Requirements for dose calculation on an active scanned proton beamline for small, shallow fields

Area of Science:

• Medical Physics
• Radiation Oncology

Background:

• Growing interest in proton pencil beam scanning (PBS) for small, shallow targets like ocular melanoma.
• Need for accurate dose prediction in these specialized treatments.

Purpose of the Study:

• To demonstrate accurate dose prediction by a commercial treatment planning system (TPS) for a synchrotron-based PBS system with a dedicated small field nozzle.
• To validate TPS accuracy with appropriate nozzle and TPS tuning.

Main Methods:

• Developed a removable nozzle extension with circular apertures (5-34mm diameter).
• Measured and calculated doses for spread-out Bragg peaks (SOBP) using GATE/Geant4 and RayStation TPS.
• Optimized parameters like dose grid and scattering resolution for accuracy and performance.

Main Results:

• Dose agreement within 1% for SOBPs >10mm depth with apertures 8-34mm.
• Deviations up to 5% for smaller apertures.
• Introduced a plastic taper to reduce scatter and improve TPS accuracy to 5% in the entrance region.

Conclusions:

• Commercial TPS and GATE/Geant4 accurately calculate dose for small, shallow proton fields using PBS and collimators.
• Validated the feasibility of using TPS for precise dose delivery in specialized proton therapy applications.