At therapeutic doses fast neutron radiotherapy alone gives significant tumor control can result in a significant increase in tumor control probability. As a fast neutron beam penetrates tissue it is partially moderated producing a cloud of thermalized neutrons. We describe measurements of this thermalized component using the therapy beam at the University of Washington and show how it is possible to increase its magnitude without appreciable degradation of the other clinical properties of the beam. We describe model calculation of the radiobiological effects of a boron neutron capture boost as well as in vitro cell culture measurements on the V-79 cell line, in vivo experiments on the 36B10 rat glioma system, and a preliminary experiment on a human melanoma nodule test system. Plans to extend this work to clinically-relevant tumor systems are discussed.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.