title: Tracking latency in image-based dynamic MLC tracking with direct image access.
abstract: OBJECTIVE
  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.
  
  
  METHODS
  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 ΔT(image) increased linearly with ΔT(image) as 148 ms + 0.58 * ΔT(image) (kV) and 162 ms + 1.1 * ΔT(image) (MV). The latency contribution from image acquisition and image transfer for kV image-based tracking was independent on ΔT(image) at 103 ± 14 ms. For MV-based tracking, it increased with ΔT(image) as 124 ms + 0.44 * ΔT(image). For ΔT(image) = 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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Oncology, Aarhus University Hospital, Denmark. walther@fledelius.com
authors.forename: Walther
authors.initials: W
authors.lastname: Fledelius
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Keall
authors.forename: Byungchul
authors.initials: B
authors.lastname: Cho
authors.forename: Xinhui
authors.initials: X
authors.lastname: Yang
authors.forename: Daniel
authors.initials: D
authors.lastname: Morf
authors.forename: Stefan
authors.initials: S
authors.lastname: Scheib
authors.forename: Per R
authors.initials: PR
authors.lastname: Poulsen
mesh_terms: D000465:Algorithms; D006801:Humans; D007091:Image Processing, Computer-Assisted; D009038:Motion; D009369:Neoplasms / Q000473:pathology / Q000532:radiotherapy*; D010315:Particle Accelerators / Q000295:instrumentation*; D019047:Phantoms, Imaging; D050397:Radiotherapy, Intensity-Modulated; D012119:Respiration; D014414:Tungsten / Q000032:analysis
chemical_list: D014414:Tungsten