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Feasibility of Adaptive MR-Guided SBRT of Lung Cancers

N Dogan

N Dogan*, K Padgett , G Simpson , A Ishkanian , University of Miami School of Medicine, Miami, FL


TU-D-FS4-7 (Tuesday, August 1, 2017) 11:00 AM - 12:15 PM Room: Four Seasons 4

Purpose: Online Adaptive Radiotherapy (ART) with frequent imaging has the potential to improve dosimetric accuracy by taking into account for anatomical and functional changes during the course of radiotherapy. The goal of this work to provide an assessment of online adaptive MRI-Guided radiotherapy (MRI-RT) for lung Stereotactic Body Radiotherapy (SBRT).

Methods: Lung SBRT patients who were treated with MRI-RT system were included. For each patient, MR images were acquired for planning and prior to each treatment fraction. Prescription dose was 50 Gy/5 fx, with 95% PTV covered by 100% of the prescription dose. First, GTV, PTV, cord, lungs, heart, and esophagus were delineated on the planning MRI. The treatment plan was generated using step-and-shoot IMRT and utilized Monte Carlo calculation including the magnetic field correction of 0.35T. Second, the contours from planning MRI was propagated to fraction MRIs. The deformed contours on fraction MRIs were reviewed and modified by the physician. Two ART strategies were explored: 1) Calculating the plan from the planning MR on each fraction MR images (NoAdapt) and 2) Creating a new optimized IMRT plan on fraction MR image (FxAdapt). The treatment plans both strategies were compared using the clinical dose volume constraints.

Results: PTV Dmax constraint (57.5 Gy) was not satisfied for all fraction plans with NoAdapt strategy as compared to FxAdapt strategy. On average, 8% reduction in (Lungs-PTV) volume receiving 12.5 Gy and 13.5 Gy respectively was achieved with FxAdapt. Furthermore, 14% reduction in Chest Wall volume receiving 40 Gy (40 Gy <30cc) was achieved with FxAdapt. The heart and esophagus Dmax were very similar with both NoAdapt and FxAdapt strategies.

Conclusion: Online adaptive MR-Guided lung SBRT provides better target conformality and homogenity and critical structure sparing compared with non-adaptive SBRT. Further studies are needed to evaluate benefits of daily online ART during MR-Guided Radiotherapy.

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