title: Image-guided radiotherapy in near real time with intensity-modulated radiotherapy megavoltage treatment beam imaging.
abstract: OBJECTIVE
  To utilize image-guided radiotherapy (IGRT) in near real time by obtaining and evaluating the online positions of implanted fiducials from continuous electronic portal imaging device (EPID) imaging of prostate intensity-modulated radiotherapy (IMRT) delivery.
  
  
  METHODS
  Upon initial setup using two orthogonal images, the three-dimensional (3D) positions of all implanted fiducial markers are obtained, and their expected two-dimensional (2D) locations in the beam's-eye-view (BEV) projection are calculated for each treatment field. During IMRT beam delivery, EPID images of the megavoltage treatment beam are acquired in cine mode and subsequently analyzed to locate 2D locations of fiducials in the BEV. Simultaneously, 3D positions are estimated according to the current EPID image, information from the setup portal images, and images acquired at other gantry angles (the completed treatment fields). The measured 2D and 3D positions of each fiducial are compared with their expected 2D and 3D setup positions, respectively. Any displacements larger than a predefined tolerance may cause the treatment system to suspend the beam delivery and direct the therapists to reposition the patient.
  
  
  RESULTS
  Phantom studies indicate that the accuracy of 2D BEV and 3D tracking are better than 1 mm and 1.4 mm, respectively. A total of 7330 images from prostate treatments were acquired and analyzed, showing a maximum 2D displacement of 6.7 mm and a maximum 3D displacement of 6.9 mm over 34 fractions.
  
  
  CONCLUSIONS
  This EPID-based, real-time IGRT method can be implemented on any external beam machine with portal imaging capabilities without purchasing any additional equipment, and there is no extra dose delivered to the patient.
authors.affiliation: Department of Radiation Oncology, Stanford University, Palo Alto, CA, USA. weihua.mao@utsouthwestern.edu
authors.forename: Weihua
authors.initials: W
authors.lastname: Mao
authors.forename: Annie
authors.initials: A
authors.lastname: Hsu
authors.forename: Nadeem
authors.initials: N
authors.lastname: Riaz
authors.forename: Louis
authors.initials: L
authors.lastname: Lee
authors.forename: Rodney
authors.initials: R
authors.lastname: Wiersma
authors.forename: Gary
authors.initials: G
authors.lastname: Luxton
authors.forename: Christopher
authors.initials: C
authors.lastname: King
authors.forename: Lei
authors.initials: L
authors.lastname: Xing
authors.forename: Timothy
authors.initials: T
authors.lastname: Solberg
mesh_terms: D000465:Algorithms; D006046:Gold; D006801:Humans; D008297:Male; D009068:Movement; D019047:Phantoms, Imaging; D019095:Point-of-Care Systems; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy*; D019736:Prostheses and Implants; D011859:Radiography; D011879:Radiotherapy Dosage; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*
chemical_list: D006046:Gold