title: The design, construction and performance of a variable collimator for epithermal neutron capture therapy beams.
abstract: A patient collimator for the fission converter based epithermal neutron beam (FCB) at the Massachusetts Institute of Technology Research Reactor (MITR-II) was built for clinical trials of boron neutron capture therapy (BNCT). A design was optimized by Monte Carlo simulations of the entire beam line and incorporates a modular construction for easy modifications in the future. The device was formed in-house by casting a mixture of lead spheres (7.6 mm diameter) in epoxy resin loaded with either 140 mg cm(-3) of boron carbide or 210 mg cm(-3) of lithium fluoride (95% enriched in 6Li). The cone shaped collimator allows easy field placement anywhere on the patient and is equipped with a laser indicator of central axis, beam's eye view optics and circular apertures of 80, 100, 120 and 160 mm diameter. Beam profiles and the collateral dose in a half-body phantom were measured for the 160 mm field using fission counters, activation foils as well as tissue equivalent (A-150) and graphite walled ionization chambers. Leakage radiation through the collimator contributes less than 10% to the total collateral dose up to 0.15 m beyond the edge of the aperture and becomes relatively more prominent with lateral displacement. The measured whole body dose equivalent of 24 +/- 2 mSv per Gy of therapeutic dose is comparable to doses received during conventional therapy and is due principally (60-80%) to thermal neutron capture reactions with boron. These findings, together with the dose distributions for the primary beam, demonstrate the suitability of this patient collimator for BNCT.
authors.affiliation: Nuclear Reactor Laboratory, Massachusetts Institute of Technology, 138 Albany St. Cambridge, MA 02139, USA.
authors.forename: K J
authors.initials: KJ
authors.lastname: Riley
authors.forename: P J
authors.initials: PJ
authors.lastname: Binns
authors.forename: S J
authors.initials: SJ
authors.lastname: Ali
authors.forename: O K
authors.initials: OK
authors.lastname: Harling
mesh_terms: D016754:Boron Neutron Capture Therapy / Q000295:instrumentation* / Q000379:methods*; D006801:Humans; D009010:Monte Carlo Method; D009502:Neutrons; D010315:Particle Accelerators; D012984:Software