SU‐E‐J‐74: Assessing Accuracy of Tumor Localization with Aperiodic Respiratory Breathing Using 4D‐CT and 4D‐CBCT Imaging

Abstract

Purpose: To assess the accuracy of tumor localization for aperiodic respiratory breathing for 4D‐CT and 4D cone‐beam CT (4D‐CBCT) imaging using a computer controlled moving phantom. Methods: We simulated an aperiodic respiratory based tumor motion using a programmable respiratory motion phantom (Quasar Modus QA). 4D‐CT scans were acquired four times at different starting points of respiratory cycle. 4D‐CT acquisitions were sorted into ten phases using time‐based binning, and the respiratory cycle reconstructed from the target motion. 4D‐CBCT scans, acquired with Elekta XVI 4DCBCT (Symmetry option), were sorted into 10 phases using amplitude‐based binning. A phased 4D‐CT dataset was selected as a reference CT image on the basis of minimum variation of tumor location over all 4 scans. This phased CT was used as reference for registration with 4D‐CBCT imaging, and the respiratory signal reconstructed. Results: For the selected aperiodic respiratory signal, phases 50% to 70% showed minimal variation amongst the four 4D‐CT scans. The 50% phase was selected as the reference for registration with 4D‐CBCT imaging. This yielded a reconstructed respiratory signal differing from the average tumor trajectory (based on the motor signal) by <2mm for the 50%‐70% region, which was similar to using the average 4D‐CT image (averaged over all phases) as the reference image. Use of other phases of 4D‐CT imaging as the reference yielded differences of up to 6mm from the average tumor motion trajectory. Conclusion: For aperiodic respiration motion considered here, this study showed that the use of phases 5 through 7 is optimal for treatment delivery, not only due to longer time of target in that position, but also because the variation in the reconstructed respiratory signals (and hence uncertainty in target position) due to different starting points in respiratory cycle in 4D‐CT imaging, is minimal.