Comparative radiobiological measurements with two high-energy cyclotron-produced neutron beams presently used for radiotherapy

Comparative radiobiological measurements were made at two of the high-energy cyclotrons currently being used for neutron therapy in the United States. They are the Texas A & M variable energy cyclotron (TAMVEC), which operates at 50 MeV, and the Naval Research Laboratory (NRL) cyclotron in Washington, D.C., which operates at 35 MeV. Using Vicia seedlings, the oxygen enhancement ratio (OER) was found to be 1–6 for both neutron beams. Further, the OER did not vary significantly with depth in a water phantom. Determinations were made of the relative biological effectiveness (RBE) for both neutron beams using mammalian cells in culture and Vicia seedlings. The 35 MeV NRL neutrons have a slightly higher RBE than the 50 MeV TAMVEC neutrons. This difference amounts to about 10 per cent at the dosage level used for individual treatment fractions in radiotherapy. Since the OER for these high-energy neutrons is no larger than for 14 MeV d→T or for the Hammersmith neutrons, and since they are produced at a high doserate with favourable depth-doses, they represent an attractive modality for radiotherapy.