An examination of some methods of improving the depth dose characteristics of cyclotron-produced fast neutron beams

The fast neutron beam currently being used at Hammersmith Hospital for radiotherapy is produced by bombarding a beryllium target with 16·7 MeV deuterons from the MRC cyclotron. From the radiotherapy point of view the dose-rate is good, being about 40–50 rads/minute, but the penetration into tissue is considerably worse than that obtained from modern high energy X-ray and tele-isotope units. Experiments have been carried out to determine whether it is possible to improve these depth dose characteristics without too great a loss in dose-rate. Techniques considered are filtration of the beam, use of thin beryllium targets and the use of alternative neutron producing reactions. While improvements in depth doses can be obtained by the use of these techniques a maximum increase in the dose at 10 cm deep in tissue (from 42 to 46 per cent) was obtained in going from a thick beryllium target to the optimum target configuration investigated. However, since the existing beryllium target is not thick enough to stop the deuterons completely, the improvement gained in going from this to the optimum thickness is overshadowed by the decrease in dose-rate. It is also shown that calculations based on the maximum energy of neutrons available from a given (d,n) reaction using the maximum energy of the projectiles and the Q value for the reaction are not a satisfactory indicator to the depth doses which would result.