Rescaling pMBRT plans for uniform target dose

Abstract Proton minibeam radiation therapy (pMBRT) is an emerging modality aimed at improving tumor control while minimizing damage to surrounding healthy tissues. Using a mechanical collimator is one of the most practical and widely used technique to generate a desired pMBRT dose distribution. However, while collimators facilitate beam shaping, they also introduce unwanted dose inhomogeneities in the target, which requires rescaling of spots’ weight (Monitor Unit) to ensure uniform dose delivery. Obtaining a homogeneous target dose in pMBRT can simplify evaluation and comparison with conventional radiotherapy. In this study, we present a combined transverse and longitudinal rescaling method to enhance dose uniformity in pMBRT. Our rescaling algorithm adjusts spot weights based on proton energy and lateral position, using data from Geant4 Monte Carlo simulations to quantify dose profiles for each energy layer, both with and without a collimator. The rescaling factors derived from these simulations are applied to correct the dose inhomogeneities introduced by the collimator. Our results demonstrate that while individual rescaling in either the transverse or longitudinal direction can improve the dose distribution, neither alone is sufficient to meet clinical standards. The fully optimized plan, which incorporates both transverse and longitudinal adjustments, achieves a uniform dose distribution within the planning target volume. The final dose lies within the clinically acceptable range of 95%–107%, with significant improvements in dose homogeneity. Additionally, dose-volume histogram and flatness analysis confirm that combined rescaling are necessary to ensure uniform dose coverage.