secondary protons [3]. Data are available for splitting of nuclear/non-nuclear terms. A comparison with Geant4 shows the underestimation of nuclear interactions in the PETRA model. [3] Medin J, Andeo P 1997. Internal Report MSF 1997-01. Stockholm University Results 10 / 11
Summary Deposition needs only protons (including nuclear-scattered ones) Lateral scattering needs multiple scattering and nuclear interactions Escaping energy is taken away by gamma particles and neutrons Processes like electron ionization or transportation are called very often even though their contribution is small. These may follow some approximations. Neutron dose beyond the peak can be neglected (more than 10-3 smaller than the peak value at 2 cm, with further exponential decay). It can be clinically relevant only when risk organ intrudes into the target. Summary 11 / 11