ESTRO 2025 - Abstract Book

S3702

Physics - Quality assurance and auditing

ESTRO 2025

4466

Digital Poster Evaluation of beam model changes following corrective maintenance in Tomotherapy Diogo Campizes 1 , Tiago Ventura 2 , Josefina Mateus 2 , Tania Santos 2 , Maria do Carmo Lopes 2 1 Physics, Faculty of Sciences and Technology of University of Coimbra, Coimbra, Portugal. 2 Medical Physics, IPO Coimbra, Coimbra, Portugal Purpose/Objective: Following corrective maintenance of the Tomotherapy unit, such as changes to the MLC leaf latency, AAPM's TG-306 proposes a method for estimating dose error based on the latency offset difference (LOD) and key planning parameters, including the modulation factor (MF) and gantry period (GP). In this study, the methodology outlined in the TG-306 was compared with the differences in 3D dose distributions before and after equipment intervention, with the aim of ensuring equivalent treatments and determining whether the dose error remains within acceptable clinical thresholds, potentially avoiding unnecessary replanning. Material/Methods: This study included 14 patients who were undergoing treatment during the maintenance phase and had PSQA plans. For each plan, the MF, GP, and LOD values were recorded, and the percentage dose error was calculated using the TG-306 formula. Tolerance and action levels of 1% and 2%, respectively, were defined to determine the need for replanning. Recommended operating intervals for the MF and GP were also established to streamline future evaluations. Since the TG-306 formula does not account for other beam model parameters, MATLAB was used to perform detailed voxel-by-voxel comparisons of the 3D dose matrices generated before and after maintenance, providing a comprehensive analysis of changes in dose distributions. Results: The TG-306 formula was successfully applied. All plans were within the tolerance level and no replanning action was required. Histograms of percentage dose differences were generated from the MATLAB analysis. Most discrepancies were within clinically acceptable limits, supporting the practical application of TG-306. The comparative analysis showed a strong correlation between the actual dose differences observed in the 3D dose distributions and the TG-306 calculated dose errors. On average, the TG-306 method produced slightly lower dose errors than those derived from direct 3D comparisons. However, the distance between dose errors narrowed as LOD values increased, suggesting that the TG-306 formula is optimised for higher latency shifts. Conclusion: The TG-306 formula was found to be adequate for assessing treatment plan equivalence following MLC adjustments, providing a reliable way to identify cases requiring replanning. This study highlights the importance of integrating TG-306-based calculations into standard workflows, potentially aided by automated MATLAB scripts for dose difference analysis. Such tools extend the applicability of the methodology to a variety of maintenance scenarios, including those unrelated to MLC latency changes. This ensures that dose deviations remain within clinical tolerance, maintaining treatment quality and patient safety, while streamlining the decision-making process concerning actions after major technical interventions.

Keywords: Tomotherapy, major intervention, equivalent plans