Epithermal neutron beams are under development in a number of locations in the U.S. and abroad. The increased penetration in tissue provided by these neutrons should circumvent problems associated with the rapid attenuation of thermal neutron beams encountered in previous clinical trials of neutron capture therapy (NCT). Physical and radiobiological experiments with two “intermediate energy” or “epithermal” beams have been reported. A comparison is made here between the 24-keV iron-filtered beam at Harwell, England, and the broad-spectrum A1203moderated beam at the Brookhaven Medical Research Reactor (BMRR). In addition, parameters which are relevant for NCT, and which are best suited for evaluation and comparison of beams, are discussed. Particular attention is paid to the mean neutron energy which can be tolerated without significant reduction of therapeutic gain (TG), where TG is the ratio of tumor dose to maximum normal tissue dose. It is suggested that the simplest and most meaningful parameters for comparison of beam intensity and purity are the epithermal neutron fluence rate, and the fast neutron dose per epithermal neutron (4.2X10-11rad/neutron for the broad-spectrum beam and 29 X 10 “11rad/neutron for the 24-keV beam). While the A12O3beam is close to optimal, the 24-keV beam produces a significant fast neutron dose which results in a lower TG. It is argued that if sufficient intensity is available, TG is the parameter which determines the potential usefulness of a beam for NCT. Data are presented to illustrate that with an epithermal fluence rate of —1.8 X 109n/cm2s, the A12O3filtered beam at the BMRR is optimized and suitable for clinical application. A TG of — 3 can be realized with currently available compounds; NCT of a brain tumor in a single exposure would take — 30 min. Further increases in TG can only be obtained from increased boron concentration and retention within tumor cells, and from improved clearance of boron from normal tissues.
- epithermal neutrons
- neutron capture therapy