Three-dimensional computational evaluation of water-to-air mass stopping power ratio for therapeutic helium-, carbon-, oxygen-, and neon-ion beams and its application to patient-specific quality assurance

Objective. Current dosimetry protocols recommend a constant water-to-air mass stopping power ratio, sw,air, of 1.126 with an uncertainty of ±1.5% for light ions, irrespective of ion species and energy. This study aims to develop a computational framework to calculate three-dimensional sw,air distributions of helium-, carbon-, oxygen-, and neon-ion beams for patient specific quality assurance (PSQA), and to systematically investigate spatial variations in sw,air under clinically relevant conditions. Approach. A computational framework was developed to calculate three-dimensional sw,air distributions based on the Bragg-Gray approximation, together with three-dimensional dose distributions for treatment planning. The framework was validated against published datasets for a mono-energetic carbon-ion beam. Treatment plans were generated using helium-, carbon-, oxygen-, and neon-ion beams for rectangular targets with varying spread-out Bragg peak (SOBP) widths and depths, as well as for bone and soft tissue, head and neck, and ocular cases. The corresponding sw,air distributions were calculated in a water phantom and compared with the recommended value of 1.126. Main Results. The mean sw,air within the target increased for shallower targets with narrower SOBP widths. For all ion species, sw,air increased toward the distal end of the SOBP, with larger variations observed for heavier ion species. In neon-ion irradiations for shallow targets, sw,air occasionally exceeded the ±1.5% uncertainty range in a limited region near the distal end of the SOBP. In all other cases, sw,air remained within the uncertainty range. Similar trends were observed in clinical cases. Based on these results, when considering the mean sw,air within the target, the suitable fixed values were 1.128 for carbon and oxygen ions and 1.129 for helium and neon ions. Significance. The proposed computational framework enables three-dimensional, plan-specific evaluation of sw,air and provides a practical approach for improving the dosimetric accuracy in charged-particle therapy with helium, carbon, oxygen, and neon ions, especially for PSQA.