A pencil beam algorithm for magnetic resonance image-guided proton therapy.

Area of Science:

• Medical Physics
• Radiation Oncology
• Computational Dosimetry

Background:

• Magnetic resonance image (MRI)-guided proton therapy requires advanced dose calculation methods.
• Current algorithms need extensions to accurately model proton beams in magnetic fields.

Purpose of the Study:

• To develop a pencil beam algorithm (PBA) for calculating proton beam doses in magnetic fields up to 3 Tesla.
• To assess the feasibility of MRI-based proton therapy through improved dose calculation.

Main Methods:

• Monte Carlo simulations (GATE/GEANT4) generated data for PBA calibration and benchmarking.
• Evaluated dose distributions in water, adipose, bone, and air phantoms with proton energies from 80-240 MeV.
• Utilized a trajectory calculation method, look-up tables, and a novel parametrization model for lateral beam profiles.

Main Results:

• PBA demonstrated close agreement with Monte Carlo simulations in water and heterogeneous phantoms across various magnetic field strengths.
• Deviations were minimal before the Bragg peak (<0.1%) and acceptable in the distal falloff region (2-8%).
• Gamma index pass rates consistently exceeded 98%, indicating high accuracy for clinical consideration.

Conclusions:

• The developed PBA accurately accounts for dose distortions caused by external magnetic fields.
• Analytical modeling significantly reduces computation time, making the PBA suitable for treatment planning systems.
• This algorithm supports the clinical implementation of MRI-guided proton therapy.