(Saturday, 3/30/2019)
Room: Exhibit Hall
Purpose: MRI guidance provides superior soft-tissue contrast, making it increasingly appealing for radiation therapy requiring high localization accuracy. Enhanced visualization of tissues (e.g. CSF and spinal cord) during patient positioning enables more exact targeting and dose escalation. This work investigates treatment plan quality and deliverability of spine Stereotactic Radiosurgery (SRS) cases planned for delivery on an MR-Linac.
Methods: In this retrospective study, six spine-SRS patients were re-planned on a commercial Monte Carlo-based MR-Linac TPS. The population included C/T/L vertebral bodies with varying degrees of epidural involvement. The treatment technique utilized 9-10 posterior step-and-shoot IMRT beams. Treatment plans were optimized to achieve coverage, constraints, and quality criteria dictated by RTOG-0631. Three cases, with varying degrees of modulation, were delivered on a solid water phantom. Deliverability and dosimetric accuracy were assessed via ion chamber and absolute film dosimetry patient-specific quality assurance (PSQA) measurements.
Results: The MC-based TPS provided IMRT plans that met all critical-tissue constraints outlined in RTOG-0631, while easily covering 90% of the target with the 18-Gy single-fraction prescription dose. Critical tissues encountered in the sampled population include the spinal cord, pharynx, esophagus, great vessels, stomach, kidneys, lungs, and liver. In all cases, the spinal cord was the most limiting organ, where the max dose encountered was kept below 10.55-Gy. Plan quality metrics controlling hot spots and high-dose spillage (e.g. <115% permitted outside the target) were achieved but provided the greatest planning challenge due to the nature of static beams. Efforts were made to minimize plan complexity (~7-segments/beam) and delivery time (~24-minutes). PSQA measurements proved accurate deliverability (average ion chamber agreement -2.34% and 3%/1mm global gamma analysis 97.11%) and were within institutional tolerances.
Conclusion: This study demonstrated the feasibility of planning RTOG-0631 compliant spine-SRS treatments via an MC-based TPS for delivery on an MR-Linac. PSQA measurements validated deliverability and dosimetric accuracy.
Funding Support, Disclosures, and Conflict of Interest: The submitting institution holds research agreements with Philips Healthcare and ViewRay, Inc.
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