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The TOPAS Monte Carlo code can accurately predict proton spot halos in phantom, crucial for precise radiation therapy. However, careful selection of physics models and phantom materials is advised for reliable simulation results.

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Area of Science:

  • Medical Physics
  • Radiation Oncology
  • Computational Dosimetry

Background:

  • Proton therapy utilizes pencil beam scanning, where spot halos can significantly increase dose output.
  • Accurate simulation of these halos is essential for precise treatment planning.

Purpose of the Study:

  • To evaluate the ability of TOPAS Monte Carlo code to predict proton spot halos at depth in phantom.
  • To assess the impact of halo modeling within the proton source distribution on simulation accuracy.

Main Methods:

  • Proton sources were modeled using 2D Gaussian functions and optimized to match surface measurements.
  • Simulations were compared with EBT3 film measurements in Solidwater® phantoms for various proton energies and depths.
  • Analysis included 2D profile agreement and Gamma criteria evaluation.

Main Results:

  • TOPAS simulations predicted measured profiles within 1 mm distance to agreement for 2D profiles to the 0.1% isodose.
  • Gamma criteria (1 mm/1%) were met for integrated spot profiles.
  • Discrepancies were observed in simulated primary spot sigma beyond the 0.1% isodose, with differences up to 15% and 1 mm.

Conclusions:

  • TOPAS Monte Carlo simulation, with a modeled proton spot halo, can predict profiles at depth with reasonable accuracy.
  • Physics model selection and the use of Solidwater® as a water phantom substitute introduce uncertainties that require careful consideration.