First experimental demonstration of VMAT combined with MLC tracking for single and multi fraction lung SBRT on an MR-linac
Uijtewaal, Prescilla; Borman, Pim T.S.; Woodhead, Peter L.; Kontaxis, Charis; Hackett, Sara L.; Verhoeff, Joost; Raaymakers, Bas W.; Fast, Martin F.
(2022) Radiotherapy & Oncology, volume 174, pp. 149 - 157
(Article)
Abstract
Background and purpose: VMAT is not currently available on MR-linacs but could maximize plan conformality. To mitigate respiration without compromising delivery efficiency, MRI-guided MLC tumour tracking was recently developed for the 1.5 T Unity MR-linac (Elekta AB, Stockholm, Sweden) in combination with IMRT. Here, we provide a first experimental demonstration
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of VMAT + MLC tracking for several lung SBRT indications. Materials and methods: We created central patient and phantom VMAT plans (8×7.5 Gy, 2 arcs) and we created peripheral phantom plans (3×18 & 1×34 Gy, 4 arcs). A motion phantom mimicked subject-recorded respiratory motion (A‾=11 mm, f‾=0.33 Hz, drift‾=0.3 mm/min). This was monitored using 2D-cine MRI at 4 Hz to continuously realign the beam with the target. VMAT + MLC tracking performance was evaluated using 2D film dosimetry and a novel motion-encoded and time-resolved pseudo-3D dosimetry approach. Results: We found an MLC leaf and jaw end-to-end latency of 328.05(±3.78) ms and 317.33(±4.64) ms, which was mitigated by a predictor. The VMAT plans required maximum MLC speeds of 12.1 cm/s and MLC tracking superimposed an additional 1.48 cm/s. A local 2%/1 mm gamma analysis with a static measurement as reference, revealed pass-rates of 28–46% without MLC tracking and 88–100% with MLC tracking for the 2D film analysis. Similarly, the pseudo-3D gamma passing-rates increased from 22–77% to 92–100%. The dose area histograms showed that MLC tracking increased the GTV D98% by 5–20% and the PTV D95% by 7–24%, giving similar target coverage as their respective static reference. Conclusion: MRI-guided VMAT + MLC tracking is technically feasible on the MR-linac and results in highly conformal dose distribution.
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Keywords: Lung cancer, MLC tracking, MR-linac, Prediction, VMAT, Radiotherapy Dosage, Magnetic Resonance Imaging, Radiotherapy Planning, Computer-Assisted/methods, Radiotherapy, Intensity-Modulated/methods, Humans, Lung, Radiosurgery/methods, Hematology, Oncology, Radiology Nuclear Medicine and imaging, Journal Article
ISSN: 0167-8140
Publisher: Elsevier Ireland Ltd
Note: Funding Information: The authors acknowledge funding by the Dutch Research Council (NWO) through project No. 17515 (BREATHE EASY). MLC tracking research at UMC Utrecht is performed under a research agreement with Elekta AB. We thank Peter Münger and Görgen Nilsson (ScandiDos) for providing research software for the Delta 4 , and we thank Jochem Wolthaus for supporting the film dosimetry and Wilfred de Vries for assisting the Delta 4 measurements. Publisher Copyright: © 2022 The Authors
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