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Optimizing Fiducial Visibility On Periodically Acquired Megavoltage and Kilovoltage Image Pairs During Volumetric Modulated Arc Therapy

P Zhang

P Zhang*, P Wang , H Laura , B Ravindranath , G Mageras , M Hunt , Mem Sloan-Kettering Cancer Ctr, New York, NY


SU-C-BRB-7 (Sunday, July 12, 2015) 1:00 PM - 1:55 PM Room: Ballroom B

Purpose: Robust detection of implanted fiducials is essential for monitoring intra-fractional motion during hypo-fractionated treatment. We developed a plan optimization strategy to ensure clear visibility of implanted fiducials, and facilitate 3D localization during volumetric modulated arc therapy (VMAT).

Method: Periodic kilovoltage (kV) images were acquired at 20° gantry intervals, and paired with simultaneously acquired 4.4° short arc megavoltage digital tomosynthesis (MV-DTS) to localize three fiducials during VMAT delivery for prostate cancer. Beginning with the original optimized plan, control point segments where fiducials (with 1cm margin to account for potential motion) were consistently blocked by MLC within each 4.4° MV-DTS interval were first identified. For each segment, MLC apertures were edited to expose the fiducial that led to the least increase in a dosimetric cost function. Subsequently, MLC apertures of all control points not involved with fiducial visualization were re-optimized to compensate for plan quality losses and match the original DVH. MV dose for each MV-DTS was also kept above 0.4 MU to ensure acceptable image quality. Different imaging (gantry) intervals and visibility margins around fiducials were also evaluated.

Result: Fiducials were consistently blocked by the MLC for, on average, 38% of the imaging control points for five hypofractionated prostate VMAT plans but properly exposed after re-optimization. Optimization resulted in negligible dosimetric differences compared with original plans, and outperformed simple aperture editing: on average, PTV D98 recovered from 87% to 94% of prescription, and PTV dose homogeneity improved from 9% to 7%. Without violating plan objectives and compromising delivery efficiency, the highest imaging frequency and largest margin that can be achieved are a 10° gantry interval, and 15mm, respectively.

Conclusion: VMAT plans can be made to accommodate MV-kV imaging of fiducials. Fiducial visualization rate and workflow efficiency are significantly improved with an automatic modification and re-optimization approach.

Funding Support, Disclosures, and Conflict of Interest: Research supported by Varian Medical System

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