On the nature and significance of the radiation outside the beam in neutron therapy

Measurements have been made of beam profiles at 10 cm deep for the neutron beam of mean energy 7.5 MeV produced by the MRC cyclotron at Hammersmith Hospital. Neutron and gamma-ray doses in water were measured with a tissue-equivalent chamber and a Geiger-Müller counter. Far outside the beam the radiation consists mostly of gamma rays penetrating the shielding. Perfect shielding would reduce this but the profile of biologically effective dose would still be worse than that from conventional X- and gamma-ray generators owing to the high RBE of small doses of neutrons. The main hazard to the patient is probably that of carcinogenesis. An approximate assessment is made of this hazard which is compared with that estimated in previous publications.