Evaluating dose uncertainty that is caused by excess CBCT frames on Elekta XVI systems using log file analysis – technical note

Introduction: This report investigates excess frame count during radiotherapy sessions using Elekta Versa HD systems with X-ray Volume Imaging (XVI) technology at Singleton Hospital. The hospital has 4 clinical linear accelerators (linacs) with XVI, which were analysed to identity variations in the number of excess frames between machines and imaging protocols. Such deviations could affect imaging dose accuracy, patient safety, and system efficiency. Method: XVI log files were gathered from each linac over an 18-month period using data backups. A Python script was created to read and link all the required data in a simple format to generate histograms and tables. Results: The excessive frames resulted in increased radiation doses. Although individual doses were negligible, the highest excess dose for a single patient was 0.7 mSv in 1 fraction, leading to a total dose of 3.4 mSv instead of the expected 2.7 mSv scans which is equivalent to 3 months of background radiation extra. The study revealed that 1.7 % of all imaging sessions were affected (417 imaging fractions). It was identified that the ‘Fast’ Breast imaging protocols were more likely to generate excess frames, likely to be due to the increased gantry speed. Conclusion: Despite the small individual doses, the findings raise concerns about system performance and patient safety for imaging, emphasising the need for further investigation to ensure optimal treatment accuracy and compliance with the Ionising Radiation (Medical Exposure) (Amendment) 2024), Regulation 11 and 12.