Design Of Beam Shaping Assembly For An Accelerator-based Boron Neutron Capture Therapy Facility And Study On Its Clinical Dosimetry Parameters

An appropriate neutron source is an essential prerequisite for boron neutron capture therapy（BNCT）.The initial neutron source often fails to meet the requirements,so the beam shaping assembly（BSA）is one of the key components in BNCT neutron source facility to produce high quality neutron beam.In order to optimize the dosimetry parameters,this paper systematically studies the initial neutron source,BSA and clinical dosimetry parameters in Snyder head phantom by using MCNPX program,which can provide reference and guidance for the design of BSA in BNCT and the development of clinical trials.Firstly,aiming at the recommended value of neutron beam parameters of BNCT by the International Atomic Energy Agency（IAEA）,the BSA is optimized.Using the parameters such as neutron yield,energy spectrum and angular distribution produced by 2.5 Me V proton bombarding lithium target as the initial source,the influences of the materials and sizes of the main structure of BSA on the neutron beam parameters are analyzed,and the optimization design of BSA is carried out.The optimized scheme can make the epithermal neutron beam parameters meet the recommended value of IAEA,and has the characteristics of high epithermal neutron flux and low pollution.Further research work evaluates the clinical dosimetry parameters of the optimized scheme.The dose distribution and clinical dosimetry parameters in the Snyder head phantom are simulated and analyzed using the optimally designed neutron beam.The results show that the advantage depth dose rate ADDR is 0.41 RBE-Gy/min,and the treatment time TT reaches 33.5 min,which has a shorter treatment time and higher treatment efficiency.Treatable depth TD and advantage depth AD reach 7.53 cm and9.39 cm respectively,proving that it can meet the treatment needs of deep tumors.The advantage ratio AR ratio also reached 5.56,which has a larger therapeutic gain.The maximum tumor dose can reach nearly 75 Gy,and the tumor dose at 7 cm can still reach30 Gy,showing good therapeutic effect.Finally,in order to provide guidance and reference for BNCT clinical trials,the effect of boron concentration distribution on clinical dosimetry parameters is quantitatively analyzed and summarized:when normal tissue has a certain affinity for boron drugs and its ¹⁰B concentration is set as 25 ppm,as the ¹⁰B concentration in tumor increases from 55 ppm to 75 ppm,AD increases from8.0 cm to 9.4 cm,AR increases from 4.0 to 5.2,and the peak tumor dose increases from48.1 RBE-Gy to 63.5 RBE-Gy;when the tumor has a certain affinity for boron drugs and its ¹⁰B concentration is set as 65 ppm,as the ¹⁰B concentration in normal tissue increases from 15 ppm to 35 ppm,AD decreases from 9.5 cm to 8.1 cm,AR changes from 8.4 to 3.8,and ADDR increases from 0.40 RBE-Gy/min to 0.68 RBE-Gy/min,TT is shortened from 31.1 min to 18.3 min,and the peak tumor dose is reduced from 75.6RBE-Gy to 44.7 RBE-Gy;when the ratio of the affinities of normal tissue for boron drugs to tumor is constant,the ¹⁰B concentration ratio is set as 1:4,as the ¹⁰B concentration in normal tissue increases from 15 ppm to 35 ppm,AD increases from9.0 cm to 9.7 cm,AR increases from 6.0 to 7.8,and peak tumor dose increases from70.0 RBE-Gy to 90.8 RBE-Gy.There are the following innovations in this research:1)The double-layer moderator design can obtain a larger epithermal neutron flux with a smaller total thickness of moderator,which has important reference value and significance for the optimization design of BSA in AB-BNCT facility;2)The influence of boron concentration distribution on clinical dosimetry parameters is comprehensively explored,and its trends and causes are analyzed and summarized,which can provide important reference for future clinical trials of BNCT.