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

Publisher Website
Google Scholar
Abstract
The fast neutron spectrum of the beam produced by a MRC cyclotron was measured in air and at several depths in a water phantom using 3 field sizes. The neutron spectra were determined both by a set of 6 threshold detectors and by a liquid scintillator spectrometer. Where a direct comparison of the 2 methods was possible the agreement was satisfactory. The shape of the spectrum, above 3 MeV, is unchanged with depth in the phantom, but for each field size the neutron fluence between 0.5-3.5 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 < 0.5% with depth in the phantom; that of carbon to ICRU muscle varied by 5%, and values of .hivin.W for methane-based tissue-equivalent gas changed by < 1%. Variations in biological response with changes in neutron spectrum are also discussed, together with the clinical significance of the results.