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

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Abstract
Measurements were made of beam profiles at 10 cm deep for the neutron beam of mean energy 7.5 MeV. Neutron and .gamma.-ray doses in water were measured with a tissue-equivalent chamber and a Geiger-Mueller 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 [relative biological effectiveness] of small doses of neutrons. The main hazard to the patient is probably that of carcinogenesis.