Tracking latency in image-based dynamic MLC tracking with direct image access

Abstract

Purpose. Target tracking is a promising method for motion compensation in radiotherapy. For image-based dynamic multileaf collimator (DMLC) tracking, latency has been shown to be the main contributor to geometrical errors in tracking of respiratory motion, specifically due to slow transfer of image data from the image acquisition system to the tracking system via image file storage on a hard disk. The purpose of the current study was to integrate direct image access with a DMLC tracking system and to quantify the tracking latency of the integrated system for both kV and MV image-based tracking. Method. A DMLC tracking system integrated with a linear accelerator was used for tracking of a motion phantom with an embedded tungsten marker. Real-time target localization was based on x-ray images acquired either with a portal imager or a kV imager mounted orthogonal to the treatment beam. Images were processed directly without intermediate disk access. Continuous portal images and system log files were stored during treatment delivery for detailed offline analysis of the tracking latency. Results. The mean tracking system latency for kV and MV image-based tracking as function of the imaging interval ΔTimage increased linearly with ΔTimage as 148 ms + 0.58 * ΔTimage (kV) and 162 ms + 1.1 * ΔTimage (MV). The latency contribution from image acquisition and image transfer for kV image-based tracking was independent on ΔTimage at 103 ± 14 ms. For MV-based tracking, it increased with ΔTimage as 124 ms + 0.44 * ΔTimage. For ΔTimage = 200 ms (5 Hz imaging), the total latency was reduced from 550 ms to 264 ms for kV image-based tracking and from 500 ms to 382 ms for MV image-based tracking as compared to the previously used indirect image transfer via image file storage on a hard disk. Conclusion. kV and MV image-based DMLC tracking was successfully integrated with direct image access. It resulted in substantial tracking latency reductions compared with image-based tracking without direct image access.