The fast neutron beam from the M.R.C. cyclotron

Abstract

A beam of fast neutrons, of mean energy about 6 MeV, has been in use for radiotherapy since August, 1966. Patients are treated in a room next to the cyclotron chamber, the beryllium target being located in the wall separating the two rooms. Treatments are given at 120 cm FSD, and at this distance the average dose-rate is about 50 rads/minute. A set of wooden collimators, which are easily changed by hand, gives a variety of field sizes up to 20 × 20 cm. A protective cone round the collimator reduces the stray radiation to an acceptable level. Depth dose characteristics are intermediate between those of 200 kVp X rays and 60Co γ rays. The dose from γ radiation amounts only to a few per cent of the dose from fast neutrons and has been measured with photographic films and thermoluminescent dosimeters. Thermal and epithermal neutrons make only small contributions to the absorbed dose. The beam has been developed sufficiently for a clinical trial to be made. The penetration of the beam is adequate for the radiotherapy of all but the deepest lying lesions.