The design of a practical fast neutron therapy equipment for routine clinical use

A 14 MeV neutron therapy equipment, incorporating a sealed tube neutron generator with an output of 1 × 1012 neutrons per second, is under construction and will be installed at the Christie Hospital, Manchester. The machine is designed for clinical use and has all the features normally provided on radiation therapy equipment. The neutron shield is 0·5 m thick and comprises an iron core surrounded by a layer of hydrogenous material. It provides a dose attenuation factor of 99 per cent. The treatment head can be rotated about a horizontal axis 0·8 m from the neutron source. At this distance the machine delivers a dose-rate of 4·5 rads/minute. An important feature of the design is the reduction to acceptable levels of γ dose-rates from neutron-induced radioactivity. A movable tungsten plug in front of the neutron source reduces the exposure rate at 1 m from the source from 157 mR/hour to 1 mR/hour. The surface exposure rate on the shield is reduced from 61 mR/hour to less than 4 mR/ hour by the use of pure iron rather than a standard grade of steel, for the shield core. Similar precautions in the construction of the treatment table and the treatment room ensure that the total activity levels in the room are such that radiographers using the equipment will not receive excessive doses.