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Impact of MLC Model Parameters On Head and Neck Plan Quality for a Novel 6 MV Flattening Filter-Free (FFF) Straight-Through Linac with Jawless Prototype MLC Collimation

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R Scheuermann

R Scheuermann*, C Kennedy , S Anamalayil , L Brady , D Mihailidis , L Dong , J Metz , University of Pennsylvania, Philadelphia, PA


TH-CD-708-3 (Thursday, August 3, 2017) 10:00 AM - 12:00 PM Room: 708

Purpose: To compare the dosimetric quality of plans produced for a prototype 6MV flattening-filter free (6FFF), jawless straight-through linac with novel MLC design with that achieved by a clinical 6MV flattened (6X) Varian TrueBeam® (TB) with Millennium MLC. Furthermore, to understand the impact of differences in MLC leaf speed (2.5cm/s vs. 5cm/s), transmission (0.015 vs. 0.007), and dosimetric-leaf gap (0.15cm vs. 0.01cm) on resulting plan quality.

Methods: Using a test version of Eclipse v15 (Varian), three definitive H&N patients were planned with 9 equally-spaced IMRT fields using a clinical TB6X model to meet clinical acceptance criteria. Final optimization objectives were saved as a template. A copy of the TB6X plan was reoptimized using the optimization objectives with 200 iterations of the PO optimizer. The same were applied for a plan using the prototype model, a TB6FFF plan with clinical MLC parameters, a TB6FFF plan with prototype MLC parameters, and TB6FFF plans with a single MLC parameter set to the prototype value. All resulting plans were normalized such that D95=100% for the PTV70Gy. Resulting mean doses to selected OARs were evaluated.

Results: The prototype system produced lower average doses to all OARs considered. The average OAR mean dose reduction was 3.5Gy for the prototype system. Applying the prototype MLC parameters to the TB model produced an average OAR mean dose reduction of 2.8Gy. Of the MLC parameters considered individually, the leaf transmission resulted in the greatest reduction in OAR doses with an average reduction of 1.9Gy. The largest average dose reduction for an OAR was 6.0Gy to the larynx.

Conclusion: The prototype system produced lower mean OAR doses compared to the clinical TB system. The majority of this effect is due to the novel MLC design. The single MLC parameter with greatest impact on the OAR doses is the leaf transmission.

Funding Support, Disclosures, and Conflict of Interest: This work was performed as part of a research agreement with Varian Medical Systems.

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