RBE OF CYCLOTRON FAST-NEUTRONS FOR A RAT-BRAIN TUMOR

The effects of cyclotron neutrons and 60Co-γ rays on an ethylnitrosourea-induced rat astrocytoma transplanted to an intracerebral site in rats have been examined. Unirradiated tumor-bearing animals succumbed from the mass effects of relentless tumor growth 21 days after implantation of 5 X 104 cells with a survival-time range of 5 days. Single-dose, whole-brain neutron or photon irradiation 1 week after transplantation delayed the onset of neurological signs and prolonged survival. This response was dose-dependent with both types of radiation. However, neutrons were clearly more effective per rad resulting in an RBE of 3.0 to 3.5 at a neutron dose per fraction of 200 rad. Results using two and five daily fractions indicate that fractionation did not appreciably change the length of survival of the RBE value from that seen with single-dose experiments. Irradiation of tumors 2 weeks after transplantation did not change the mean survival time or the RBE values from those observed for tumors irradiated 1 week after transplantation. Histologically, the effects of irradiation appeared to be localized to the tumor site or tumor-involved brain tissue. Irradiated brain tissue remote from the tumor appeared normal histologically. Comparison of these results with other reports of normal central nervous tissue responses to fractionated neutron radiation in the clinical dose range suggests that there may be no advantage in using neutrons for the treatment of brain tumors.