Markerless Tumor Tracking using Short Arc Digital Tomosynthesis With Fast-kV Switching Dual Energy Imaging

Abstract

Markerless tumor tracking (MTT) using template matching is being considered for real-time lung tumor tracking using single energy (SE) and dual energy (DE) fluoroscopy. While MTT results using DE are generally superior vs. SE imaging, tracking small targets (< 10 mm) is challenging for both techniques. Short arc digital tomosynthesis (DTS), in which projections over a short (6 deg) sliding arc are combined to form a tomosynthesis image, may serve as a preprocessing technique to improve tracking. The goal of this study is to evaluate MTT with DTS using SE imaging and fast-kV switching DE imaging with the on-board imager (OBI) of a commercial linear accelerator. To evaluate these techniques, a motion phantom, consisting of a torso with embedded ribs and spine along with a cavity having lung-equivalent density, was used. A 5 mm simulated tumor was placed inside the lung equivalent compartment of the phantom. The simulated tumor was placed at three different static positions representing the two extremes and the mid-point of the respiratory cycle. Full rotation cone beam computed tomography (CBCT) acquisitions were obtained using SE and DE imaging (15 frames/sec) with a gantry speed of 6 deg/sec. DE images were processed using weighted logarithmic subtraction on consecutive high-low projections to produce soft tissue images. DTS image sequences were generated from both SE and DE image sequences. A template-based matching algorithm was used to track target motion on SE and DE fluoroscopy and SE- and DE-DTS images. Tumor tracking coordinates were evaluated against ground truth motion using the receiver operating characteristics (ROC) and root-mean-squared error (RMSE) on the combined results for all three tumor positions. The ROC area-under-the-curve (AUC) was the lowest for SE (0.82) and DE fluoroscopy (0.87). The use of SE- and DE-DTS increased the AUC values to 0.93 and 0.98, respectively. For a 95% specificity, the sensitivities for SE and DE fluoroscopy were 37% and 50%, respectively. The inclusion of SE-DTS increased this sensitivity to 81%. However, the highest sensitivity was observed for DE-DTS at 96%. With respect to local accuracy, the RMSE was evaluated based on a 95% specificity. For SE and DE fluoroscopy, the RMSE were 1.57 mm and 1.13 mm, respectively, while SE- and DE-DTS had RMSE of 0.58 mm and 0.54 mm, respectively. While DE fluoroscopy improves MTT vs. SE, both techniques have low sensitivity for small targets. The use of DTS significantly improves sensitivity and tracking accuracy vs. these fluoroscopic techniques. However, the most significant improvements were noted for DE-DTS, suggesting that this technique can be a useful preprocessing technique to make markerless tracking of small targets more robust and accurate.