Detailed analysis of latencies in image-based dynamic MLC racking

Purpose: Previous measurements of the accuracy of image-based real-time dynamic multileaf collimator (DMLC) tracking show that the major contributor to errors is latency, i.e., the delay between target motion and MLC response. Therefore the purpose of this work was to develop a method for detailed analysis of latency contributions during image-based DMLC tracking. Methods: A prototype DMLC tracking system integrated with a linear accelerator was used for tracking a phantom with an embedded fiducial marker during treatment delivery. The phantom performed a sinusoidal motion. 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. Each image was stored in a file on the imaging workstation. A marker segmentation program opened the image file, determined the marker position in the image, and transferred it to the DMLC tracking program. This program estimated the three-dimensional target position by a single-imager method and adjusted the MLC aperture to the target position. Imaging intervals Δ Timage from 150 to 1000 ms were investigated for both kV and MV imaging. After the experiments, the recorded images were synchronized with MLC log files generated by the MLC controller and tracking log files generated by the tracking program. This synchronization allowed temporal analysis of the information flow for each individual image from acquisition to completed MLC adjustment. The synchronization also allowed investigation of the MLC adjustment dynamics on a considerably finer time scale than the 50 ms time resolution of the MLC log files. Results: For Δ Timage =150 ms, the total time from image acquisition to completed MLC adjustment was 380±9 ms for MV and 420±12 ms for kV images. The main part of this time was from image acquisition to completed image file writing (272 ms for MV and 309 ms for kV). Image file opening (38 ms), marker segmentation (4 ms), MLC position calculation (16 ms), and MLC adjustment (52 ms) were considerably faster. For Δ Timage =1000 ms, the total time from image acquisition to completed MLC adjustment increased to 1030±62 ms (MV) and 1330±52 ms (kV) mainly because of delayed image file writing. The MLC adjustment duration was constant 52 ms (±3 ms) for MLC adjustments below 1.1 mm and increased linearly for larger MLC adjustments. Conclusions: A method for detailed time analysis of each individual real-time position signal for DMLC tracking has been developed and applied to image-based tracking. The method allows identification of the major contributors to latency and therefore a focus for reducing this latency. The method could be an important tool for the reconstruction of the delivered target dose during DMLC tracking as it provides synchronization between target motion and MLC motion.