Initial Clinical Evaluation of Fast-kV Dual Energy Imaging for Markerless Tumor Tracking of Lung Tumors in Stereotactic Body Radiation Therapy (SBRT)

Abstract

Dual energy (DE) fluoroscopy is being considered for markerless tumor tracking (MTT) of lung tumors. The principal advantage of this technique is that it can be used to remove overlaying bony anatomy, and hence improve the tracking accuracy vs. single energy (SE) fluoroscopy. Previous studies using phantoms have demonstrated the superiority of DE vs. SE imaging for MTT. The goal of this study is to clinically evaluate MTT with fast-kV switching DE imaging on a cohort of SBRT lung cancer patients using the on-board imager (OBI) of a commercial linear accelerator.In this Institutional Review Board (IRB)-approved prospective study, DE images were acquired for 10 SBRT lung cancer patients. For each patient, DE images were obtained over 180o arc using fast-kV switching implemented on the OBI of a commercial linear accelerator. This fast-kV switching technique produced x-ray pulses that alternate between programmed tube voltages (i.e., 120 and 60 kVp) at a rate of 15 Hz. Weighted logarithmic subtraction was performed offline on consecutive high-low energy projections to produce soft tissue images. For comparison, SE image sequences were obtained by using the 120 kVp images. Separately, a template was derived from the contoured gross tumor volume (GTV) on the CT simulation scan. A template-based matching algorithm was then used to track target motion on both DE and SE image sequences. Successful matching was defined as any instance in which the algorithm matched the template on the respective fluoroscopic image. Ground truth positions were estimated from both the DE and SE images using Bayesian inference.A total of 2278 SE and DE image frames were analyzed. DE imaging resulted in significantly improved tracking vs. SE on 884/2278 (38.8%) of image frames. In those frames, the average tracking error was 1.78 +/- 2.02 mm vs. 2.21 +/- 2.38 mm, for DE vs. SE imaging, respectively (P < 0.001). For 4/10 patients, the average reduction in tracking error with DE vs. SE tracking was > 0.7 mm. No correlation was observed between tumor volume/location and improved tracking accuracy with DE imaging. Additionally, the fraction of images that could not be tracked with SE was 91/2278 (4.0%). The addition of DE imaging decreased this number to 33/2278 (1.4%) (P < 0.001).This is the first prospective study to evaluate fast-kV switching DE imaging for MTT in a cohort of SBRT lung cancer patients. This study has demonstrated that by removal of overlapping bony anatomy, DE imaging increases tracking accuracy and decreases the number of images where tracking fails. Future work includes optimization of template parameters to further improve DE tracking accuracy.