title: Automatic prostate tracking and motion assessment in volumetric modulated arc therapy with an electronic portal imaging device.
abstract: OBJECTIVE
  To assess the prostate intrafraction motion in volumetric modulated arc therapy treatments using cine megavoltage (MV) images acquired with an electronic portal imaging device (EPID).
  
  
  METHODS
  Ten prostate cancer patients were treated with volumetric modulated arc therapy using a Varian TrueBeam linear accelerator equipped with an EPID for acquiring cine MV images during treatment. Cine MV images acquisition was scheduled for single or multiple treatment fractions (between 1 and 8). A novel automatic fiducial detection algorithm that can handle irregular multileaf collimator apertures, field edges, fast leaf and gantry movement, and MV image noise and artifacts in patient anatomy was used. All sets of images (approximately 25,000 images in total) were analyzed to measure the positioning accuracy of implanted fiducial markers and assess the prostate movement.
  
  
  RESULTS
  Prostate motion can vary greatly in magnitude among different patients. Different motion patterns were identified, showing its unpredictability. The mean displacement and standard deviation of the intrafraction motion was generally less than 2.0 ± 2.0 mm in each of the spatial directions. In certain patients, however, the percentage of the treatment time in which the prostate is displaced more than 5 mm from its planned position in at least 1 spatial direction was 10% or more. The maximum prostate displacement observed was 13.3 mm.
  
  
  CONCLUSIONS
  Prostate tracking and motion assessment was performed with MV imaging and an EPID. The amount of prostate motion observed suggests that patients will benefit from its real-time monitoring. Megavoltage imaging can provide the basis for real-time prostate tracking using conventional linear accelerators.
authors.affiliation: Department of Radiation Oncology, Stanford University, Stanford, California 94305, USA. jdazcona@stanford.edu
authors.forename: Juan Diego
authors.initials: JD
authors.lastname: Azcona
authors.forename: Ruijiang
authors.initials: R
authors.lastname: Li
authors.forename: Edward
authors.initials: E
authors.lastname: Mok
authors.forename: Steven
authors.initials: S
authors.lastname: Hancock
authors.forename: Lei
authors.initials: L
authors.lastname: Xing
mesh_terms: D000465:Algorithms; D019583:Dose Fractionation, Radiation; D057918:Fiducial Markers; D006801:Humans; D008297:Male; D009068:Movement; D010315:Particle Accelerators / Q000295:instrumentation*; D011467:Prostate; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation*; D050397:Radiotherapy, Intensity-Modulated / Q000295:instrumentation* / Q000379:methods; D062206:Spatial Analysis