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 combination with IMRT. Here, we provide a first experimental demonstration 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 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.