Real-Time 4D Tumor Tracking and Modeling from Internal and External Fiducials in Fluoroscopy

Fluoroscopy is currently used in treatment planning for patients undergoing radiation therapy. Radiation oncologists would like to maximize the amount of dose the tumor receives and minimize the amount delivered to the surrounding tissues. During treatment, patients breathe freely and so the tumor location will not be fixed. This makes calculating the amount of dose delivered to the tumor, and verifying that the tumor actually receives that dose, difficult. We describe a correlationbased method of tracking the two-dimensional (2D) motion of internal markers (surgical clips) placed around the tumor. We established ground truth and evaluated the accuracy of the tracker for 10 data sets of 5 patients. The root mean squared error in estimating 2D marker position was 0.47 mm on average. We also developed a method to model the average and maximum three-dimensional (3D) motion of the clips given two orthogonal fluoroscopy videos of the same patients that were taken sequentially. On average, the error was 3.0 mm for four pairs of trajectories. If imaging is possible during treatment, such motion models may be used for beam guided radiation; otherwise, they may be correlated to a set of external markers for use in respiratory gating.