Development of a Clinical Neutron Source for Boron Neutron Capture Therapy

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<h3>Purpose/Objective(s)</h3> In boron neutron capture therapy (BNCT), a source of epithermal (1 eV to 30 keV) neutrons activates high concentrations of ¹⁰B, selectively accumulated in tumor cells from infusion of a non-radioactive pharmaceutical, whose high LET reaction products fully deposit their energy within 10 microns of the reaction site. Historically, only nuclear reactors provided sufficient neutron intensity and beam quality for clinically acceptable BNCT treatments, but hospital-based BNCT is now feasible using compact accelerator sources. The present work has designed a system to produce an epithermal neutron beam using a tandem electrostatic accelerator, proton beam line, neutron-producing targets, neutron beam shaping assembly (BSA) and collimators. Testing was required to verify that this design can produce a clinically viable neutron source for BNCT in terms of total neutron yield, output reproducibility and operational stability. <h3>Materials/Methods</h3> The present neutron source uses neutrons from the ⁷Li(p,n)⁷Be reaction at nominal proton energy and current of 2.5 MeV and 10 mA. A negative hydrogen ion source is pre-accelerated to 200 keV and injected into the tandem accelerator, which increases the negative ion kinetic energy to half the goal proton energy at its center. A charge exchange target strips the electrons to create positively-charged protons that are further accelerated to the full operational energy at the tandem exit. The proton beam is focused and may be directed to up to three treatment rooms, each with a copper-backed target of natural lithium metal under vacuum. The maximum neutron energy from 2.5 MeV protons slowing down in lithium is 800 keV and requires moderation to the desired epithermal range. A static beam BSA surrounds the target and provides efficient neutron moderation and reflection using carefully chosen materials. The BSA's 25 cm circular exit port may be collimated to small circular epithermal beam diameters as small as 5-8 cm. <h3>Results</h3> A tandem accelerator, proton beam line and lithium target have been installed and began initial testing. Proton energy and current up to 2.3 MeV and 7 mA have been achieved for continuous operating periods exceeding 30 minutes with no energy or current degradation. Raster patterns have been designed to scan the 1-cm diameter proton beam over a 10-cm circular area on the lithium target, and water cooling of the copper backing has maintained maximum target temperatures below 150°C. The measured lithium deposition uniformity is expected to translate to a neutron yield uniformity better than 5%. <h3>Conclusion</h3> This BNCT neutron source has demonstrated operational stability and reproducibility of proton energy, proton current and neutron production. These measurements support an expectation that the equipment currently undergoing evaluation, in combination with the planned BSA, will meet the necessary requirements for a hospital-based, clinical BNCT system.