SU‐FF‐T‐338: Comparison Between Proton and Neutron Dose Distributions From Single‐Scattering and Dual‐Scattering Systems in Ocular Proton Therapy

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Purpose: Typically, ocular proton treatment nozzles use a single‐scattering (SS) flattening filter design to achieve lateral spreading of a raw pencil‐beam to create a flat field. This approach has the advantage of simplicity but at the cost of proton efficiency, η. It is expected, though never demonstrated, that a more efficient, dual‐scattering (DS) flattening filter design may possess several advantages. Method and Materials: A proton nozzle with the SS filter design was modeled in Monte Carlo (MC) radiation transport software. Simulations of this nozzle provided in‐field proton absorbed dose distributions in water, D, and the neutron dose equivalent values, H, outside of the field, which were subsequently benchmarked to published measurements. Then, a proposed nozzle was modeled that uses a DS filter design. Simulations of the SS and DS nozzles were conducted to investigate differences in several figures‐of‐merit including the distal 80%–20% falloff lD80–20 and neutron dose equivalence per therapy Gray (H/D) distributions. Other figures‐of‐merit presented will include the field uniformity U, therapeutic dose rate Ddot, and the 80–20% lateral penumbral width lL80–20. Results: The simulations and measurements of the proton absorbed dose distributions from the SS nozzle agreed to within 2% or 0.1 mm. The shape of the measured and simulated H/D values as a function of distance from isocenter perpendicular to the beam agreed within 3%. The distal falloff width was expectedly narrower from the DS nozzle by 2.5 mm. The simulations revealed that the DS design yields substantially lower H/D values (between 0.3–0.6 times the SS values). This is partially attributed to the DS design's increased peak dose per proton. Conclusion: The DS flattening filter design may offer clinical advantages when compared to the SS filter design, including a sharper distal falloff, Ddot, and decreased H/D values. © 2005, American Association of Physicists in Medicine. All rights reserved.

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Medical Physics

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