Electron beam output of an ELEKTA Sli-Plus linear accelerator for irregular shaped fields and for extended SSD

Abstract

Introduction A dosimetric study of the electron beam output of the ELEKTA Sli-Plus linear accelerator is being presented: (a) when customized electron cutouts are used in order to shape the field, (b) when the treatment is performed in extended Source-Skin Distance. Purpose We have investigated how the machine output varies when electron cutouts of different shapes and sizes are inserted into the beam with the aim to establish a method to predict the beam output in similar circumstances. In order to determine the effective SSDs and the position of the effective point source, the Inverse Square Law (ISL) method has been used for all electron energies and applicators in order to correct the output at extended SSDs applying the ISL correctly. Materials and methods An extended set of output measurements has been performed using electron beams blocked by customized electron cutouts. Measurements have also been carried out in a water phantom at the depth of maximum dose as a function of the air gap g between the far end of the applicator and the phantom surface. Measurements have been compared to calculations with the Oncentra Masterplan treatment planning system. Results & conclusion Our results indicate that the electron beam output does not vary significantly with the insertion of an electron cutout into the beam and therefore, the monitor units determined for a given electron applicator can be used for the blocked beam as well. Monitor Units can also be safely determined by using the SSD eff in order to account for the change in output and depth-dose distribution. Disclosure The authors declare that they have no conflict of interest or any relationship that may bias their presentation.