Phantom scatter factors for small MV photon fields.

Abstract

A table of small field (<4 cm width) S(p) values was created from measured S(cp) and S(c) data and compared with other authors' work. S(cp) and S(c) values measured with three types of diode and a diamond were used to calculate S(p). The results from measurements with different detectors were examined to determine the reasons for the differences and these were found to be due to detector and phantom-specific problems in the experimental determination of S(cp) and S(c). Small field S(p) were shown to be independent of beam defining system and linac design and dependent only on measurement depth and beam area irradiated. S(p) values obtained from S(cp) and S(c) measurements with a photon diode were found to be unreliable, due to problems with S(cp) measurements. Although the electron and stereotactic diodes and the diamond are suitable detectors for S(cp) and S(c) measurements, the differences between S(p) values calculated from diamond measurements and those calculated from measurements with unshielded diodes still require clarification through Monte Carlo modelling. A final table of S(p) was produced, for fields >0.5 cm width.