ESTRO 2025 - Abstract Book

S2924

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2025

4180

Digital Poster Dosimetric assessment of different treatment planning and delivery systems in complex spine SABR. Mahmoud Younes 1 , Abdul Mroue 2 1 Physics, Genesiscare UK, London, United Kingdom. 2 Physics, Genesiscare UK, Oxford, United Kingdom Purpose/Objective: Spine metastases are a common site for advanced cancer spread. Stereotactic Ablative Radiotherapy (SABR) has emerged as a highly effective treatment modality for spine lesions. This study assessed three different treatment planning and delivery systems for cases involving tumors in the posterior vertebral body. The first system used BrainLab Elements with Elekta Agility (5 mm MLC, 6 MV FFF), the second used Eclipse with TrueBeam (5 mm MLC, 6 MV FFF), and the third employed Eclipse with Varian Edge (2.5 mm MLC-HD, 6 MV FFF). Material/Methods: Ten spine SABR cases with Gross target volume (GTV) at the posterior part of the vertebral body were planned on the three systems. To ensure compliance with dose constraints, the planning target volume (PTV) is typically cropped by 2 mm from the spinal cord planning risk volume (PRV). All plans aimed to maximize PTV dose coverage while limiting the spinal cord PRV maximum dose (D0.035cc) to ≤20.3 Gy, as recommended by the UK SABR consensus constraints. The other plan quality indices were compared for the plans on each system. The number of Monitor units and optimization time for each plan was investigated.

Fig 1. shows the GTV location (blue) in the posterior part of Vertebral body.

Results: The results showed that the spinal cord PRV D0.035cc was consistently 18 Gy (±1 Gy) across all plans. Gross tumor volume (GTV) coverage was 95% (±2%) with the Edge system, and 94% (±3%) with both the TrueBeam and Elekta systems. For thoracic spine plans, the oesophageal dose (D0.1cc) was 17.3 Gy (±1 Gy) with the 2.5mm MLC and 20.6 Gy (±1 Gy) with the 5 mm MLC systems. The gradient index, a measure of dose fall-off, was superior in the 2.5mm MLC system (3.2 ± 0.5) compared to the 5 mm MLC plans (4.3 ± 0.8). Additionally, optimization times averaged at 30 minutes for plans using Acuros for dose calculation, but it extended to 70 minutes for Elements plans using the Monte Carlo algorithm. The total number of monitor units also investigated for all systems and there was insignificant difference between them for the same plan.

Conclusion: In conclusion, MLC leaf width has a significant impact on dosimetric outcomes in spine SABR. Plans using the 2.5