Coin-structured tunable beam shaping assembly design for accelerator-based boron neutron capture therapy for tumors at different depths and sizes

In the past decade, boron neutron capture therapy utilizing an accelerator-based neutron source (ABNS) designed primarily for producing epithermal neutrons has been implemented in the treatment of brain tumors and other cancers. The specifications for designing an epithermal beam are primarily based on the IAEA-TECODC-1223 report, issued in 2001 for reactor neutron sources. Based on this report, the latest perspectives and clinical requirements, we designed an ABNS capable of adjusting the average neutron beam energy. The design was based on a 2.8 MeV, 20 mA proton beam bombarding a lithium target to produce neutrons that were subsequently moderated and tuned through a tunable beam shaping assembly (BSA) which can modify the thicknesses and materials of the coin-shaped moderators, back reflectors, filters, and collimators. The simulation results demonstrated that epithermal neutron beams for deep seated tumor treatment, which were generated by utilizing magnesium fluoride with lengths ranging between 28 and 36 cm as the moderator, possessed a treatment depth of 5.6 cm although the neutron flux peak shifts from 4.5 to 1.0 keV. When utilizing a thinner moderator, a less accelerated beam power can meet the treatment requirements. However, higher powers reduced the treatment time. In contrast, employing a thick moderator can reduce the skin dose. In scenarios that required relatively low energy neutron beams, the removal of the thermal neutron filter can raise the thermal neutron flux at the beam port. And the depth of the dose rate peak could be adjusted between 0.25 and 2.20 cm by combining magnesium fluoride and polyethylene coins of different thicknesses. Hence, this device has a better adaptability for the treatment of superficial tumors. Overall, the tunable BSA provides greater flexibility for clinical treatment than common BSA designs that can only adjust the port size.