Effects of neutron sources and moderator materials on therapeutic dose delivery to deep-seated lesions in proton accelerator-based boron neutron capture therapy

This study aimed to identify the optimal conditions for delivering sufficient doses to deep-seated lesions within short irradiation times for two boron carriers of different T / N ratios. The therapeutic depth and irradiation time of a neutron beam for beam shaping assemblies (BSAs) with a Li or Be target and a MgF2 2 or CaF2 2 moderator were examined with the fast-neutron dose per epithermal neutron (FNR) as a parameter. When T / N = 3.61, the therapeutic depth was almost saturated at an FNR of about 10 x 10-13 Gy cm2; 2 ; when the FNR value was about 10 x 10-13 Gy cm2, 2 , the therapeutic depth of the neutron beam for the BSA with a Be target and a MgF2 2 moderator was almost identical to that for the neutron beam for the BSA with a Be target and a CaF2 2 moderator, and slightly greater than those for the neutron beams for the BSAs with a Li target and a MgF2 2 or CaF2 2 moderator; moreover, the irradiation time of the neutron beam for the BSA with a Be target and a MgF2 2 moderator was shorter than that for the neutron beam for the BSA with a Be target and a CaF2 2 moderator. When T / N =100, the therapeutic depths of the neutron beams for the BSAs varied greatly depending on the FNR, and were greater than the corresponding values for T / N = 3.61. We therefore concluded that the BSA with a Be target and a MgF2 2 moderator that produced a neutron beam with an FNR of about 10 x 10-13 Gy cm2 2 is optimal for delivering sufficient doses to deep-seated lesions in short irradiation times when T / N = 3.61, and stricter control over FNR is required when T / N = 100.