Research on the impact of the extended collimator on the beam characteristics of the boron neutron capture therapy
Objective To investigate beam distribution characteristics through a phantom with conventional or extended collimators designed based on conventional collimators in boron neutron capture therapy(BNCT).Methods By Monte Carlo simulation,we calculated the neutron beam distributions along the beam direction with a conventional collimator,5 cm-extended collimator,and 10 cm-extended collimator;calculated the irradiation time and average depth using 10 cm-extended collimators with no air gap comprised of lithium fluoride(LiF)+polyethylene or boron carbide(B4C)+polyethylene at different mass ratios;and calculated the irradiation time,advantage depth,and off-axis dose with conventional or extended collimators at without air gap or certain air gaps.Results For the 10 cm-extended collimator without air gap,the thermal neutron flux density,gamma ray dose rate,and fast neutron dose rate were highest,and their peaks were 1.0×109 n/(cm2·s),5.3 cGy/min,and 9.1 cGy/min,respectively.Collimators comprised of polyethylene and LiF were superior to those of polyethylene and B4C in advantage depth and irradiation time.For five types of collimators made of polyethylene and LiF,the combination of 20 wt％ polyethylene and 80 wt％ LiF exhibited the greatest advantage depth(8.7 cm),but with a longer irradiation time(20.5 minutes);and the combination of 80 wt％ polyethylene and 20 wt％ LiF achieved the shortest irradiation time(19.0 minutes),with an advantage depth of 8.5 cm.Compared with the conventional collimator,the use of 5 cm-and 10 cm-extended collimators reduced treatment time by 26.4％ and 40.3％ ,respectively,with small changes in advantage depth;and for the same collimator,the off-axis dose increased with the increase in the air gap.Conclusions The use of 5 cm-and 10 cm-extended collimators can increase neutron beam intensity and reduce irradiation time,with a small impact on advantage depth and off-axis dose,which can solve the problem of prolonged treatment time caused by an air gap between patient's tumor surface and the beam aperture when head and neck movement is limited.BNCT can be equipped with appropriate extended collimators according to actual clinical needs.
Boron neutron capture therapyExtended collimatorBeam distribution
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