Study Of Beam Interactions With Devices And Human Body In Proton Therapy Based On Monte Carlo Methods

Cancer has become a fatal disease threatening human life and health,and developing cancer treatment technologies is always the research frontier in the world.The dose Bragg peak of proton beams enables precision cancer radiotherapy.The merits of high-conformity,low-side effect,and non-invasive treatment modality drive the development of proton therapy technologies quickly.In proton therapy,proton beams will experience interactions with various substances during the process of generation,acceleration,extraction,collimation,energy-degradation,transmission,and finally irradiation.These interactions profoundly affect the behaviors of proton beams throughout therapy devices and bring about various useful or detrimental outcomes.It can be said that proton therapy research is inseparable from the interaction between protons and matter.The current proton therapy researches mainly focus on the development of therapy devices or the bio-medical field.Based on the Huazhong University of Science and Technology Proton Therapy Facility(HUST-PTF),this paper,in view of the essence of proton therapy,systematically studies the overall interactions between proton beams and matter using the Monte Carlo method.The overall interactions include the interactions of proton beams with devices and proton beams with human tissues.In a cyclotron-based proton therapy facility,the beam is extracted from the accelerator,passes through the energy selection system,and reaches the treatment terminal finally,during which protons interact with the devices along their path.Further studies on these interactions are vital to ensure the normal operation of the proton therapy facility.This paper completed the overall analysis of the interactions between proton beams and devices.The paper proposed a specific mixing ratio of boron carbide and graphite composite to compose the energy degrader,a vital beam loss device of the facility,which effectively improved the beam transmission efficiency of the energy degrader system,and reduced its secondary particle yield.This paper also calculated the secondary particle generation from the overall interactions and completed its shielding design.The biological effect of proton beam with human tissues is the foundation of proton therapy.Based on the phenomenological models of the Relative Biological Effectiveness(RBE),this paper proposed a deterministic optimization algorithm to accurately calculate the averaged Linear Energy Transfer(LET)of proton beams,the main physical parameter affecting the RBE.The new algorithm included the contribution of secondary particles generated from the interactions between protons and tissues.Besides,the new algorithm directly adopted the electron stopping power parameter instead of the random ratio of energy deposition and track length at each step to calculate the proton beam’s averaged LET.The four types of averaged LETs calculated by the new algorithm did not vary with the model’s product cut values,where the relative deviation did not exceed 1.1%.Meanwhile,the calculated results were comparable to the experimental measurement results.This new LET algorithm had good robustness and reliability,laying an important realistic foundation for clinically predicting the proton beams’ RBE,and providing a possibility for the precision dose planning with variable RBE.The interactions between proton beams and human body determine the dose distribution and the absorbed dose rate in the tumor target,consequently directly affecting the clinical therapeutic effect.This paper proposed a new shoot-through irradiation mode with a focused proton beam for the biologically superior FLASH therapy by enhancing the clinical dose rate and optimizing the dose distribution in the body.The new irradiation mode improved the proton beam’s biological responses to tumors and reduced the harm to normal tissues.Compared to the conventional parallel beam,the focused beam could reduce the dose by 34% to the normal tissues when maintaining the tumor target’s dose same,which means that the focused proton beam could generate a pseudo Bragg peak at the tumor target by optimizing the dose distribution.Meanwhile,the on-axis dose rate could exceed 40 Gy/s,adequately meeting the needs of FLASH therapy under the 20 n A beam current extracted from the cyclotron.Based on the HUST-PTF,the new irradiation mode could be realized by closing the energy selection system and adding a set of focusing magnets at the end of the beamline.This new irradiation mode has a potential advantage improving the clinical therapeutic effect due to its unique dose pseudo-Bragg-peak characteristic and ultra-high on-axis dose rate.