ESTRO 2024 - Abstract Book

S4295

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

ESTRO 2024

In the second part of the study, participants were asked to assess the motion amplitude based on the provided 4DCT data for the distributed case and plan the case based on the internal protocols for such an indication. Concerning motion assessment, significant variations were observed between institutions for both motion amplitude, as well as the methodology used to calculate it. Depending on the institute, motion amplitude was defined as i) a 3D vector displacement, ii) motion trajectory along the three anatomical axes, iii) the largest displacement between extreme phases (peak-to-peak measurements) or iv) the result of beam-specific changes in water equivalent thickness. Such evaluations were based on visual inspection of 4DCT, calculated from deformable image registration, or from in house developed scripts. Due to this wide variety of approaches to assessing motion extent, maximum difference in reported 3D mean target motion for the same 4DCT data set was up to 4.3 mm. From the dosimetric perspective, clinically acceptable target coverage was achieved in all plans, using on average 4 beams (min: 2, max: 6), and only two centers reported deviations from clinical goals for V95% and D max . Analysis of cardiac substructures showed significant differences in mean dose to the left atrium and maximum doses to the coronary arteries. Outliers in mean dose to the chest (L/R) and ventricles (L/R) were reported by one center, as well as the V5 dose metric for both lungs. In addition to potential differences in technical capabilities across the centers, these variations may be the result of the target used for plan optimization (CTV, ITV, PTV), the use of robust optimization and/or different priorities given to spare specific OARs.

Conclusion:

The presented work was the first study as far as we are aware comparing multi-institutional treatment strategies for 4D treatments in particle therapy. Results from questionnaires, motion assessment and preliminary dosimetric analysis reveal significant variations in techniques, leading to differences in the chosen planning and delivery approaches, particularly in the assessment of motion amplitude and sparing of specific OARs. Indeed, an unexpected large discrepancy was reported for the motion estimation, indicating the necessity of the 4D treatments standardization, starting from the trivial steps. In summary, the study underscores the importance of addressing such inconsistencies as they are vital in order to ensure a consistent approach to particle therapy procedures worldwide.

Keywords: moving targets, 4D treatment, 4D planning