Exhibit Hall | Forum 8
Purpose: The superior soft-tissue contrast of MRI-guided radiotherapy offers enhanced localization accuracy of the spinal cord in spine Stereotactic Body Radiotherapy (SBRT). This work includes a treatment plan quality and deliverability study for spine-SBRT on an MR-Linac. Additionally, an online adaptive radiation therapy (ART) workflow was used to clinically treat a spine metastasis, we report our initial experience of targeting accuracy, image-guided localization, and treatment.
Methods: Six spine-SBRT patients were retrospectively re-planned to 18 Gy in 1-fraction on a commercial, Monte Carlo-based MR-Linac treatment planning system. Plans were generated using 9-13 step-and-shoot intensity-modulated radiation therapy 6MV-flattening-filter-free beams and optimized to plan quality metrics recommended by RTOG-0631. One thoracic vertebral body was clinically treated to 27 Gy in 3-fractions, where daily anatomical changes were accounted for via re-planning and treatment utilizing an ART workflow. Deliverability and dosimetric accuracy were evaluated via ion-chamber and film patient specific quality assurance measurements.
Results: Plans met all critical-tissue constraints outlined in RTOG-0631 and AAPM Task Group-101 (spinal cord maximum dose <10.6 Gy in 1-fraction and <21.9 Gy in 3-fractions), while covering 90% of the target with the prescription dose. Plan quality metrics controlling hot spots and high-dose spillage were achieved but provided the greatest challenge. Clinically, the inter-fraction visibility of the cord allowed for patient setup focusing on cord-alignment, while re-contouring of the target ensured sufficient coverage. Real-time tracking and gating on sagittal MR-cine images provided sufficient quality for intra-fraction tracking of the spinal canal utilizing a 3.0 mm gating boundary and 1-2% region of excursion allowance. Average ion-chamber and film agreement were -3.01±1.70% and 96.55±2.44%, utilizing a 3%1mm gamma-analysis criterion.
Conclusion: Achieved plan quality and deliverability was within acceptable standards. MR-guidance with an on-line ART workflow offered increased accuracy in localization of the spinal cord to enhance tissue sparing and target volume coverage.
Funding Support, Disclosures, and Conflict of Interest: The submitting institution holds research agreements with ViewRay Inc.
Stereotactic Radiosurgery, MRI, Image-guided Therapy