Clinical Considerations in the Use of Thermal and Epithermal Neutron Beams for Neutron Capture Therapy
Two important developments in the field of neutron capture therapy (NCT) in recent years have been the ultra-wide thermal neutron beam1 and the epithermal neutron beam.2,3 Both these maneuvers improve the depth in tissue at which the therapeutic advantage falls to unity, often referred to as the “advantage depth”.3,4 Both neutron beams have contaminating dose components, i.e., those that are not tumor-cell specific. These include: incident gamma rays (those originating in the reactor core and those produced by neutron capture in the beam line’s structural materials); induced gamma rays (produced mainly by thermal neutron capture by hydrogen within the target tissue itself); background thermal neutrons (which interact mainly with tissue nitrogen by neutron capture, producing proton emission); and incident epithermal and fast neutrons (producing mainly recoil protons by scattering with tissue hydrogen). Whereas incident gammas and epithermal and fast neutrons (above about 30 keV) can be reduced to acceptably low levels by judicious beam design, the induced gamma and background thermal neutron doses are largely irreducible, although higher concentrations of 10 B in tumor diminishes their magnitude on a relative basis. Incident epithermal neutrons of 0.5 eV–30 keV, however, are the desirable components of an epithermal neutron beam. Such neutrons thermalize at depth in tissue, thereby producing the desired B reactions. Very simplistically, an epithermal beam may be considered equivalent to a thermal beam “injected” at a couple of centimeters depth in tissue.
KeywordsFast Neutron Neutron Beam Linear Energy Transfer Boron Neutron Capture Therapy Relative Biological Effectiveness
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