Validation of a simple analytical model for in-air outputfactor calculation for SL-15 Philips-Elekta linear accelerator

A simple aralytical approach to model extrafocal radiation (EFR) and monitor chamber backscatter (MBS)-and consequently collimator scattar factor-is investigated. The model has been applied to 6 and 10 MV photon beams produced by a Philips-Elekta SL-15 medical linear accelerator. Both EFR and MBS are determined simultaneously using conventional measured data at the isocenter and the calculated in-air output factors (Sc) were in good agreement with the measured values. When the square field size changes from 4×4 to 40×40 cm2, the total intensities of EFR were 17.6% and 13%, while the MBS contributions to Sc were 0.1% and 0.2% for 6 and 10 mv, respectively. The model was also used to calculate Sc for symmetric or asymmetric rectangular jaws-defined fields with an accuracy of less than 0.2% at extended or shortened source detector distances Moreover, the model was verified for both very small field sizes (2×2 cm2 down to 0.6×0.6 cm2) and for field sizes defined by micro multi-leaf collimator to check its applicability for stereotactic radiotherapy dose calculations. A simple programme is designed to facilitate the calculation process of Sc for a medical linear accelerator at different situations either for commissioning or verification of the model at different energies.