PURPOSE: For many years neutron radiation has been used to treat malignant disease both as fast neutron radiotherapy and as thermal neutron induced boron neutron capture therapy (BNCT). To date, these two approaches have been used independently of one another due to the large difference in neutron energies each employs. In this paper we discuss the potential application of BNCT to enhance the therapeutic effectiveness of a fast neutron radiotherapy beam.

METHODS AND MATERIALS: Measurements are presented for the thermal neutron component that is spontaneously developed as the University of Washington fast neutron radiotherapy beam penetrates a water phantom. The biological effect of this thermalized component on cells "tagged" with boron-10 (10B) is modeled mathematically and the expected change in cell survival calculated. The model is then extended to estimate the effect this enhanced cell killing would have for increased tumor control.

RESULTS: The basic predictions of the model on changes in cell survival are verified with in vitro measurements using the V-79 cell line. An additional factor of 10-100 in tumor cell killing appears achievable with currently available 10B carriers using our present neutron beam. A Poisson model is then used to estimate the change in tumor control this enhanced cell killing would produce in various clinical situations and the effect is sufficiently large so as to be clinically relevant. It is also demonstrated that the magnitude of the thermalized component can be increased by a factor of 2-3 with relatively simple changes in the beam generating conditions.

CONCLUSION: BNCT may provide a means of enhancing the therapeutic effectiveness of fast neutron radiotherapy in a wide variety of clinical situations and is an area of research that should be aggressively pursued.