Aim: The aim of this study is to evaluate the effect of soft tissue composition on dose distribution and spread-out Bragg peak (SOBP) characteristics in proton therapy. Subjects and Methods: Proton beams with nominal energies of 70, 120 and 210 MeV were considered. The soft tissues and tissue equivalent materials implemented in this study are: 9-component soft tissue, 4-component soft tissue, adipose tissue, muscle (skeletal), lung tissue, breast tissue, A-150 tissue equivalent plastic, perspex and water. Each material was separately defined inside a 20 cm × 20 cm × 40 cm phantom. A multilayer phantom was evaluated as well. The effect of tissue composition on the relative dose in SOBP region (relative to the dose in SOBP region in water), range of SOBP, length of SOBP, and uniformity index of SOBP was evaluated. Results: Various soft tissues and tissue equivalent materials have shown different dose level in SOBPs, ranges of SOBPs, lengths of SOBPs and uniformity indices. Conclusions: Based on the obtained results, various soft tissues and tissue equivalent materials have quite different SOBP characteristics. Since in clinical practice with proton therapy, only the range of SOBP is corrected for various tissues, omission of the above effects may result in major discrepancies in proton beam radiotherapy. To improve treatment accuracy, it is necessary to introduce such effects in treatment planning in proton therapy
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