MA13.10 Magnetic Resonance Imaging-Guided Delivery of Lung Stereotactic Radiotherapy Using Patient-Controlled Visual Guidance

Abstract

Treatment-related toxicity is more common following stereotactic ablative radiotherapy (SABR) for central lung tumors, than is the case for peripheral tumors [Tekatli 2015]. Further reductions in doses to critical central structures are possible using respiration-gated SABR delivery, but insertion of fiducial markers for gating is also associated with toxicity. We describe a novel approach for clinical delivery of breath-hold gated SABR under continuous MRI-guidance. The MRIdian® system permits tumor visualization at 4 frames/second during treatment delivery, with radiation beam-holds whenever the target is outside a prespecified gating window. The gating procedure is as follows: a 17 second inspiration breath-hold MR scan is performed for planning before each SABR fraction (resolution 1.6×1.6×3.0 mm). Image registration is performed, and contours adapted when necessary. A 3mm PTV margin is added, and planned dose distribution recalculated for the ‘anatomy of the day’, and reoptimized. A sagittal plane is chosen for tumor tracking and gating, with a planning target margin of 3 mm. The sagittal tracking view from the MRIdian console is projected on a MR-safe monitor (Cambridge Research), and patients can continuously observe the tracking image using a mirror inside the bore. Since May 2016, 30 fractions of MR-guided gated delivery have been performed in 5 cancer patients with 6 central tumors. All MR-based breath-hold PTV’s were smaller (mean 19.8 ± 13.3 cc) than a conventional free-breathing, motion-encompassing approach (mean 36.1 ± 21.9 cc). Plans of a single case are shown in Figure 1. Video-assisted visual feedback achieved a breath-hold gating efficiency of 52% (range 27-88%). For high-risk SABR cases, use of MR-guided, video-assisted breath-hold gated SABR delivery constitutes a novel treatment method, allowing for minimization of mobility- and setup margins, and for improved verification of SABR delivery. Data from additional patients undergoing treatment will be presented.