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Use of a Dynamic KV X-Ray Collimator for Reduced-Dose Fluoroscopic Fiducial Tracking

A Belcher

E Pearson, A Belcher*, Z Grelewicz, R Wiersma, C Pelizzari, Univ Chicago, Chicago, IL

WE-G-141-4 Wednesday 4:30PM - 6:00PM Room: 141

Purpose: Intra-fraction organ motion can be problematic for highly conformal radiation therapy techniques. Fluoroscopy with the on-board imager available on many linear accelerators can provide real time tumor position information however potentially high radiation dose can be a concern. The target may only account for a few percent of the image area, thus using a dynamic collimation device to restrict the field of view to an appropriate region-of-interest and follow its motions the imaging dose can be substantially reduced.

Methods: A prototype collimator has been developed which can be mounted to the kV x-ray source of a Varian Trilogy linear accelerator in place of the standard bowtie filter. The collimator tracking was tested using realistic motion profiles derived from patient data and a 3-axis motion stage with a CT spot as the target fiducial. The clinical system does not currently provide real time access to the image data so the collimator was fed the fiducial position from the motion control system with controllable lag to simulate image-processing time.

Results: In the acquired fluoroscopic data the collimator blade edges were identified using a hough line transform on the thresholded gradient magnitude image and the fiducial position was identified with a template matching technique. Over four minutes of continuous tracking with intermittent burst of fluoroscopic imaging the fiducial was never more than 1.7mm from the aperture center and the root mean square distance was 0.8mm.

Conclusion: The dynamic collimator exhibits accuracy that could enable highly conformal fluoroscopic imaging to provide real-time target position information with greatly reduced imaging dose.

Funding Support, Disclosures, and Conflict of Interest: This work was funded, in part, by Varian Medical Systems, Palo Alto CA. The contents of this work are solely the responsibility of the authors and do not necessarily represent the official views of any supporting organizations.

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