(Saturday, 4/4/2020) [Mountain Time (GMT-6)]
Purpose: Diagnostic MRI scans are often fused with primary planning kVCT scans to delineate target and organs at risk (OAR). Up until now MRI is not used to generate clinical radiation treatment plans, mainly due to its inability to provide electron density data. Recently developed MRI-guided radiotherapy systems allow for real-time visualization and tracking of target and region of interest (ROI). In this study, we show workflow of ViewRay adaptive plans generated for gastric lymphoma.
Methods: Maximum inhale breath hold (MIBH) scans were acquired on ViewRay system as well as on Siemens Somatom CT scanner. On MRIdian linac system, a cine was also acquired for 120 seconds. Planning target volume (PTV) was generated by expanding clinical target volume (CTV). Finally, all images and contours were exported to ViewRay MRIdian linac treatment planning system (TPS). Simulation CT scan was registered with MR planning scan to derive electron density for planning. A 16-beam step and shoot treatment plan was generated using TPS. Treatment plan was optimized so that 95% of the PTV received 95% of the prescribed dose.
Results: All planning objectives were satisfied by the generated treatment plan. IMRT QA passed the clinically set gamma criteria of 3%±3 mm. On the day of treatment, patient setup scan was registered to planning MR. Necessary edits were done for tumor to accommodate the CTV shrinkage and OARs while patient was on treatment couch. Superior target coverage and sparing of heart and left kidney was obtained by reoptimized plan.
Conclusion: In order to account for anatomical changes during radiotherapy, necessary setup modifications as well as dose recalculations are done. By incorporating onboard MR scanning in daily setup, we can eliminate the CT based registration and hence can avoid imaging related extra dose to patient for adaptive radiotherapy treatments.
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