A consistent formalism for the application of phantom and collimator scatter factors

A coherent system for the use of scatter correction factors, determined at 10 cm depth, is described for dose calculations on the central axis of arbi trarily shaped photon beams. The system is suitable for application in both the fixed source-surface distance (SSD) and in the isocentric treatment se t-up. This is in contrast to some other proposals where only one of these a pproaches forms the basis of the calculation system or where distinct quant ities and data sets are needed. In order to derive the relations in the for malism, we introduced a separation of the phenomena related to the energy f luence in air and to the phantom scatter contribution to the dose. Both are used relative to quantities defined for the reference irradiation set-up. It is shown that dose calculations can be performed with only one set of ba sic beam data, obtained at a reference depth of 10 cm. These data consist f or each photon beam quality of measured collimator and phantom scatter corr ection factors, in combination with a set of (percentage/relative) depth-do se or tissue-phantom ratio values measured along the central axis of the be am. Problems related to measurements performed at the depth of maximum abso rbed dose, due to the electron contamination of the beam, are avoided in th is way. Collimator scatter correction factors are obtained by using a mini- phantom, while phantom scatter correction factors are derived from measurem ents in a full scatter phantom in combination with the results of the mini- phantom measurements. For practical reasons the fixed SSD system was chosen to determine the data. Then, dose calculations in a fixed SSD treatment se t-up itself are straightforward. Application in the isocentric treatment se t-up needs simple conversion steps, while the inverse approach, from isocen tric to fixed SSD, is described as well. Differences between the two approa ches are discussed and the equations for the conversions are given.