Modification of the University of Washington Neutron Radiotherapy Facilityfor optimization of neutron capture enhanced fast-neutron therapy

A modified neutron production target assembly has been developed to provide improved performance of the proton-cyclotron-based neutron radiotherapy fa cility at the University of Washington for applications involving neutron c apture enhanced fast-neutron therapy. The new target produces a neutron bea m that yields essentially the same fast-neutron physical depth-dose distrib ution as is produced by the current UW clinical system, but that also has a n increased fraction of BNCT enhancement relative to the total therapeutic dose. The modified target is composed of a 5-millimeter layer of beryllium, followed by a 2.5-millimeter layer of tungsten, with a water-cooled copper backing. Measurements of the free-field neutron spectrum of the beam produ ced by the new target were performed using activation foils with a direct s pectral unfolding technique. Water phantom measurements were performed usin g a tissue-equivalent ion chamber to characterize the fast-neutron depth-do se curve and sodium activation in soda-lime glass beads to characterize the thermal-neutron flux (and thus the expected neutron capture dose enhanceme nt) as a function of depth. The results of the various measurements were qu ite consistent with expectations based on the design calculations for the m odified target. The spectrum of the neutron beam produced by the new target features an enhanced low-energy flux component relative to the spectrum of the beam produced by the standard UW target. However, it has essentially t he same high-energy neutron flux, with a reduced flux component in the mid- range of the energy spectrum. As a result, the measured physical depth-dose curve in a large water phantom has the same shape compared to the case of the standard UW clinical beam, but approximately twice the level of BNCT en hancement per unit background neutron dose at depths of clinical interest. In-vivo clinical testing of BNCT-enhanced fast-neutron therapy for canine l ung tumors using the new beam was recently initiated. (C) 2000 American Ass ociation of Physicists in Medicine. [S0094-2405(00)00802-6].