Boron neutron capture therapy for the treatment of cerebral gliomas. I: Theoretical evaluation of the efficacy of various neutron beams

Abstract

The technique of boron neutron capture therapy in the treatment of cerebral gliomas depends upon the selective loading of the tumor with a ¹⁰B-enriched compound and subsequent irradiation of the brain with low-energy neutrons. The charged particles produced in the ¹⁰B (n,α) ⁷Li reaction have ranges in tissue of less than 10 μm so that the dose distribution closely follows the ¹⁰B distribution even to the cellular level. The effectiveness of this therapy procedure is dependent not only on the ¹⁰B compound but on the spectral characteristics of the neutron source as well. Hence, an optimization of these characteristics will increase the chances of therapeutic success. Transport calculations using a neutral particle transport code have been made to determine the dose–depth distributions within a simple head phantom for five different incident neutron beams. Comparison of these beams to determine their relative therapeutic efficacy was made by the use of a maximum useable depth criterion. In particular, with presently available compounds, the MIT reactor (MITR) therapy beam (a) is not inferior to a pure thermal neutron beam, (b) would be marginally improved if its gamma-ray contamination were eliminated, (c) is superior to a partially ¹⁰B-filtered MITR beam, and (d) produces a maximum useable depth which is strongly dependent upon the tumor-to-blood ratio of ¹⁰B concentrations and weakly dependent upon the absolute ¹⁰B concentration in tumor. A pure epithermal neutron beam with a mean energy of 37 eV is shown to have close to the optimal characteristics for boron neutron capture therapy. Furthermore, these optimal characteristics can be approximated by a judiciously D₂O moderated and ¹⁰B-filtered ²⁵²Cf neutron source. This tailored ²⁵²Cf source would have at least a 1.5 cm greater maximum useable depth than the MITR therapy beam for realistic ¹⁰B concentrations. However, at least one gram of ²⁵²Cf would be needed to make this a practical therapy source. If the moderated ²⁵²Cf source is not ¹⁰B filtered, the resultant neutron beam has characteristics similar to those of the MITR beam with no gamma-ray contamination. For such a beam, 100 mg of ²⁵²Cf would produce a flux of 2.4×10⁸ neutrons/(cm² sec), which is an intensity suitable for therapy applications.