Development of an extended Macro Monte Carlo method for efficient and accurate dose calculation in magnetic fields.

Küng, R.; Guyer, G.; Volken, W.; Frei, D.; Stabel, F; Stampanoni, M F M; Manser, Peter; Fix, M K (2020). Development of an extended Macro Monte Carlo method for efficient and accurate dose calculation in magnetic fields. Medical physics, 47(12), pp. 6519-6530. American Association of Physicists in Medicine AAPM 10.1002/mp.14542

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MOTIVATION

Progress in the field of MR-guided radiotherapy has triggered the need for fast and accurate dose calculation in presence of magnetic fields. The aim of this work is to satisfy this need by extending the Macro Monte Carlo (MMC) method to enable dose calculation for photon, electron and proton beams in a magnetic field.

METHODS

The MMC method is based on the transport of particles in macroscopic steps through an absorber by sampling the relevant physical quantities from a pre-calculated database containing probability distribution functions. To enable MMC particle transport in a magnetic field, a transformation accounting for the Lorentz force is applied for each macro step by rotating the sampled position and direction around the magnetic field vector. The transformed position and direction distributions on local geometries are validated against full MC for electron and proton pencil beams. To enable photon dose calculation, an in-house MC algorithm is used for photon transport and interaction. Emerging secondary charged particles are passed to MMC for transport and energy deposition. The extended MMC dose calculation accuracy and efficiency is assessed by comparison with EGSnrc (photon and electron beams) and Geant4 (proton beam) calculated dose distributions of different energies and homogeneous magnetic fields for broad beams impinging on water phantoms with bone and lung inhomogeneities.

RESULTS

The geometric transformation on the local geometries is able to reproduce the results of full MC for all investigated settings (difference in mean value and standard deviation < 1%). MMC calculated dose distributions in a homogeneous magnetic field are in agreement with EGSnrc and Geant4, respectively, with gamma passing rates > 99.6% (global 2%, 2 mm and 10% threshold criteria) for all situations. MMC achieves a substantial efficiency gain of up to a factor of 21 (photon beam), 66 (electron beam) and 356 (proton beam) compared to EGSnrc or Geant4.

CONCLUSION

Efficient and accurate dose calculation in magnetic fields was successfully enabled by utilizing the developed extended MMC transport method for photon, electron and proton beams.

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5 Citations in Web of Science ®
6 Citations in Scopus

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