Purpose: Fast kilovoltage (kV) switching is a method for creating dual energy (DE) images to increase the visibility of lung tumors for markerless tumor tracking (MTT). However, scatter from the MV beam during the treatment degrades the quality of kV images. The goal of this study is to determine the effect of MV scatter on the accuracy of MTT using DE images.
Methods: A Varian TrueBeam was used to acquire a series of interleaved 60 and 120 kVp images in Developer mode. Using a thoracic phantom with simulated tumors (10 and 15 mm diameter), two sets of kV projections were acquired - one with and one without simultaneous MV irradiation. The MV beam parameters consisted of 5.0 × 5.0 cm2 field size, 6MV-FFF with a dose rate of 1400 MU/min, and kV frame rate of 15 fps. Weighted logarithmic subtraction was performed on consecutive high-low projections to produce soft-tissue images for kV only (DEkV) and kV with MV beam scatter (DEkV+MV). A template-based matching algorithm was used to track the targets on both image sequences. Tracked tumor location coordinates were evaluated against ground truth using the receiver operating characteristics (ROC), root-mean-squared error (RMSE), and percent missing frames (%MFs).
Results: The ROC area-under-the-curve (AUC) was 0.95 vs. 0.91 (10 mm target) and 0.92 vs. 0.87 (15 mm target) for DEkV vs. DEkV+MV. RMSE, based on a 95% specificity, was 0.65 mm vs. 0.88 mm (10 mm target) and 0.55 mm vs. 0.74 mm (15 mm target) for DEkV vs. DEkV+MV. The %MFs for DEkV were 0.43% and 0.44%, while for DEkV+MV, they were 1.31%, and 3.49%, for 10 and 15 mm targets, respectively.
Conclusion: This study indicates that MV scatter degrades tracking accuracy in DE images. In the future, methods will be considered to mitigate the MV scatter on kV images.
Funding Support, Disclosures, and Conflict of Interest: Research reported in this publication was supported by the National Cancer Institute of the National Institutes of Health under Award Number R01-CA207483. M. Lehmann, D. Morf, L. Zhu, and M. Walczak are employees at Varian Medical Systems.
Dual-energy Imaging, Scatter, ROC Analysis
TH- External Beam- Photons: Motion management - intrafraction