Dual Energy Fluoroscopy Improves the Accuracy of Template-Based Tracking in Lung Cancer Patients Receiving Stereotactic Body Radiation Therapy

Abstract

Template-based matching algorithms are currently being considered for markerless motion tracking (MMT) of lung tumors using fluoroscopy. The accuracy of this match is quantified through a metric known as peak-to-side lobe ratio (PSR). A previous study has shown that template matches with PSR≥3 are significantly more accurate than those with PSR<3. The goal of this study is to evaluate the fraction of single energy (SE) fluoroscopic images, obtained in SBRT lung cancer patients, that have PSR≥3, and if the addition of dual energy (DE) subtraction fluoroscopy can increase the PSR value. Moreover, radiomic parameters from CT images are evaluated to determine if any are predictive of PSR. In this prospective study, a total of 2,448 fluoroscopy frames were acquired, at multiple imaging angles, from 8 SBRT patients with Stage IA - IIA lung cancer. Sequential 60 kVp and 120 kVp fluoroscopic sequences were acquired and aligned based on respiratory signal. DE fluoroscopic sequences were created using a frame-by-frame weighted logarithmic subtraction. Separately, a template was derived from the contoured gross tumor volume (GTV) on the CT simulation scan. A template-based matching algorithm was used to track tumor motion throughout the DE and SE (120 kVp) fluoroscopy sequences. This algorithm shifts the template across the image and calculates the normalized cross correlation (NCC) between the template and image, resulting in a match score surface. The offset at which the NCC has the maximum value (i.e., the position of the peak of the surface) represents the potential target position. The strength of this peak relative to NCC values away from the peak, called side lobe values, is quantified by the PSR. Radiomic parameters using the GTV from the planning CT scans were examined to determine correlation with the PSR values. Approximately 68% of the SE fluoroscopic images had PSR ≥3. Of the SE images with PSR <3, 79.5% were obtained at an oblique imaging angle, while the remaining were obtained at an anterior imaging angle. By adding DE fluoroscopy, over 86% of the images obtained PSR ≥3 (P < 0.001). Of the 23 unique imaging sessions, SE imaging alone allowed for tracking in over 80% of the images in 12/23 (52%) sessions vs. 18/23 (78%) when combined with DE fluoroscopy (P = 0.12). In those cases where DE imaging increased the PSR to ≥3, the change in template positioning was 1.14 +/- 0.61 mm. Of the 200 unique radiomic parameters analyzed, “energy” was predictive of PSR value, having a Pearson’s correlation coefficient = 0.846 and 0.841 for SE and DE, respectively. The PSR value provides an independent metric for assessing the accuracy of a template match when used in conjunction with MMT. The addition of DE imaging to SE fluoroscopy increases the fraction of images with PSR >3, thus improving the overall accuracy of template matching.