Purpose: Real time visualization of fiducial markers and their movement during radiation therapy treatment may allow for more accurate dose delivery. The purpose of this study was to optimize techniques for online fiducial marker tracking by detecting the Compton-scattered treatment beam through a coded aperture (CA). CA imaging is a novel technique that may allow real-time fiducial localization during treatment.
Methods: MCNP6.2 was used to model a radiotherapy beam through phantoms containing gold fiducial markers. Orthogonal scatter radiographs were collected through a CA geometry. After decoding the simulated radiograph data, the centroid location and FWHM of the fiducial signals were analyzed. We investigated the effects of properties related to the CA (rank, pattern, and physical dimensions), detector (dimensions and pixel count), position (CA and phantom), and the incident beam (spectrum and direction). These variables were evaluated by quantifying the positional accuracy, resolution, and SNR of the fiducialsâ€™ signal. We also sought to reduce the effects of phantom scatter and decoding artifacts.
Results: The method was able to accurately localize the markers to within 1 pixel. A 5x5x2cm tungsten mask was chosen to attenuate 99% of incident scatter through opaque elements, while minimizing collimation artifacts which arise from vignetting of the coded radiograph. Adjusting mask pattern allows for resolution enhancement. Increasing the rank decreased the aperture element size from 1.52x to 0.53x the detectorâ€™s pixel. This corresponded to a reduction in FWHM of the fiducialâ€™s signal by 50.9% and 9.2% in the 2 planar directions. Clear separation of centroids from fiducial signals with 2.5mm separation was maintained.
Conclusion: Current results show a proof of concept for a novel real-time imaging method. CAI is a promising technique for extracting the fiducial scatter signal from a broader Compton-scatter background. These results can be used to guide fabrication and testing of a clinical device.
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.