Practical beam steering of X-ray beams on Elekta accelerators: The effect of focal spot alignment on beam (symmetry and position) and radiation isocentre (size and position).

Acceptance and commissioning of a linear accelerator is the process of preparing it for clinical use. One of the initial important dosimetric tasks for X-ray beam set-up and use is to optimise the trajectory of the electron beam before it hits the target (focal spot). The main purpose of this study is to characterise the effect of the focal spot position (offset) on the photon beam symmetry and centre position, as well as on linac radiation isocentre size and position for an Elekta Synergy® linac. For this machine, the initial electron beam steering control items 2T and Bending F were altered to steer the beam in both transverse and radial directions respectively. The IC Profiler™ was utilised to measure the photon beam symmetry and centre position; the electronic portal imaging device (EPID) and the authors' published ready-to-go procedure were used to measure the focal spot offset; and the radiation isocentre size and position were measured using the EPID, the Elekta ball-bearing phantom and in-house software. It was observed that for the 6MV beam investigated, beam symmetry shows a high dependency on the focal spot position, with correlation coefficients of 8.6%/mm and 5.6%/mm in transverse and radial directions respectively. The radiation isocentre size shows dependency of 1.7 mm/mm on focal spot position in the transverse direction only. The radiation isocentre longitudinal position shows dependency of - 1.8 mm/mm on the focal spot position in the radial direction only. The beam centre position is directly correlated with the focal spot position in both directions, but the correlation coefficient depends on the collimation used in a given direction i.e. MLC (- 1.5 mm/mm) or diaphragms (- 0.8 mm/mm). Based on the results, a fast beam steering method was proposed and used successfully on an Elekta Versa HD™ linac, utilizing the IC Profiler™ and its associated Gantry Mounting Fixture™ (GMF) to efficiently and effectively optimise beam steering parameters for clinical use. Independent validation of the method showed that focal spot offsets and beam symmetries in terms of absolute deviations were on average 0.08 ± 0.05 mm (1SD) and 0.70 ± 0.27% (1SD) respectively.