S2325
Physics - Quality assurance and auditing
ESTRO 2026
1 Radiotherapy, UMC Utrecht, Utrecht, Netherlands. 2 Radiation Oncology, University of Iowa, Iowa, USA. 3 Elekta, Elekta AB, Stockholm, Sweden Purpose/Objective: The MR-linac combines MRI with radiotherapy delivery, enabling adaptive treatments with real-time visualization. While volumetric modulated arc therapy (VMAT) has already been shown to be feasible on the MR-linac1,2 and offers many advantages, commissioning tests are essential for its clinical implementation. This abstract highlights some commissioning tests developed and performed on a 1.5T MR-linac, focusing on verifying beam stability and MLC behaviour under dynamic gantry conditions. These tests address the unique challenges posed by the mechanical characteristics of the MR-linac as well as the presence of the magnetic field. By establishing a comprehensive set of tests, this study aims to support the safe integration of VMAT into clinical practice once it becomes commercially available for the MR-linac. Material/Methods: The measurements were done on the Unity MR-linac (Elekta, Sweden) system with VMAT enabled in research mode. Beam profiles were continuously acquired using the portable imager, while the gantry was rotating at 7.0˚/s, 10.6 ˚/s, and 21.3˚/s. A global gamma analysis was performed on the data with a 3%/3mm criterion across all gantry angles to evaluate the consistency of beam delivery under dynamic conditions. A modified strip test3 was developed to measure the MLC performance under dynamic gantry and MLC conditions. The MLC performance was measured under static gantry conditions at gantry 0˚, with either static or dynamic leaf conditions, and this was compared to dynamic gantry conditions with dynamic leaf conditions. Metrics evaluated included the Absolute and Relative Positioning Error (APE and RPE), gain, and the Y1/Y2 leaf bank offsets. Results: As shown in Figure 1, the beam profile remained stable under dynamic conditions. The gamma passing rate remained above 95% across nearly all angles, with no gantry speed-dependent effect, demonstrating the robustness of beam delivery across different rotational speeds. Strip tests confirmed precise MLC behaviour under rotation: the APE remained well below ±0.5 mm, the RPE matched the intended leaf gap of 1 mm, and the Y1/Y2 offsets were centered around 0 mm and stayed within the ±0.5 mm tolerance (Figure 2). No differences were observed between static and dynamic gantry delivery, indicating that rotational motion does not influence MLC behaviour.
Conclusion: This work provides the first proposed commissioning tests to facilitate the introduction of VMAT on the 1.5T MR-linac when it becomes commercially available. The highlighted developed tests demonstrate that both the beam delivery and the MLC performance remain stable under rotation, ensuring stable behaviour under dynamic gantry conditions. References: 1.Snyder JE, Fast MF, Uijtewaal P, et al. Enhancing delivery efficiency on the magnetic resonance-linac: a comprehensive evaluation of prostate stereotactic body radiation therapy using volumetric modulated arc therapy. Int J Radiat Oncol Biol Phys. 2025;122(4):976–985. 2.Uijtewaal P, Borman PTS, Woodhead PL, et al. First experimental demonstration of VMAT combined with MLC tracking for single and multi-fraction lung SBRT on an MR-linac. Radiother Oncol. 2022;174:149–157.3.Sastre-Padro R, Azario- Lopez M, Reig J, et al. Characterization of a novel 2D array dosimeter for verification of treatments in radiotherapy. Phys Med Biol. 2008;53(17):4565–4576. Keywords: MR linac, VMAT, commissioning Digital Poster Highlight 206 Methodology development for a multicentre audit of small field output factors Diana L. Silva 1,2 , Catarina Souto 1,2 , José Mesquita 1,2 , Joana Lencart 1,2 , Anabela G. Dias 1,2 1 Medical Physics Department, Portuguese Oncology Institute of Porto (IPO-Porto), Porto, Portugal. 2 Medical
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