Beam Profiles Of An Epithermal Neutron Beam

The thermal neutron fluence profiles, the gamma-ray dose profiles and the fast neutron dose profiles have been measured free in-air as well as in phantoms irradiated with the clinical epithermal neutron beam for boron neutron capture therapy at the High Flux Reactor in Petten, The Netherlands. For this purpose a set of well characterized detectors has been used: a PN diode detector, a magnesium ionization chamber flushed with argon and a tissue-equivalent ionization chamber flushed with tissue-equivalent gas. The influence of collimator size on the various dose profiles has been determined in cubical water phantoms of different size for 8 cm, 12 cm and 15 cm diameter circular beam collimators. A minor influence of the phantom size on the various beam profiles was observed for the 8-cm and 12-cm field. For the 15-cm field, a profound influence of the phantom size was observed on the thermal neutron fluence profiles. A minor influence on the gamma-ray dose profiles was seen for this field size. A treatment planning system is under development to use the results of these measurements to calculate in a semi-empirical way, the various dose components at all clinically relevant points in patients irradiated with the epithermal neutron beam.