Room: Track 3
To enable efficient and accurate dose calculation of photon, electron and proton beams in presence of magnetic fields by extending the Macro Monte Carlo (MMC) method for applications in MR-guided radiotherapy.
In the MMC methodology, charged particles are transported in macroscopic steps through an absorber by sampling from a database containing probability distribution functions for all relevant physical quantities scored on the surface of spheres (electrons) or slabs (protons) of different sizes, media and energies. In the presence of a magnetic field, a deterministic transformation accounting for the Lorentz force is applied for each macro step by rotating the sampled position and direction around the magnetic field vector. Calculation of photon beams is enabled by connecting an in-house photon MC algorithm with the MMC algorithm for delta electron transport. The extended MMC dose calculation is validated against EGSnrc (photon and electron beams) and Geant4 (proton beam) for academic situations by investigating a mono-energetic 5x5 cm² field for a 6 MeV photon beam, a 15 MeV electron beam and a 100 MeV proton beam in a magnetic field of 1.5 T perpendicular to the incident beam direction by means of 2D gamma evaluation.
The adapted MMC dose calculation algorithm is in excellent agreement with EGSnrc and Geant4, respectively, with gamma passing rates >99.8% (global 2%, 2 mm and 10% threshold criteria). For the investigated situations, MMC achieves an efficiency gain of a factor of 18 (photon beam), 42 (electron beam) and 295 (proton beam) compared to EGSnrc or Geant4.
The extended MMC transport algorithm enables fast and accurate dose calculation for photon, electron and proton beams in a magnetic field that is of relevance for MR-guided radiotherapy.
Funding Support, Disclosures, and Conflict of Interest: This work was partially supported by Varian Medical Systems.