Experimental central nervous system injury from fast neutrons

Therapeutic use of neutrons has entered a new phase. New, dedicated machines can produce high energy neutron beams that penetrate tissue at a depth equivalent to 6 mV of X rays. It should now be possible to decide whether the unacceptably high levels of late damage seen in patients in earlier trials was caused by the large dose necessary to irradiate a given volume of tissue with lower energy neutron beams or was caused by the nature of the neutron radiation itself. While the tolerance to neutron irradiation will eventually be defined in a clinical setting, until then the relative biological effectiveness (RBE) will have to be determined from the results of experiments on animals in which dose, fractionation schedules, and overall length of treatment can be controlled more easily than in a clinical setting. These data should provide a useful guide to clinicians. The clinical trials begun in the early 1960s were performed with cyclotrons that produced neutron beams (d[16]Be and d[22]Be) with poor penetration similar to that of 250kV X rays. This property limited trials to tumors of the head and neck; bilateral fixed nodes in the neck and glioblastoma multiform were included in this trail because of the poor prognosis for patients receiving conventional photon irradiation. At both sites, dose is limited by radiation damage to the CNS, but when the trials were begun information on the sensitivity of the CNS to neutrons was not available. In this paper experimental results for RBE for various energies of neutron beams are compared with the RBE values estimated from clinical experience, and possible reasons for the high RBE found for irradiation of the CNS are discussed