A fast-neutron source for radiotherapy.

Abstract

IN a recent note, Lomer and Greene1 have recommended the development of a 14-MeV fast-neutron source for radiotherapy. We feel that there are disadvantages to this approach. The relative biological effectiveness of fast neutrons is related to their linear energy transfer, which in turn varies inversely with the energy of the neutron2,3. It has been shown that cell-killing by 1—2 MeV neutrons (linear energy transfer, 50–60 keV/µ) is relatively independent of oxygen3. As the energy of the neutrons is increased, the effect becomes more oxygen-dependent4—30-MeV neutrons would have a linear energy transfer similar to 250-kV X-rays, and hence a similar oxygen-dependence. If one is to expect an improvement in the results of radiotherapy due to the relative oxygen-independence of the effect of fast neutrons, then one must attempt to use those neutrons the effects of which are relatively oxygen-independent; this cannot be claimed for 14-MeV neutrons (linear energy transfer 20 KeV/µ). A useful increase in effective linear energy transfer with depth of penetration in tissue has yet to be demonstrated: the neutrons will lose energy and become more densely ionizing but the γ-ray component of the total dose rises.