Monte Carlo Simulation Of Secondary Particles Produced By Heavy Ions In Beam Modulation Equipment

The purpose of tumor radiotherapy is to let the tumor target receive lethal dose of radiation,and at the same time make the normal tissue around the tumor target area and the dangerous organs receive the lowest possible radiation dose.Compared with traditional photon radiotherapy,heavy ion beam has unique physical and biological characteristics.This makes the heavy ion beam deposit less energy in the surrounding normal tissues while killing the tumor cells.Based on the above two advantages,the carbon ion beam is currently the best radiation therapy radiation.In carbon ion radiotherapy,secondary particles produced through the interaction of carbon ion beam with the treatment head devices and the patient's body can reach many areas in the patient's body.Among the secondary particles,the yield of neutrons is largest.Without affecting the beam delivery function,reducing secondary neutrons and ?-rays produced in carbon ion radiotherapy is very important to decrease the normal tissue complications and secondary cancer risk after radiotherapy,but also has a direct impact on the service life of the electronic equipment in the beam distribution system.In the present work,the Monte Carlo method was used to simulate and calculate the beam modulation devices in passive beam distribution system,such as Primary Collimator(PC),Ridge filter(RF),Range shifter,Multi-Leaf Collimator(MLC),and Compensator,etc.Under the irradiation of 400MeV/u carbon ion beam,the average equivalent dose of the secondary particle deposition generated in the volume of 200cm×200cm×200cm centered on the device,so as to find out the contribution of each device to the secondary particle dose.Secondly,RF is commonly used in passive beam distribution system and active beam distribution system.This paper simulates the average equivalent dose deposited by secondary neutrons and ?-rays generated by Al,Fe,and Cu as RF materials when the widths are 3cm,6cm,9cm,and 12 cm.Finally,the average equivalent dose and spatial distribution characteristics of the secondary neutrons and ?-rays produced by the 400MeV/u carbon ion beam irradiated by the typical 10cm×10cm square and 5cm diameter circular fields formed by MLCare simulated.The simulation results showed that the secondary particles produced by the tungsten MLC leaves material are the main contributors of the secondary particles in the HIMM passive beam distribution system.As the atomic number of the RF material increases,the secondary particles generated in the volume centered on RF gradually increase,and the secondary particles generated in the water phantom remain unchanged.The dose of the secondary particles produced by RF of the same material in the center volume increases gradually with the increase of the RF broadening SOBP width,and the dose deposited in the water phantom gradually decreases as the RF broadening increases.The secondary neutrons produced were mainly distributed at the incident end in the water phantom when the carbon ion beam passed through the MLC,while the secondary ?-rays were uniformly distributed in the entire water phantom and more dose of the secondary ?-rays appeared in the plateau when the irradiation field with SOBP travelled in the water phantom.And the amount of secondary particles deposited in the volume of 200cm×200cm×200cm centered on MLC is less than that deposited in the water phantom.Research conclusion: In this paper,the Monte Carlo method is used to simulate the generation of secondary particles in each beam modulation device in the HIMM passive beam distribution system.It is suggested that the material with high atomic number,such as Cu,can be used as RF material to replace the existing material,Al,thereby reducing the difficulty of equipment processing.Considering the thickness of MLC blade and secondary particles,tungsten copper alloy is a better choice when the compactness of equipment is required.Fe,Ni,Cu and Cu Zn alloy are the best choice for the secondary particles.In practical application,the choice of MLC blade materials and other beam equipment materials should be determined according to the specific requirements.