Yana Zlateva1, Bryan R Muir2, Issam El Naqa3, Jan Seuntjens4. 1. Medical Physics Unit, McGill University, Montreal, QC H4A 3J1, Canada; Department of Radiation Oncology, Duke University, Durham, NC 27710, USA. Electronic address: yana.zlateva@duke.edu. 2. Metrology Research Centre, National Research Council, Ottawa, ON K1A 0R6, Canada. 3. Department of Radiation Oncology, University of Michigan, Ann Arbor, MI 48103-4943, USA. 4. Medical Physics Unit, McGill University, Montreal, QC H4A 3J1, Canada.
Abstract
PURPOSE: Previous work presented and validated in-water Cherenkov emission (CE)-based radiotherapy dosimetry. Condensed history Monte Carlo (MC)-calculated electron beam CE-to-dose conversion with <4π CE detection, however, could exhibit step-size dependence. This work presents a physics update and numerical study of this step-size dependence in photon and electron beams, elucidates the CE generation physics, and guides further research. METHODS: The CE-to-dose conversion, kCθ±δθ, is calculated for photons (6X, 15X) and electrons (6E, 20E) on-axis in-water with: θ±δθ∈{90°±90°(4π),90°±5°,45°±45°,90°±45°}, 10 cm equivalent square, 100 cm SSD, 1cm voxel radius and beam-dependent length. Relative deviation from single-scattering (SS) simulation is evaluated on maximum fractional electron step energy loss ESTEPE∈0.01-0.25. Standard uncertainties (k=1, 108histories) are reported. A simplified method considering only the straight step direction is also implemented. RESULTS: No significant step-size effect (>0.1%) was observed for dose and all kCθ±δθ, except for surface dosimetry at 90°±5° (-1.6%±0.5%, 20E), which is not recommended. Electron SS deviation uncertainties (k=1), otherwise, varied from <0.2% overall to <0.1% with large apertures. Photon uncertainties varied from <1.1% overall to <0.2% non-superficially with large apertures. The simplified straight-step method exhibited overall greater deviation from SS, most notably -2.8%±0.1% (6E) and -2.5%±0.4% (20E) superficially with 90°±45°, and -1.4%±0.3% (6X) and -0.6%±0.2% (15X) non-superficially with 90°±5° for ESTEPE∈[0.10,0.25]. CONCLUSIONS: We demonstrate step-size independence of newly-implemented correction in EGSnrc directional Cherenkov calculations. This advances clinical CE-based dosimetry and is useful for the general Monte Carlo community.
PURPOSE: Previous work presented and validated in-water Cherenkov emission (CE)-based radiotherapy dosimetry. Condensed history Monte Carlo (MC)-calculated electron beam CE-to-dose conversion with <4π CE detection, however, could exhibit step-size dependence. This work presents a physics update and numerical study of this step-size dependence in photon and electron beams, elucidates the CE generation physics, and guides further research. METHODS: The CE-to-dose conversion, kCθ±δθ, is calculated for photons (6X, 15X) and electrons (6E, 20E) on-axis in-water with: θ±δθ∈{90°±90°(4π),90°±5°,45°±45°,90°±45°}, 10 cm equivalent square, 100 cm SSD, 1cm voxel radius and beam-dependent length. Relative deviation from single-scattering (SS) simulation is evaluated on maximum fractional electron step energy loss ESTEPE∈0.01-0.25. Standard uncertainties (k=1, 108histories) are reported. A simplified method considering only the straight step direction is also implemented. RESULTS: No significant step-size effect (>0.1%) was observed for dose and all kCθ±δθ, except for surface dosimetry at 90°±5° (-1.6%±0.5%, 20E), which is not recommended. Electron SS deviation uncertainties (k=1), otherwise, varied from <0.2% overall to <0.1% with large apertures. Photon uncertainties varied from <1.1% overall to <0.2% non-superficially with large apertures. The simplified straight-step method exhibited overall greater deviation from SS, most notably -2.8%±0.1% (6E) and -2.5%±0.4% (20E) superficially with 90°±45°, and -1.4%±0.3% (6X) and -0.6%±0.2% (15X) non-superficially with 90°±5° for ESTEPE∈[0.10,0.25]. CONCLUSIONS: We demonstrate step-size independence of newly-implemented correction in EGSnrc directional Cherenkov calculations. This advances clinical CE-based dosimetry and is useful for the general Monte Carlo community.
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