S1952
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
Radiotherapy, UT Southwestern Medical Center, Dallas, USA
MPTV and geometric uncertainties, the larger this difference. Using larger MPTV-recipes, these differences becomes less, decreasing from 5%/mm to 3%/mm.
Purpose/Objective: Treating early-stage glottic cancer with stereotactic ablative radiotherapy (SABR) significantly reduces treatment time (from 25 to 5 fractions). Limiting the dose on the laryngeal cartilages (arytenoids, thyroid, and cricoid cartilages) seems promising, since Sher et al. did not report any severe late toxicities at a follow- up of 2 years [1]. These patients were treated with non-coplanar Volumetric Modulated Arc Therapy (VMAT) on a CBCT Linac. MR-guided radiotherapy has several advantages that make it an attractive alternative. The superior soft-tissue contrast and real- time motion monitoring with gated delivery could further reduce treatment margins. This study aims to evaluate plan quality of MR-linac plans for glottic SABR and compare them to non-coplanar VMAT treatment plans. Material/Methods: Twenty early-stage glottic cancer patients (cT1-2), who were treated at UTSW Medical Center with a 5 x 8.5 Gy fractionation schedule, were used for a comparison between MR-Linac and non-coplanar CBCT Linac VMAT plans. For non-coplanar VMAT, the iGTV-PTV margins were 3 mm in anterior-posterior and left-right direction, and 5 mm in cranio-caudal direction. For MR-Linac plans, the GTV-PTV margin was 2 mm in anterior-posterior and left-right direction, and 4 mm in cranio-caudal direction. For the MR-Linac plans, we used three beam arrangements of 11 coplanar beams, chosen according to the laterality of the tumor (left, right or bilateral). The non-coplanar beam setup was two coplanar partial arcs and two non-coplanar partial arcs (at 45° and 315° couch angle). The dose constraints were the same for both techniques. Results: An example of the delineation and dose distribution of an MR-Linac plan is visualized in Figure 1. The mean dose difference between techniques on the laryngeal cartilages never exceeded 3 Gy. Two out of twenty MR- Linac plans did not meet all the constraints for the laryngeal cartilages, of which one was also not met for the non-coplanar VMAT plan (Figure 2). For MR-Linac plans, GTV coverage was > 99.5% for 17/20 patients. For one of these patients, the coverage was not sufficient for both MR-Linac and non-coplanar VMAT plans. Variation in the dose distribution is observed between patients.
Conclusion: We demonstrated a relation between two tumor safety margins, a dosimetric margin accounting mainly for radiobiological and dosimetric variations, and a geometric margin for ditto variations. Future research will include the theory and development of a TCP- validated CTV margin recipe. References: 1Stroom JC, Vieira SC, Greco C, Nijsten SMJJG. Accuracy-dependent dose-constraints and dose-based safety margins for organs-at-risk in radiotherapy. Physics and Imaging in Radiation Oncology 2025;33:100713. https://doi.org/10.1016/j.phro.2025.100713. Keywords: uncertainty, margins, dose constraints, CTV
Digital Poster 3800
MR-Linac as an alternative to non-coplanar VMAT in treating early-stage glottic cancer with SABR Sabine M.L. Linden 1 , Marielle E.P. Philippens 1 , Eric Brand 1 , David J. Sher 2 , Mu-Han Lin 2 , Mischa de Ridder 1 1 Department of Radiotherapy, University Medical Center Utrecht, Utrecht, Netherlands. 2 Department of
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