Abstract
Purpose: A cyclotron-based proton beam is under development to accommodate the geometric and dosimetric requirements of small-animal research, namely a sharp pristine Bragg peak at ∼8mm depth in tissue. The goal of this project is to develop a Monte Carlo model to support further development of the existing beam, provide data on physical dose in complex tissues or phantoms, and for biological research including microdosimetric analysis of LET and RBE effects. Method: A Monte Carlo simulation of the beam was implemented using the GEANT-4 based TOPAS software, under a beta-test user agreement with the developers. This system allows for creation of geometrical objects, voxel dose scoring within those objects as well as input of DICOM image data. We developed simulations of dose distributions in water phantoms, small ion chambers and in air. We compare these simulations to measured dosimetry data, and predictions of a previously published analytical model of dose deposition named the ‘Proton Energy Loss’ model. Results: The Monte Carlo model with best-fit parameters of incident energy 28.9 MeV and energy spread 1.5% gives less than 2% error in percent depth dose compared to the experimental data. Simulated lateral profiles also match the EBT3-based film measurements to better than 3.5%. Run times for simulations are approximately one hour per million particles. Multiple runs can be processed simultaneously on a 16-core server without noticeable degradation in performance. Conclusion: Monte Carlo simulation of the proton beam provides a foundation for exploring more complex geometrical dose patterns such as a high dose rings or spread-out Bragg peaks. Modeling will be further developed to calculate dose distributions in various experimental arrangements and the complex tissue structures presented in small-animal CT scans. The precision proton radiotherapy platform is expected to enable novel radiobiological research, as have dedicated small-animal X-ray irradiation systems.
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