ESTRO 2024 - Abstract Book

S4390

Physics - Intra-fraction motion management and real-time adaptive radiotherapy

ESTRO 2024

Change in plan complexity scores during online adaptive treatments on Ethos linear accelerator

Domonkos Szegedi 1,2 , Tamás Pócza 1 , Tibor Major 1,3 , Csilla Pesznyák 1,2

1 National Institute of Oncology, Radiotherapy Center, Budapest, Hungary. 2 Budapest University for Technology and Economics, Physical Doctoral School, Budapest, Hungary. 3 Semmelweis University, Department for Oncology, Budapest, Hungary

Purpose/Objective:

Our aim was to investigate retrospectively the complexity scores of the dual layer MLC system for multi-field sliding window Intensity Modulated Radiation Therapy (sw-IMRT) technique and their change between reference („scheduled”) and online adapted („adapted”) plans on Ethos linear accelerator.

Material/Methods:

Multi-field sw-IMRT technique is a common choice for online adaptation in clinical practice on Ethos linear accelerator. Although the plan quality is comparable with Volumetric Modulated Arc Therapy (VMAT) technique optimized in Ethos TPS, the plan generation is significantly faster for sw-IMRT during the on-couch sessions. In this investigation 15 patients treated in 2022-2023 with online adaptive radiotherapy in the pelvic region were included using 9 or 12 field sw-IMRT technique. Reference and adapted plans were exported from Ethos TPS 2.1 to Eclipse 16.1 system where an in-house C# based plug-in script was used to extract aperture area and aperture irregularity complexity scores for proximal and distal MLC banks separately[1]. Correlation between complexity scores of the two layers of MLC and between the two types of complexity scores were evaluated, as well as the change of the plan averaged aperture irregularity score between the reference and the adapted plans during the treatment course. Since target volume and irradiation field sizes can vary among patients all scores were normalized to the maximal values in the reference plans. Evaluating 3 879 treatment fields of scheduled and adapted plans in 403 fractions for 15 patients, strong correlation between beam weighted aperture area for proximal and distal MLC-s was found, as well as between beam-weighted proximal and distal aperture irregularity complexity scores. Furthermore, beam weighted aperture area and aperture irregularity were also correlated for proximal and distal layer of MLCs. All correlations had larger Pearson’s coefficient (r2) than 0.97. Correlation between distal and proximal scores were expected to be high, as neighbouring MLCs of the two layers should move with the lowest possible interleaf leakage in addition to increasing the resolution of the 1 cm thick MLC system. The relative difference of the plan-averaged aperture irregularity scores between the scheduled and adapted plans was 1.4% on average, but with a significant standard deviation of 13.2% (greatest relative increase of 40%, and decrease of -33%). Three of the 15 patients never had the same complexity score in adapted plans as in the original scheduled plans possibly indicating some systematic difference in the conditions between the treatment preparation and the online adaptive sessions. Results:

Conclusion: