Relative biological effectiveness for damage to the central nervous system by neutrons

The relationship between Relative biological effectiveness (RBE) and dose/fraction for damage to the spinal cord and brain has been established for the Hammersmith neutron beam (Ed = 16 MeV-Be). Results of RBE for fast neutrons from other centers are reviewed. The Ed = 16 MeV-Be (Hammersmith) and Ed = 22 MeV-Be (Seattle) neutrons were 1.15 more effective than 15 MeV d-T (Rijswijk) and 50 MeV-Be (Houston). With these results and the formula TD = NSD N 38 T 02 (which has been established for x rays on nervous tissue), it is possible that the neutron doses which have been used in clinical trials of brain tumors were all above tolerance. This hypothesis is consistent with the clinical findings. If RBE is plotted against γ-ray dose/fraction, the RBE relative to an acceptable x ray schedule can be read off directly; if RBE is plotted against neutron dose/fraction, the equivalent γ-ray dose can be obtained. At 200 rad/fraction of γ-rays, the RBE for Ed = 16 MeV-Be neutrons is 5.2. On this basis a total neutron dose of about 1100 rad to a large volume of the brain probably would be the maximum that could be tolerated