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Developing a MLC Modifier Program to Improve Fiducial Detection for MV/kV Imaging During Hypofractionated Prostate Volumetric Modulated Arc Therapy

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L Happersett

L Happersett*, P Wang , P Zhang , J Mechalakos , H Pham , G Li , M Zelefsky , G Mageras , M Hunt , Mem Sloan-Kettering Cancer Ctr, New York, NY


SU-J-CAMPUS-JT-4 (Sunday, July 30, 2017) 4:00 PM - 5:00 PM Room: Joint Imaging-Therapy Theater

Purpose: To develop an Eclipse plug-in (MLC-modifier) that modifies control points to expose fiducials otherwise obscured by the MLC during VMAT, thereby facilitating tracking using periodic MV/kV imaging during hypofractionated prostate radiotherapy.

Methods: 3D fiducial tracking was performed during TrueBeam VMAT by pairing short-arc (3°) MV digital tomosynthesis (DTS) images obtained every 20° to triggered kV images. To ensure visualization of one fiducial for each DTS image, MLC apertures were evaluated at appropriate angles for visibility. If necessary, angles subtended by control points were compressed and low-dose “imaging” control points were inserted to expose one fiducial with 1 cm margin. Dose delivered over DTS short-arc segment remained unchanged. Modified VMAT beams were imported into Eclipse for re-calculation and comparison to original plan. To assess MLC-modifier efficacy, images for 27 patients were retrospectively reviewed to determine rate of MV DTS registration failures. Failures were categorized as being due to a) DTS image quality, b) MLC blockage of all fiducials and c) unknown reasons. Statistical significance was evaluated using Fisher’s exact test.

Results: For 27 patients, MLC apertures had been manually modified without change to control point dose for 12 and modified with MLC-modifer and control point compression for 15. All plans met clincial dose criteria. MV registration failures due to poor image quality, MLC blockage, unknown reasons and overall failure were 8% versus 6.4% (p<0.05), 16.5% versus 0.1% (p<0.0001) 7.6% versus 2.4% (p<0.0001) and 32% versus 8.9% (p<0.0001) for plans manually and application modified, respectively.

Conclusion: Increasing fiducial visibility via an automated process comprised of angular compression of control points and insertion of additional “imaging” control points is feasible. Degradation of plan quality is minimal. Fiducial detection and registration success rates are significantly improved compared to manually edited apertures. MLC-modifier is now routinely used for simultaneous MV/kV tracking during prostate SBRT.

Funding Support, Disclosures, and Conflict of Interest: This research has been supported by Varian medical system.

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