TU‐F‐17A‐05: Calculating Tumor Trajectory and Dose‐Of‐The‐Day for Highly Mobile Tumors Using Cone‐Beam CT Projections

Abstract

Purpose:Cone‐beam CT (CBCT) projection images provide anatomical data in real‐time over several respiratory cycles, forming a comprehensive picture of tumor movement. We developed a method using these projections to determine the trajectory and dose of highly mobile tumors during each fraction of treatment.Methods:CBCT images of a respiration phantom were acquired, where the trajectory mimicked a lung tumor with high amplitude (2.4 cm) and hysteresis. A template‐matching algorithm was used to identify the location of a steel BB in each projection. A Gaussian probability density function for tumor position was calculated which best fit the observed trajectory of the BB in the imager geometry. Two methods to improve the accuracy of tumor track reconstruction were investigated: first, using respiratory phase information to refine the trajectory estimation, and second, using the Monte Carlo method to sample the estimated Gaussian tumor position distribution. 15 clinically‐drawn abdominal/lung CTV volumes were used to evaluate the accuracy of the proposed methods by comparing the known and calculated BB trajectories.Results:With all methods, the mean position of the BB was determined with accuracy better than 0.1 mm, and root‐mean‐square (RMS) trajectory errors were lower than 5% of marker amplitude. Use of respiratory phase information decreased RMS errors by 30%, and decreased the fraction of large errors (>3 mm) by half. Mean dose to the clinical volumes was calculated with an average error of 0.1% and average absolute error of 0.3%. Dosimetric parameters D90/D95 were determined within 0.5% of maximum dose. Monte‐Carlo sampling increased RMS trajectory and dosimetric errors slightly, but prevented over‐estimation of dose in trajectories with high noise.Conclusions:Tumor trajectory and dose‐of‐the‐day were accurately calculated using CBCT projections. This technique provides a widely‐available method to evaluate highly‐mobile tumors, and could facilitate better strategies to mitigate or compensate for motion during SBRT.