IN-PHANTOM DOSIMETRY AND SPECTROMETRY OF PHOTONEUTRONS FROM AN 18 MV LINEAR-ACCELERATOR

A combination of three superheated drop detectors with different neutr on energy responses was developed to evaluate dose-equivalent and ener gy distributions of photoneutrons in a phantom irradiated by radiother apy high-energy x-ray beams. One of the three detectors measures the t otal neutron dose equivalent and the other two measure the contributio ns from fast neutrons above 1 and 5.5 MeV, respectively. In order to t est the new method, the neutron field produced by the 10 cm X 10 cm x- ray beam of an 18 MV radiotherapy accelerator was studied. Measurement s were performed inside a tissue-equivalent liquid phantom, at depths of 1, 5, 10 and 15 cm and at lateral distances of 0, 10, and 20 cm fro m the central axis. These data were used to calculate the average inte gral dose to the radiotherapy patient from direct neutrons as well as from neutrons transmitted through the accelerator head. The characteri stics of the dosimeters were confirmed by results in excellent agreeme nt with those of prior studies. Track etch detectors were also used an d provided an independent verification of the validity of this new tec hnique. Within the primary beam, we measured a neutron entrance dose e quivalent of 4.5 mSv per Gy of photons. It was observed that fast neut rons above 1 MeV deliver most of the total neutron dose along the beam axis. Their relative contribution increases with depth, from about 60 % at the entrance to over 90% at a depth of 10 cm. Thus, the average e nergy increases with depth in the phantom as neutron spectra harden. ( C) 1998 American Association of Physicists in Medicine.