Evaluation of the effect of soft tissue composition on the characteristics of spread-out Bragg peak in proton therapy

Mahdi Ghorbani¹, Sayyed Bijan Jia², Mohsen Khosroabadi³, Hamid Reza Sadoughi³, Courtney Knaup^4
1 Medical Physics Research Center, Mashhad University of Medical Sciences, Mashhad, Iran
² Department of Physics, University of Bojnord, Bojnord, Iran
³ Faculty of Medicine, North Khorasan University of Medical Sciences, Bojnurd, Iran
⁴ Comprehensive Cancer Centers of Nevada, Las Vegas, Nevada, USA

Correspondence Address:
Sayyed Bijan Jia,
Department of Physics, University of Bojnord, Bojnord
Iran

Source of Support: None, Conflict of Interest: None

DOI: 10.4103/0973-1482.180615

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 nine-component soft tissue, four-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: The beam properties have been obtained in a way to have the flat dose regions with the desired modulation ranges and penetration depths in the water phantom. Then, these were used to construct flat dose regions in the tissues with density near water. The SOBPs in various soft tissues and tissue-equivalent materials that had their middle correspond to about 70, 120, and 210 MeV proton beam energies are evaluated.
Conclusions: Various soft tissues and tissue-equivalent materials have shown different dose levels in SOBPs, ranges of SOBPs, lengths of SOBPs, and uniformity indices. 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 the treatment planning of proton therapy.

Keywords: Dose distribution, proton therapy, soft tissue, spread-out Bragg peak, tissue-equivalent material