Evaluation of the dose and flux of secondary particles in the lung tissue in breast proton therapy using the Monte Carlo simulation code

Unlike proton therapy, conventional radiation therapy directs X-rays not only at the tumor but also unavoidably at nearby healthy tissue. Protons deliver radiation to tumor tissue while the healthy structures will be spared during proton therapy. When protons travel through matter, secondary particles like neutrons and photons are produced. It is believed that the secondary dose can lead to secondary cancer. It is not generally possible to measure directly the absorbed dose in a Human body. Therefore, Monte Carlo simulations and phantoms are useful for estimating the absorbed dose in organs. In this study, the MCNPX and Analytical phantom of the human body, ORNL-female phantom were used for breast proton therapy. We considered a characteristic tumor with the same composition of breast tissue. Measurement was performed with the energy range of 60-70 MeV proton beams accelerated. Secondary productions generated in the body of the patient can affect the organs surrounding the target region, so the equivalent dose of these secondary radiations was calculated in vital organs including Right-Lung and Left-Lung. For all energies, the equivalent dose of photons in vital organs is lower than neutrons. Our Monte Carlo results show that the equivalent dose of the Neutrons and Photons in the left lung is 0.1051 mSV and 0.0178 mSV in the right lung per 1mSV of 70 MeV energy incident proton beam. It is shown that for high-energy proton beams, most of the absorbed dose by organs is due to secondary neutrons but those are low enough to be neglected. Breast proton therapy provides satisfactory target coverage and enhances normal tissue sparing that can limit unnecessary doses delivered to the lung, though there has been a significant development in Radiation therapy technology, there still remain concerns on treatment related to long-term side effects. This problem is more pronounced in pediatric cases.