ESTRO 2024 - Abstract Book

S4946

Physics - Quality assurance and auditing

ESTRO 2024

By plotting the unadjusted output data, the trends in outputs over time were visualised where the predominate trend observed is the gradual decrease in annual output variation with time. The initial steep climb followed by a plateau in output is consistent with that described previously in literature 1,2,5 . Although curve fitting was initially employed to quantify these trends, significant inter-chamber variability was observed and therefore, no single model could be developed to quantify the data.

Figure 1: Unadjusted output curve, where adjustments to the output have been artificially removed. This curve was obtained for machine with local identifier LA6, for beam energy 6MV.

For each clinical photon energy, the annual percentage change in output was determined and is depicted visually in Figure 2. It was observed that in the first year of use the gain in output is largest, with initial changes between 2-6%. Consequently, most machines required 1-2 output adjustments in their first year of use, with 3 machines requiring 3 output adjustments across all available energies in this period.Two chambers on separate linear accelerators were found to have a greater percentage change in output in their second year of use, where one chamber was then replaced after two years. Across all four energies, one chamber had the largest percentage change in output over the first year of use, identified by the outliers in Figure 1. Following a drop in percentage output change in the second year, this chamber then demonstrated the greatest output change in the third year across three out of the four energies.The frequency of output adjustments made annually was found to vary between energies for each chamber.