MR-guided Gated Stereotactic Radiation Therapy Delivery for Lung, Adrenal, and Pancreatic Tumors: A Geometric Analysis

We implemented magnetic resonance–guided breath-hold stereotactic body radiation therapy in combination with visual feedback using the MRIdian system. Both accuracy of gated delivery and reproducibility of tumor positions were studied. Tumor tracking is realized through repeated magnetic resonance imaging in a single sagittal plane at 4 frames per second with deformable image registration. An in-room monitor allowed visualization of the tracked gross tumor volume (GTV) contour and the planning target volume (PTV) (GTV + 3 mm), which was the gating boundary. For each delivery, a predefined threshold-region of interest percentage (ROI%) allows a percentage of GTV area to be outside the gating boundary before a beam-hold is triggered. Accuracy of gated delivery and tumor position reproducibility during breath-holds was analyzed for 15 patients (87 fractions) with lung, adrenal, and pancreas tumors. For each fraction, we analyzed (1) reproducibility of system-tracked GTV centroid position within the PTV; (2) geometric coverage of GTV area within the PTV; (3) treatment duty cycle efficiency; (4) effects of threshold ROI% settings on treatment duty cycle efficiency and GTV area coverage; and (5) beam-off latency effect on mean GTV coverage. For lung, adrenal, and pancreatic tumors, grouped 5th to 95th percentile distributions of GTV centroid positions in the dorsoventral direction, relative to PTV-center of mass (COM), were, respectively, −3.3 mm to 2.8 mm, −2.5 mm to 3.7 mm, and −4.4 mm to 2.9 mm. Corresponding distributions in the craniocaudal direction were −2.6 mm to 4.6 mm, −4.1 mm to 4.4 mm, and −4.4 mm to 4.5 mm, respectively. Mean GTV areas encompassed during beam-on for all fractions were 94.6%, 94.3%, and 95.3% for lung, adrenal, and pancreas tumors, respectively. Mean treatment duty cycle efficiency ranged from 67% to 87% for these tumors. Use of higher threshold-ROI% resulted in increased duty cycle efficiency, at the cost of a small decrease in GTV area coverage. The beam-off latency had a marginal impact on the GTV coverage. Gated stereotactic body radiation therapy delivery during breath-hold, real-time magnetic resonance guidance resulted in at least 95% geometric GTV coverage in lung, adrenal, and pancreatic tumors.