Room: Exhibit Hall
Purpose: To accurately simulate novel spot scanning beam system with vacuum chamber with Monte Carlo (MC) multi-particle transport codes, correct representation of the beam at the entrance to the nozzle is required. The results of MC simulation will provide data for commissioning the Eclipse (Eclipse v13.7.15, Varian) treatment planning system.
Methods: A proton therapy system (Probeat-V, Hitachi) to produce primary beam (sigma ~2.5 mm at 221 MeV) and mini-beams (spot sigma of 1.4 mm at 221 MeV at isocenter) was simulated based on design information from vendor using the TOPAS 2 Monte Carlo code. Divergent beam was simulated with the representation of the fluence and angular distributions with 2D Gaussian functions. Convergent beam was simulated using emittance and twiss parameters. The Monte Carlo model was validated with the commissioning measurements of lateral spot profiles in air and integrated depth doses (IDDs) in water. The Eclipse TPS models with the data from divergent and convergent model were created and compared. The QA fields were calculated with both models and measured with Matrixx.
Results: The spot sizes (sigma) in air for divergent and convergent model at the isocenter and distal from the isocenter are compatible. At the proximal distances the convergent model better describes sigma than divergent model. The convergent beam model is required for the minibeam spot at all distances. The computed sigma agree within 0.1 mm with microDiamond detector (PTW) measurements for minibeam and Lynx (IBA) and film for primary beam option. Therapeutic ranges (R90) are within 0.15 mm across all energies for both type of beams. The gamma analysis of the QA measurements and TPS generated fields shows the higher pass rate for the convergent beam model.
Conclusion: The convergent beam model should be used in Monte Carlo simulation for the proton beam machines with spot scanning beam.