Effect of variation in the energy spectrum of a cyclotron-produced fast neutron beam in a phantom relevant to its application in radiotherapy.

The fast neutron spectrum of the beam produced by the MRC cyclotron at the Hammersmith Hospital has been measured in air and at several depths in a water phantom using three field sizes. The neutron spectra were determined both by a set of six threshold detectors and by a liquid scintillator spectrometer. Where a direct comparison of the two methods was possible the agreement was satisfactory. It was found that the shape of the spectrum, above 3 MeV, is unchanged with depth in the phantom, but for each field size the neutron fluence between 0.5 and 3.5 MeV increases significantly and reaches a maximum and the mean energy a minimum. This minimum decreases with increase in field size. Values of the kerma ratio in ICRU muscle to both that in A--150 plastic and bone were calculated and found to vary by less than 0.5% with depth in the phantom: that of carbon to ICRU muscle varied by 5%, and values of W for methane-based tissue-equivalent gas were found to change by less than 1%. Variations in biological response with changes in neutron spectrum are also discussed, together with the clinical significance of the results.