Extended Abstract
Introduction
The main goal to choose the most appropriate method for cancer treatment is the achievement of high dose deposition in the tumor while preserving the surrounding healthy tissue as much as possible. Protons have different dosimetric characteristics than photons used in conventional radiation therapy. After a short build-up region, conventional radiation shows an exponentially decreasing energy deposition with increasing depth in tissue. In contrast, protons show an increasing energy deposition with penetration distance leading to a maximum (the “Bragg peak”) near the end of range of the proton beam [1-5]. In this study, it has been attempted to predict appropriate conditions for proton therapy of liver cancer. Although different studies use water or soft tissue phantoms to perform dosimetry, here three phantoms such as soft tissue, water phantom and, a phantom consists of liver realistic material were simulated to observe their dosimetric differences.
Materials and Methods
The target was simulated as a spherical tumor with a radius of 2 cm in the liver, which is located inside the complete phantom of the human body. At the first step to obtain Bragg peaks in the tumor region, the proton source was considered as a single energy perpendicular to the phantom and the energy was changed by 2 MeV steps. Phantoms 1 to 3 are made of soft tissue, liver realistic elements, and water, respectively. By adding a set of beams with different weight factors, a Spread-Out Bragg Peak (SOBP) is generated, which delivers the desired dose to the whole of the treatment target [10, 11]. The Bragg peaks to cover the tumor were calculated and then the SOBP designed by calculating the weighting factors. The dose distribution in the tumor and surrounding areas, as well as the dose of the protons, secondary neutrons, and photons absorbed in the tumor and healthy organs around the tumor, were calculated and the results are compared for three phantoms. All of the simulations were carried by MCNPX.

Results
The results of this study consist of Bragg peak dose distributions, Bragg peak positions, determination of weighting factors, creating SOBP, and evaluation of deposited dose in tumor and healthy tissues. The proper proton beam energies to cover tumor region and the Bragg peak positions for three phantoms are shown in Figure 1.

For phantoms 1 and 2, the coverage of tumor volume is done by using of Bragg peak energies about 90 MeV-120 MeV. For phantom 3 the energy of first Bragg peak at the tumor surface is about 88 MeV and the last one is about 116 MeV. A uniform dose region is created by adding Bragg peaks for different proton energies by considering the appropriate weights to obtain a flat SOBP. The resultant SOBPs are presented in Figure 2.

The evaluated dose of protons, neutrons, and photons in the tumor and some organs around that for three phantoms were calculated. The ratio of deposited dose in non-involved organs to the tumor was calculated and shown in Figure 3.

Discussion
In this study, the simulation of liver proton therapy for three phantoms consisting of soft tissue, realistic liver, and water was investigated to find the dosimetric differences. Suitable Bragg peaks were calculated to cover the tumor volume for the three phantoms. The results showed that for the soft tissue phantom and the phantom consisting of the realistic liver tissue, Bragg peaks to cover the tumor volume were in range of 90 MeV-120 MeV and for the water phantom it was about 88 MeV-116 MeV. The dose deposited point for the water phantom at each energy was different from the other two phantoms and this difference was about 4.5 mm at each point. In order to cover the whole tumor volume, we have created SOBPs for three phantoms by using properly optimized weighting factors. It would be better to take in to account the realistic composition of different tissues of the phantom. Finally, the total dose of proton and secondary particles in the tumor and 22 non-involved organs were calculated. Dose calculations in different organs of the body showed that most parts of the body received dose and organs close to the liver such as the heart, stomach, pancreas, etc. received more doses than other organs. But this deposited dose is insignificant compared to the dose received by the tumor.
Ethical Considerations
Compliance with ethical guidelines
Since there was no experiment on human or animal samples in this study, there was no need for ethical approval.
Funding
The present paper was extracted from a PhD. thesis of the first author, Zahra Ahmadi ganjeh, approved by the Department of Physics, School of Science, Yazd University, Yazd, Iran.

Authors' contributions
All authors contributed equally in preparing all parts of the research.
Conflicts of interest
The authors declared no conflict of interest.