K Snyder*, J Kim, J Dolan, Y Huang, A Doemer, N Wen, I Chetty, P Parikh, M Shah, S Siddiqui, Henry Ford Health System, Detroit, MI

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  K Snyder

PO-GePV-M-181 (Sunday, 7/25/2021)   [Eastern Time (GMT-4)]

Purpose: To investigate treatment plan quality for intracranial post-operative tumor beds treated with fractionated stereotactic radiotherapy (FSRT) for delivery on a Magnetic Resonance Image Guided Linear Accelerator (MR-Linac).

Methods: Ten intracranial FSRT patients were replanned on a Monte Carlo-based commercial treatment planning system (TPS), MRIdian v5.4.0.97 (ViewRay, Mountain View CA). The MR-linac system combines a 0.345T magnet and a 6MV flattening filter free (FFF) treatment beam. Plans were generated using a step-and-shoot IMRT technique, generated with a double-stacked MLC with an equivalent leaf width of 4.15mm. Eleven beams were utilized, spaced 20-35 degrees apart. Target volumes were prescribed to 24-30Gy in 3 fractions (8-10Gy per fraction) and normalized so that 95% of the target volume received prescription dose. Plans were optimized to achieve AAPM TG 101 organ at risk tolerances and sharp dose fall off. Dose fall off parameters were evaluated including conformality index (CI), gradient index (GI), volume of Normal Brain (Brain-Target volume) receiving 12Gy (NormBrainV12Gy), 18Gy (NormBrainV18Gy), and 24Gy (NormBrainV24Gy) as well as maximum dose to the target volume (MaxTargetD0.03cc). Deliverability of five of the ten cases was verified with ion chamber and film measurements.

Results: The average target volume was 26.20cc (range: 10.57cc-59.36cc), with several planned with a simultaneous integrated boost (SIB). The average MaxTargetD0.03cc normalized to the SIB dose was 120% (range: 112%-126%). The corresponding CI and GI were 1.15 and 3.69, respectively. Average volumes to NormBrainV12Gy, NormBrainV18Gy, NormBrainV24Gy were 65.87cc, 22.09cc, and 5.38cc, respectively. Average delivery time was 12.2 minutes. Average ion chamber point dose difference from planned values was -1.92% and average gamma pass rate utilizing 3%1mm criteria was 97.94%.

Conclusion: This study demonstrates the feasibility of planning intracranial FSRT on an MR-Linac. Delivery was verified and dose distributions were deemed clinically acceptable and accomplished without sacrificing maximum target dose.

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