Purpose: Scattered treatment photons contain information about patient anatomy, yet the extremely low SNR makes scatter imaging challenging. A coded aperture is a device that encodes spatial information into the scatter image to boost the SNR and enable tracking of tumor motion without adding imaging dose to the patient. The purpose of this study was to explore the feasibility of Coded Aperture Scatter Imaging (CASI) for markerless tracking of lung tumors.
Methods: 4DCT data of 10 lung cancer patients were imported into an MCNP6.2 radiotherapy model that included an MV treatment beam and an orthogonal kV on-board imager (kV-OBI). A MURA Coded Aperture was modeled between the patient and kV-OBI, and simulated scatter images were generated for different respiratory phases. These data were decoded, and the signals originating from the patients’ tumors were analyzed. The tumor position was measured throughout the breathing cycle by a template matching algorithm and compared to the reference position.
Results: Using the CASI method, tumor motion could be detected and tracked within the ITV throughout the patients’ respiratory cycle, without adjusting the relative aperture position. Calculated deviations of the template correlation for large, mobile tumor positions were quantified with RMSE values in a range of [0.188-0.302] indicating the capability for accurate identification. Other anatomical features such as the diaphragm, chest wall, and ribs were visualized, along with their motion within the visible field edges.
Conclusion: CASI is a novel markerless imaging technique which allows for visualizing tumor location and motion during radiation treatment. Results show that the CASI method is effective in visualizing a variety of tumor sizes’ and different patient specific respiratory motion. Real-time, markerless tracking of lung tumor motion using scattered treatment photons during radiotherapy delivery 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.