Towards zero radiation isocentre size: minimising radiation beam isocentricity on Elekta linear accelerators by means of optimising look-up tables

Abstract

The most important geometric characteristics of SRS SBRT treatments are precise target localisation and precise aiming of the radiation beam at the target The AAPM-RSS Medical Physics Practice Guideline 9 a for SRS SBRT recommends that the radiation isocentricity i e beam deviation from the isocentre should not exceed 1 mm for SRS and 1 5 mm for SBRT Minimising the beam deviations from the treatment target largely due to the gantry sag can improve the accuracy of radiosurgery and stereotactic treatments and commonly beam steering parameters are optimised to achieve this objective This study aims to investigate as a proof of concept if it is possible to eliminate the beam deviations on Elekta linear accelerators altogether by optimising gantry angle dependent beam steering parameters as stored in look-up tables The investigation used the EPID-based Winston-Lutz test at 13 gantry angles separated every 30 from - 180 to 180 Elekta linacs have two look-up tables that can be customised explicitly for radial beam angle and transverse beam position Modifications of the radial look-up table were limited by the radial beam asymmetry inhibit of more than 5 as measured by the linac in-built ionisation chamber Therefore only small radial beam deviation reductions of 0 1 mm were achieved on average from 0 37 to 0 26 mm while radial beam symmetry changed significantly by up to 7 depending on the gantry angle as measured by the IC Profiler The optimised transverse look-up table resulted in reduction of transverse beam deviations to almost zero on average from 0 20 to 0 03 mm however that changed the transverse beam symmetry by almost a constant value of 1 as measured by the IC Profiler Ideally two additional look-up tables are needed for effective beam steering one for radial beam position and one for transverse beam angle Four look-up tables in total would enable customising beam centre position and beam symmetry at any gantry angle that would minimize radiation isocentre size without compromising beam symmetry