(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: Develop and investigate step-size dependence of Monte Carlo code for Cherenkov emission (CE)-based dosimetry calculations with photons and electrons.
Methods:
We previously presented setup and formalism, EGSnrc-based MC code, and relative experimental validation for CE-based dosimetry in water. Here, we implement a step-size correction to reduce errors by splitting steps into two equal-length substeps, assigned to respective initial and final directions. Parallel monoenergetic broad 6 MeV, 20 MeV, 6 MV, and 15 MV electron and photon beams are simulated normally-incident on water. The conversion (energy deposited per CE energy-normalized counts) is calculated for previously-proposed potential detection configurations. Relative deviations from single-scattering (SS) simulations are examined in terms of fractional charged-particle energy loss-per-step restriction, ESTEPE, as a function of (i)beam quality, (ii)depth, and (iii)angle relative to beam. Results are compared to Geant4 by scoring CE relative to overall step direction.
Results:
No significant step-size effect was observed (>0.1%) for dose and CE conversion, except for 20 MeV superficially at 90° (-1.6%±0.5%). This could be noise or greater CE at 90° than approximated by our condensed-history (CH) simulation. The latter could partially explain previous experimental CE overestimation superficially. Nonetheless, electron-beam surface CE-based dosimetry at 90° is not recommended. Electron SS deviation uncertainties (k=1) were <0.2% overall and <0.1% with large apertures for clinically-relevant configurations. Photon uncertainties were =1% overall, <0.4% with large apertures, and <0.2% non-superficially with large apertures. The Geant4 method exhibited larger SS deviation of up to -2.8%±0.1% with electrons and -1.4%±0.3% with photons. This stronger SS effect is expected for longer CH steps since straight step approximation is less representative of the true curved trajectory.
Conclusion:
New step-size correction is implemented in EGSnrc directional Cherenkov calculations and step-size independence is demonstrated. This not only motivates clinical CE-based dosimetry, but is also useful for the general Monte Carlo community.
Keywords
Monte Carlo, Optical Dosimetry, Multiple Scattering
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