Focal spot estimation of an Elekta dedicated stereotactic linear accelerator Monte Carlo model

The most challenging task in the Monte Carlo modelling of linear accelerators (linacs) is an accurate determination of the electron beam parameters striking the target which are characterised by the mean energy of incident electron beam and the electron beam shape, referred to as the focal spot. This work aims to determine the optimum focal spot size and shape of Elekta Axesse linac equipped with the Beam Modulator. A BEAMnrc Monte-Carlo linac model has been developed to produce a 6 MV photon beam. Different square field sizes of 2.4 cm, 4 cm and 10.4 cm were simulated in a simple water phantom with a source-to-surface distance of 100 cm. The simulation was performed with the incident electron beam energy of 6.2 MeV with the focal spot size varied between 0.1 and 0.3 cm with an increment of 0.05 cm. The field width (50% relative dose) and penumbra width (distance between 80% - 20% relative dose) of the simulated profiles were compared with the measured profiles. This work found that an elliptical shape of the focal spot results in a better match with the measured data with the size of 0.2 cm in X-axis and 0.3 cm in Y-axis direction.