Phantom scatter factors for small MV photon fields

Abstract

Background and Purpose 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. Methods and materials S cp and S c values measured with three types of diode and a diamond were used to calculate S p. Results 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. Conclusion 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.