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Current Modulated Volume-Of-Interest Imaging for Kilovoltage Intrafraction Monitoring of the Prostate

D Parsons

D Parsons1*, J Robar1,2 , M T R Stevens3 , (1) Dalhousie University, Halifax, NS (2) Nova Scotia Health Authority, Halifax, NS, (3) University of Chicago, Chicago, IL


TH-AB-205-4 (Thursday, August 3, 2017) 7:30 AM - 9:30 AM Room: 205

Purpose: This work aims to improve the available kV image quality for continuous intrafraction monitoring during VMAT. This is investigated using a novel dynamic blade collimation system enabling tube current modulated (CM) volume-of-interest (VOI) imaging of prostate fiducial markers during radiotherapy.

Methods: Fiducial markers embedded in a dynamic pelvis phantom during gantry rotation were imaged using a VOI. For each fiducial, a VOI margin around each marker was set to be 2σ of the population covariance matrix characterizing prostate motion. This was used to conform to a single or several fiducials and compared to a static field. kV attenuation was calculated for each VOI as a function of angle and used to optimize tube current during acquisition. Image quality was assessed with regard to CNR, fiducial detectability and imaging dose.

Results: Fiducials can be accurately located using a VOI containing a single or several fiducials using a relatively high constant kV output. The imaging dose can be mitigated through tailoring the imaging field to a single or several fiducials, in which the integral dose is reduced by a factor of 15.6 and 3.7, respectively, compared to a 6x6cm2 static field. CM can be used to overcome the variability in image quality throughout the rotation and therefore improve fiducial CNR and detectability during periods of high kV attenuation.

Conclusion: The combination of VOI and CM introduces an advantageous approach in intrafraction monitoring of the prostate during radiotherapy by both reducing and localizing the imaging dose, while improving image quality and fiducial detectability during periods of high kV attenuation.

Funding Support, Disclosures, and Conflict of Interest: This work was support by a research grant with Varian Medical Systems, the Canadian Institutes of Health Research and the Canadian Nuclear Safety Commission.

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