Effects of modulation materials for lung dose distribution in proton therapy

Exploration of the optimal combination of range modulation materials (RMMs) for proton beam irradiation in lung heterogeneous media using Monte Carlo simulation was the focus of study. Physical and dosimetric properties of each homogenous RMM irradiated with proton beam were investigated. The water equivalent ratio (WER) and full width at half maximum (FWHM) were compared to determine the primary RMM. The optimized trade-off between the effects of thickness needed for range modulation and FWHMs for selecting optimal RMM combinations was found. The optimal 2nd RMM was determined based on figure-of-merit (FOM) analyses in the water-lung-water media irradiated by proton beams. This study created an easier approach and successfully created the spread-out Bragg peak (SOBP) in lung heterogeneous media, using statistically fitted matrix with uniformity of SOBP better than 2%. The weighting factors matrix for creating SOBP may theoretically be applied to any multiple range modulation systems. The methodology for the 150 MeV proton beams' range modulation in the lung heterogeneous media can be similarly applied to other energy of proton as well as for heavy ion therapy.