Monte Carlo Study Of Secondary Neutron Radiation In Heavy Ion Passive Beam Delivery System

In recent years,due to the unique advantages of heavy ion beam in physical and biological,and encouraged by the excellent efficacy of heavy ion human clinical trials,heavy ion(such as carbon ion)radiotherapy has attracted more and more attention.In carbon ion radiotherapy,the carbon ion beam will interact with beam line components during the dose delivery process,which inevitably generates secondary radiation such as neutrons,electrons,and photons.The formation of secondary external radiation in the treatment room creates additional radiation exposure for patients and operators.The photons and charged particles are mostly absorbed when they pass through the shielding material,and the main constituent of the secondary radiation field in the treatment room is neutron radiation.Because of the relatively high relative biological effect(RBE)of neutrons,it is generally believed that neutrons contribute significantly to the increase in the incidence of normal tissue complications and the increase in secondary tumors after radiotherapy.Therefore,it is important to explore and clarify the distribution and variation of neutron radiation in the secondary external radiation field in carbon ion radiotherapy for radiation protection.In this paper,the Monte Carlo method was used to simulate and analyze the external secondary neutron radiation generated from the passive beam delivery system in Wuwei Heavy Ion Hospital.We obtained the fluence spectrums at different locations around the isocenter under the conservative case(i.e.the beam energy is 4000)(1/,the spread-out Bragg peak(Spread-out Bragg Peak,SOBP)width is 6(88),the multi-leaf collimator(Multi-leaf Collimator,MLC)is fully closed)by simulation.Then,the corresponding dose-equivalent spectrums were calculated using the extended fluence-dose-equivalent conversion coefficient.The secondary neutron energy and spatial distribution characteristics were analyzed.And the high-energy neutron dose equivalent contribution at different locations were calculated.Secondly,according to the actual clinical irradiation situation,we simulated the secondary neutron dose equivalent when the treatment dose was delivered under different conditions.And the influence of various treatment parameter settings(such as beam energy,measurement position,field size,and SOBP width)on the/value(secondary neutron dose equivalent produced by the delivered unit therapeutic dose)was explored.At the same time,the feasibility of establishing a facility-specific secondary neutron dose analysis model was verified using empirical formulas.Finally,the simulation results in this paper were compared with the secondary neutron dose levels of different radiotherapy equipment to evaluate the external secondary neutron levels of the passive beam delivery system of the Heavy Ion Medical Machine(HIMM).The simulation results showed that in the patient environment of the passive treatment head of the carbon ion therapy system,the energies of external secondary neutrons span several orders of magnitude.There are mainly two significant neutron peaks.Low-energy neutrons have an isotropic distribution,and high-energy neutrons have a forward impulse distribution.The contribution rate of high-energy neutrons at the far end of the beam direction to the fluence and dose equivalent can reach more than20%,and its contribution cannot be ignored.When the carbon ion beam energy is4000)(1/(maximum treatment depth),and the SOBP width is 6(88)(standard broadening width),the?value at the isocenter was 79.878)/.The/value increases with the increase of the beam energy,decreases sharply with the increase of the distance,declines slowly with increasing SOBP width and rapidly decreases with the rise in the aperture size.The error of the empirical formulas within the typical size of the beam direction field(10×10(88)~2)was about 20%,which is within the uncertainty of the actual neutron measurement.Conclusion:This paper used the Monte Carlo method to simulate the characteristics of external secondary neutrons in the HIMM passive beam delivery system and its relationship with treatment settings.And this work validated the feasibility of predicting secondary neutron dose using an empirical formula.Since current commercial treatment planning systems cannot calculate external secondary neutron doses,/values are usually studied using the Monte Carlo method,which is difficult to apply in clinical practice.The empirical formula can provide faster calculation speed and satisfactory accuracy,and it is recommended to use the empirical formula to enhance the monitoring of secondary neutrons during clinical particle radiation therapy.Comprehensively consider the influence of different treatment settings on the/value and other beamline parameters of particle radiotherapy facilities,some strategies for optimizing the beam line design to reduce secondary neutron radiation were obtained,such as installing an adjustable primary collimator and increasing thickness of the multi-leaf collimator.