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Performance of a Multi Leaf Collimator System for MR-Guided Radiation Therapy

B Cai

B Cai*, H Li , D Yang , V Rodriguez , A Curcuru , Y Wang , J Wen , R Kashani , S Mutic , O Green , Washington University School of Medicine, St. Louis, MO


MO-F-FS1-7 (Monday, July 31, 2017) 4:30 PM - 6:00 PM Room: Four Seasons 1

Purpose: To characterize the performance of a Multi Leaf Collimator (MLC) designed for Cobalt-60 based MR-guided radiation therapy (MRgRT) system in a 0.35T magnetic field.

Methods: The RF cage shielding of MLC motor and cables were first evaluated using ACR phantoms with real time imaging and quantified by signal-to-noise ratio. The dosimetric characterizations, including the leaf transmission, leaf penumbra, tongue and groove effect, were then investigated using radiosensitive films. The output factor were measured with ionization chambers for both symmetric fields from 2.1 ×2.1 cm² to 27.3 × 27.3 cm² and asymmetric fields from 10.5 × 10.5 cm² to 10.5 × 2.0 cm². MLC positional accuracy was assessed by delivering a picket-fence (PF) style pattern on radiochromic films with wire-jig phantom. MLC reproducibility and reliability were tested by repeatedly delivering both open fields and fields with irregular shaped segments over one-month period.

Results: Comparable SNRs within 4% were observed for MLC moving and stationary plans. The maximum leaf transmission was less than 0.35%. Both the leaf edge and leaf end penumbra showed comparable values within 0.05 cm, and the measured values are within 0.1 cm with TPS values. The leaf-edge TG effect indicated a 10% underdose and the leaf end TG showed a shifted dose distribution with 0.6 cm offset. The leaf positioning test showed a 0.2 cm accuracy in the PF style test. The measured and TPS calculated output factors were within 2% for squared fields and within 3% for rectangular fields. The reproducibility test showed the deviation of output factors were well within 2% for square fields and the gamma passing rate within 1.5% for fields with irregular segments.

Conclusion: The novel MLC equipped in this MRgRT system has a good RF noise shielding design, low radiation leakage, good positioning accuracy, comparable TG effect and great reproducibility.

Funding Support, Disclosures, and Conflict of Interest: Dr. Mutic reports grants and personal fees from ViewRay, Inc, grants and other from Varian Medical Systems, other from TreatSafely, LLC, other from Radialogica, LLC. Dr. Green receives honoraria and travel expenses from ViewRay. Dr. Yang received research funding and licensing fee from Viewray.

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