Feasibility study on epithermal neutron field for cyclotron‐based boron neutron capture therapy

Abstract

To realize the accelerator-based boron neutron capture therapy (BNCT) at the Cyclotron and Radioisotope Center of Tohoku University, the feasibility of a cyclotron-based BNCT was evaluated. This study focuses on optimizing the epithermal neutron field with an energy spectrum and intensity suitable for BNCT for various combinations of neutron-producing reactions and moderator materials. Neutrons emitted at 90 degrees from a thick (stopping-length) Ta target, bombarded by 50 MeV protons of 300 microA beam current, were selected as a neutron source, based on the measurement of angular distributions and neutron energy spectra. As assembly composed of iron, AlF3/Al/6LiF, and lead was chosen as moderators, based on the simulation trials using the MCNPX code. The depth dose distributions in a cylindrical phantom, calculated with the MCNPX code, showed that, within 1 h of therapeutic time, the best moderator assembly, which is 30-cm-thick iron, 39-cm-thick AlF3/Al/6LiF, and 1-cm-thick lead, provides an epithermal neutron flux of 0.7 x 10(9) [n cm(-2) s(-1)]. This results in a tumor dose of 20.9 Gy-eq at a depth of 8 cm in the phantom, which is 6.4 Gy-eq higher than that of the Brookhaven Medical Research Reactor at the equivalent condition of maximum normal tissue tolerance. The beam power of the cyclotron is 15 kW, which is much lower than other accelerator-based BNCT proposals.

Influences of Neutron Source Spectrum and Thermal Neutron Scattering Law Data on the MCNPX Simulation of a Cyclotron-Based Neutron Field for Boron Neutron Capture Therapy

“…1, the measured neutron energy spectrum for the Ta(p,n) reaction is compared with the calculations using the LA150 cross section data. 6) As shown in this figure, the calculation overestimates the measurement by a factor of about three in the energy range higher than $5 MeV.…”

“…10) The experimental setup shown in Fig. 1 Comparison of the measured neutron energy spectrum for the Ta(p,n) reaction at 90 bombarded by 50-MeV protons with the calculation using the LA150 cross section data library 6) a As written in Ref. 9), we used the LA150 data of the W(p,n) reaction instead of the Ta(p,n) reaction, because the LA150 data does not include the Ta(p,n) reaction.…”

“…In our previous work, 6) we investigated the feasibility of a cyclotron-based BNCT by using a 50 MeV proton beam of 300 mA from the AVF cyclotron at the Cyclotron and Radioisotope Center (CYRIC), Tohoku University through simulations using a state-of-the-art radiation transport code, MCNPX, 7) and concluded that this neutron field is very promising for BNCT owing to 1) the higher neutron-production yield 8) and 2) the resultant lower heat load and heat density in the target achieved by using higher-energy incident particles. We further found that a higher fraction of high-energy neutron components produced from the target by bombarding high-energy protons can be solved by the use of 1) neutrons in the backward direction from the (p,n) spallation reaction on heavy target elements, and 2) an additional iron moderator to moderate high-energy neutrons.…”

Increase in Irradiation Beam Intensity by Using a Hybrid Target System in Cyclotron-Based Neutron Capture Therapy

“…Some design studies of accelerator-based epithermal neutron generators for BNCT [4][5][6][7][8][9][10][11] showed that an irradiation treatment took a half hour or more at the level of epithermal neutron flux obtained with the available proton current by using the latest state-of-the-art technology. Experience has shown that it is difficult to keep a patient motionless over a treatment period lasting more than a half hour.…”

“…8) Tantalum can be used as an alternative target material to W because the characteristics of neutron production are similar. 9) A thick W target has the advantage of a lower ratio of high-energy neutron production to total neutrons than a thick Be target. On the other hand, a thick Be target has the advantage of larger total neutron production than a thick W target.…”