S1958
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
lesions using advanced dose calculation algorithms. Radiat Oncol. 2014;9:223. 2. Guckenberger M, Andratschke N, Dieckmann K, et al. ESTRO ACROP consensus guideline on implementation and practice of stereotactic body radiotherapy for peripherally located early-stage non-small cell lung cancer. Rad and Onc. 2017;124(1):11-17. 3. Seuntjens J, Goetsch S. PRESCRIBING, RECORDING, AND REPORTING OF STEREOTACTIC TREATMENTS WITH SMALL PHOTON BEAMS Report Committee.4. Sarudis S, Karlsson A, Nyman J, Bäck A. Dosimetric effects of respiratory motion during stereotactic body radiation therapy of lung tumors. Acta Oncol (Madr). 2022;61(8):1004- 1011. Keywords: GTV-based prescription, Lung SBRT, 4DCT The impact of electron streaming in MR-guided SABR treatment of early-stage glottic laryngeal cancer Sabine M.L. Linden 1 , Marielle E.P. Philippens 1 , David J. Sher 2 , Mu-Han Lin 2 , Cornelis P.J. Raaijmakers 1 , Mischa de Ridder 1 1 Department of Radiotherapy, University Medical Center Utrecht, Utrecht, Netherlands. 2 Department of Radiotherapy, UT Southwestern Medical Center, Dallas, USA Digital Poster 3878 Purpose/Objective: Stereotactic ablative radiotherapy (SABR) could significantly reduce the treatment time of early-stage glottic cancer (from 25 to 5 fractions). MR-guided radiotherapy is a promising approach for this treatment, since it enables margin reduction and real- time motion monitoring. This could reduce the dose on the laryngeal cartilage structures, which is important to prevent chondronecrosis after treatment. However, the magnetic field can cause electron streaming and consequently dose deposition outside the treated volumes. The impact of this effect is not yet known for the head and neck region. This study aims to assess the clinical impact of the electron streaming effect for MR-guided SABR of early-stage glottic cancer. Material/Methods: Three MR-Linac SABR plans (5x8.5 Gy) were compared to plans without magnetic field that were re-optimized using the same constraints. The setup of these re- optimized plans was similar to the MR-Linac plans. A high dose area around the target was defined as the area with a dose >10 Gy. The dose in the skin was calculated in the superficial 5mm of the skin, cranial and caudal to the high dose area. The dose in the mucosa of the trachea and pharynx (5mm) was evaluated at a distance of 0-10mm, 10-20mm, 20-
density override for optimisation (PTVD95%, GTVD98%), followed by recalculation on original densities and normalisation to GTVD50%. The same fluence was then recalculated on three 4DCT phases. For each phase, GTVs were delineated individually. GTV dose metrics (min, mean, max, median, D98%) were analysed per phase, and average absolute deviations calculated to assess inter-phase robustness. Results: The median PTV volume on free breathing CT was 13.1 cm3 [8.0-14.0], and GTV 0.7 cm3 [0.3-1.5]. Phase- specific GTVs showed variability, with a median Δ V of 26.3 % [10.9-57.4]. The GTV delineated on the free- breathing CT was consistently larger compared to phase-specific GTVs, reflecting motion blurring; and it only was used as the reference for dose evaluation. The largest inter-phase variation was observed in GTVDmax 3.9% (one case) (Figure 1). Patient 2 showed ≈ 2% variation for all dose metrics at maximum inhale, likely due to anatomical placement of the target. The median average inter-phase deviation for all metrics was 0.8% [0.3-1.6] (Table 1), indicating robust GTV coverage despite tumour motion.
Conclusion: Treatment optimisation using PTV density override followed by GTV median dose normalisation provides a robust approach for challenging lung SBRT in this 5 patients cohort. Despite motion-induced anatomical variations, target dose remained consistent across extreme phases, reflecting the effect of initially optimised uniform fluence. This methodology supports the standardisation of dose prescription and reporting, offering a transparent framework for GTV- based planning. Future work may assess non- modulated arcs to further enhance respiratory motion
robustness. References:
1. Lacornerie T, Lisbona A, Mirabel X, Lartigau E, Reynaert N. GTV-based prescription in SBRT for lung
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