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Coded Aperture Scatter Imaging for Real-Time, Markerless Tracking of Lung Tumors

A Mahl*, B Miller, C Altunbas, B Kavanagh, M Miften, B Jones, University of Colorado Anschutz Medical Campus, Aurora, CO


(Wednesday, 7/15/2020) 11:30 AM - 12:30 PM [Eastern Time (GMT-4)]

Room: Track 2

Purpose: Coded Aperture Scatter Imaging (CASI) is a novel technique that allows for real-time imaging and markerless tracking of tumor motion during treatment and requires no extra dose to the patient. The purpose of this study was to demonstrate the potential for tracking lung tumors with CASI using only scattered radiation from the treatment beam.

Methods: A radiotherapy beam incident on voxelized reference phantoms containing lung tumors within detailed normal structure anatomy was modeled using the MCNP6.2 code. Orthogonal scatter radiographs were simulated through a CASI geometry. These generated radiograph data which were decoded, and the signals originating from the tumor were analyzed. The effects of coded aperture properties (rank and physical dimensions), tumor position and motion, phantom anatomy, and the incident beam (spectrum and leaf pattern) were investigated. These variables were evaluated by quantifying positional accuracy, resolution (FWHM), and SNR of the tumors signal within the scattered background noise. A 10x10x2cm tungsten mask that attenuates 99% of incident scatter transmission across common energy spectra was evaluated for impact on SNR.

Results: Using the CASI method, craniocaudal motion could accurately be tracked across a 4cm range without adjusting the relative aperture position, localizing the center of a 1.6cm tumor’s position to within one pixel (~1.5mm). The FWHM (4.1±0.4) and SNR (24.7±2.0) of the signal were consistent along the tracked range. The tungsten mask minimized artifacts and optimized SNR of signals within the decoded radiograph.

Conclusion: CASI is a novel markerless imaging technique for extracting the tumor’s scatter signal from a broader Compton-scatter background. Results show that the CASI method can be used to visualize and track lung tumors in real-time without adding imaging dose. Real-time, markerless tracking of lung tumor motion during radiotherapy treatment might enable more accurate dose delivery while avoiding the risk of invasive fiducial marker placement.

Funding Support, Disclosures, and Conflict of Interest: Funding in part from the National Institutes of Health under award number K12CA086913, the University of Colorado Cancer Center/ACS IRG #57-001-53 from the American Cancer Society, the Boettcher Foundation, and Varian Medical Systems. Jones/Miften report grants from Varian Medical Systems during the conduct of study, outside of the submitted work.


MCNP, Lung, Image Guidance


IM- X-Ray: Development (new technology and techniques)

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