First experimental demonstration of VMAT combined with MLC tracking for single and multi fraction lung SBRT on an MR-linac

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 combi-nation with IMRT. Here, we provide a first experimental demonstration of VMAT + MLC tracking for sev-eral lung SBRT indications. Materials and methods: We created central patient and phantom VMAT plans (8x7.5 Gy, 2 arcs) and we created peripheral phantom plans (3x18 & 1x34 Gy, 4 arcs). A motion phantom mimicked subject -recorded respiratory motion ((A)overbar=11 mm, (f)overbar =0.33 Hz, (drift)overbar=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 mea-surement 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 D-98% by 5- 20% and the PTV D(95% )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. (C) 2022 The Authors. Published by Elsevier B.V.

Automated summary

This study provides an experimental demonstration of using VMAT + MLC tracking technology on MR-linacs for several lung SBRT indications. The results show that this technique is technically feasible and results in highly conformal dose distribution, potentially becoming an effective tool for lung cancer treatment.