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An Investigation of KV CBCT Image Quality and Dose Reduction for Volume-Of-Interest Imaging Using Dynamic Collimation

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D Parsons

D Parsons1*, J Robar1,2 , (1)Department of Physics and Atmospheric Science, Dalhousie University, Halifax, NS,(2) Capital District Health Authority, Halifax, NS

Presentations

TH-A-18C-11 Thursday 7:30AM - 9:30AM Room: 18C

Purpose:The focus of this work was to investigate the improvements in image quality and dose reduction for volume-of-interest (VOI) kV-CBCT using dynamic collimation.

Methods:A prototype iris aperture was used to track a VOI during a CBCT acquisition. The current aperture design is capable of one-dimensional translation as a function of gantry angle and dynamic adjustment of the iris radius. The aperture occupies the location of the bow-tie filter on a Varian OBI system. CBCT and planar image quality was investigated as a function of aperture radius, while maintaining the same dose to the VOI, for a 20 cm diameter cylindrical water phantom with a 9 mm diameter bone insert centered on isocenter. Corresponding scatter-to-primary ratios (SPR) were determined at the detector plane with Monte Carlo simulation using EGSnrc. Dose distributions for various anatomical sites were modeled using a dynamic BEAMnrc library and DOSXYZnrc. The resulting VOI dose distributions were compared to full-field distributions.

Results:SPR was reduced by a factor of 8.4 when decreasing iris diameter from 21.2 cm to 2.4 cm (at isocenter). Similarly, this change in iris diameter corresponds to a factor increase of approximately 1.4 and 1.5 in image contrast for CBCT and planar images, respectively, and similarly a factor decrease in image noise of approximately 1.7 and 1.5. This results in a measured gain in contrast-to-noise ratio of a factor of approximately 2.3 for both CBCT and planar images. Depending upon the anatomical site, dose was reduced to 10%-70% of the full field value along the central axis plane and down to 2% along the axial planes, while maintaining the same dose to the VOI compared to full-field techniques.

Conclusion:The presented VOI technique offers improved image quality for image-guided radiotherapy while sparing the surrounding volume of unnecessary dose compared to full-field techniques.


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