Proton Stopping Power in Tissue-Equivalent and Polymeric Materials: A Systematic Review

Abstract

Proton stopping power and water-equivalent characteristics are important factors in proton dosimetry, treatment planning, phantom development, and quality assurance. Tissue-equivalent and polymeric materials are widely used as substitutes for water in experimental and clinical applications; however, their radiological equivalence depends on material composition, proton energy, density, and the method used for characterization. This systematic review evaluates the available evidence on proton stopping power and water-equivalent characteristics of tissue-equivalent and polymeric materials used in proton dosimetry and proton therapy. Relevant studies involving experimental measurements, theoretical calculations, Monte Carlo simulations, and established stopping-power databases were reviewed, with particular attention to relative stopping power (RSP), water-equivalent ratio (WER), and water-equivalent thickness (WET). The reviewed evidence indicates that PMMA, polyethylene, and polystyrene are among the most extensively investigated polymeric materials and generally exhibit favorable water-equivalent characteristics under the conditions investigated. However, measurable differences exist among materials, proton energy ranges, and experimental or computational approaches. The findings further demonstrate that material density alone is insufficient to predict proton equivalence, as elemental composition, electron density, proton energy, and the applied physical model also influence stopping power-related quantities. Overall, appropriately characterized tissue-equivalent and polymeric materials can provide useful solid alternatives to water for selected proton dosimetry and phantom applications. Nevertheless, no material should be assumed to be universally water equivalent, and experimental or independently validated computational characterization remains necessary for applications requiring accurate proton range estimation.