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Session: Early-Career Investigator Symposium [Return to Session]

Target Visibility On Planar Radiographs On the Radixact Synchrony System

W Ferris*, L DeWerd, W Culberson, University of Wisconsin-Madison, Madison, WI


(Saturday, 3/26/2022) 10:30 AM - 12:30 PM [Central Time (GMT-5)]

Room: Celestin D-E

Purpose: The Synchrony motion tracking system on the Radixact uses planar radiographs to localize the target and build a respiratory model to be used to compensate the treatment delivery in real time. The purpose of this work is to investigate target visibility as a function of patient thickness, imaging protocol, and Radixact configuration (there are two configurations with different source-to-panel distances (SPDs)).

Methods: Measurements of signal-to-noise ratio (SNR) and fiducial detection accuracy were performed using gold fiducials inside a water-equivalent phantom of varying thickness. Radiographs were acquired with the imaging axis perpendicular to the face of the phantom using various imaging protocols.

Results: Synchrony accurately located the fiducials within 0.7 mm on the radiograph when the SNR on the image was larger than 5. Fiducial size and orientation with respect to the imaging axis had minor effects on SNR. No fiducials were detected when the water-equivalent material was 50 cm thick, even for the highest output imaging protocol. Blurring is expected to be small since all imaging protocols have a tube on time of 10 ms and therefore a target moving at 20 mm/s would only move 0.2 mm during the image.

Conclusion: A higher mAs protocol should be added for use with thicker patients, as missing fiducials can lead to inaccurate target tracking and steering of the therapeutic beam or an inability to treat the patient. Similar SNR was achieved on the two different Radixact configurations despite the differences in SPDs. It is expected that higher output protocols may be required for fiducial free tracking targets, which will be investigated in future work.


Tomotherapy, Image-guided Therapy, Target Localization


TH- External Beam- Photons: Motion management - intrafraction

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