Optimisation of on-board imaging fluoroscopy

On-board imaging fluoroscopy is used for localisation of the target immediately prior to external beam radiotherapy. The feature is clinically used for lung stereotactic ablative radiotherapy involving a moving target and high dose per fraction. Use of fluoroscopy is clearly justified, allowing verification that the target is within the field over its range of motion, ensuring the tumour is not under-treated and there is not excessive dose to normal tissue. Commissioning the fluoroscopy features on Varian Clinacs and Varian Truebeams demonstrated that systems are not optimal at installation. The ability to adjust the automatic brightness control settings is not readily available and access required extensive discussions with the manufacturer. The TrueBeam linear accelerator had additional dose reduction features not available on the Clinac. Such features available in clinical mode included the ability to adjust pulse rate settings and add additional titanium filtration This paper will detail the potential for significant patient dose reduction (35 mGy/min to 16 mGy/min Clinac, 75 mGy/min to 8 mGy/min Truebeam) without a clinically significant reduction in image quality. The doses post optimisation are below what would be expected from an image intensifier and slightly higher than what would be typical for a modern flat-panel interventional system. The dose comparisons are made to interventional systems due to the lack of published data in radiotherapy systems. Historically the focus has been on the considerably higher radiotherapy dose, but recent developments and increased use of imaging in therapy has seen increased attention to imaging doses.