Comparison of parametrization methods of the collimator scatter correction factor for open rectangular fields of 6–25 MV photon beams

Purpose: To facilitate the use of the collimator scatter correction factor, S c, parametrization methods that relate S c to the field size by fitting were investigated. Materials and methods: S c was measured with a mini-phantom for five types of dual photon energy accelerators with energies varying between 6 and 25 MV. Using these S c-data six methods of parametrizing S c for square fields were compared, including a third-order polynomial of the natural logarithm of the field size normalized to the field size of 10 cm 2. Also five methods of determining S c for rectangular fields were considered, including one which determines the equivalent field size by extending Sterling's method. Results: The deviations between measured and calculated S c-values were determined for all photon beams and methods investigated in this study. The resulting deviations of the most accurate method varied between 0.07 and 0.42% for square fields and between 0.26 and 0.79% for rectangular fields. A recommendation is given as to how to limit the number of fields for which S c should be measured in order to be able to accurately predict it for an arbitrary field size.