Biological dosimetry for epithermal neutron beams

The radiobiological effectiveness of an epithermal neutron beam is describe d using cell survival as the end point. The M67 epithermal neutron beam at the Nuclear Reactor Laboratory, Massachusetts Institute of Technology, that was used for clinical trials of boron neutron capture therapy was used to irradiate Chinese hamster ovary cells at seven depths in a water-filled pha ntom that simulated healthy tissue. No boron was added to the samples. Ther efore, this experiment evaluates the biological effectiveness of the neutro n and photon components, which comprise 80-95% of the dose to healthy tissu e. Cell survival was dependent upon the depth in the phantom, as a result o f moderation and attenuation of the epithermal neutron beam components by t he overlying water. The results were compared with 250 kVp X irradiations t o determine relative biological effectiveness values. Cell survival as a fu nction of the dose delivered was lowest at the most shallow depth of 0.5 cm , and increased at depths of 1.5, 3, 4, 5.6, 6.6 and 8.1 cm. The gradual in crease in cell survival with increasing depth in the phantom is due to the exponential drop of the fast-neutron intensity of the beam. These results a re applicable to clinical boron neutron capture therapy Phase VII trials in which healthy tissue toxicity was an end point. (C) 2001 by Radiation Rese arch Society.