Simulation Of Scanning Irradiation And Parallel Acceleration In Proton Therapy

According to the World Health Organization,cancer is the leading cause of death worldwide.Radiation therapy is one of the three major modalities used in the treatment of cancer,the other two being surgery and chemotherapy.Compared with conventional photon radiotherapy,proton therapy offers a substantial advantage.That is the unique depth-dose characteristics,which can be utilized to achieve significant reduction in normal tissue doses proximal and distal to the target volume.The maximum energy loss per unit path is near the end of the range,when proton traversing a medium.It is well recognized that protons are extremely valuable to treat most common tumors.So proton therapy is gaining more and more attention in clinical application.Nowadays two beam delivery techniques used in proton therapy are passive scattering and beam scanning.At Shanghai Institute of Applied Physics(SINAP),Chinese Academy of Sciences(CAS),the first domestic prototype proton therapy system based on synchrotron accelerator is being developed.In order to optimize the system design for the development and estimate the performances of the proton therapy system using advanced active beam scanning techniques,a sophisticated simulation tool is needed.The contents of this thesis are as followings :(1)A computer program which can simulate the irradiation process for static and moving targets by using beam scanning technique is developed.The simulation program is composed of three major parts,namely the data preparation,irradiation action simulation,and weighted superposition of the dose contribution from all scanned beam spots.The data preparation includes setting target location,reading accelerator beam and scanning parameters,generating 3-dimensional energy-normalized Bragg curve,allocating scanning beam spots inside the target volume,preparing relative beam weights of each scanning spot and computing the absolute particle number to be irradiated to each spot.Then scanning irradiation is simulated by a spot-wise time integration of the beam spill mocking the function of a beam dose monitor taking the respiratory gating and the beam shutoff delay encountered in the scanning system into account.The desired dose distribution and dose rate as well as irradiation time of each spot could be outputted.(2)Using the developed simulation code,irradiation processes with discrete spot scanning of static cuboid and sphere target inside a water phantom are successfully simulated.With assumed parameters including prescribed dose,scanning speed,beam current time structure,homogeneous 3D dose distribution conformal can be obtained.The dose rate of irradiating the given cuboid and sphere target is 1.66 Gy/min,1.85Gy/min respectively.The simulation also shows that beam shutoff delay on the order of 200 ?s can significantly spoil the dose distribution due to large delivered number of particles to each beam spot as compared with the planned values unless the beam current from the accelerator is kept sufficiently low.(3)In irradiation of a moving target affected by breathing movement,the respiratory gating is an effective method to reduce the interplay effects caused by the motions of the scanning beam and the target.(4)Utilizing the OpenMP parallel,the time in the process of dose superposition is significantly reduced to 1/3 of the original one.A simulation program is successfully developed and used to study beam scanning irradiation of static as well as moving target.Impact of beam shutoff delay and beam intensity on the dose distribution and dose rate is quantitatively clarified.These results will be considered in works aiming at optimizing the design parameters of the proton therapy systems.It is shown that respiratory gating technique can effectively mitigate the influence of target motion and distinctly improves the dose distribution.The parallel computing with OpenMP shorten the simulation time.