title: Kilovoltage intrafraction monitoring for prostate intensity modulated arc therapy: first clinical results.
abstract: OBJECTIVE
  Most linear accelerators purchased today are equipped with a gantry-mounted kilovoltage X-ray imager which is typically used for patient imaging prior to therapy. A novel application of the X-ray system is kilovoltage intrafraction monitoring (KIM), in which the 3-dimensional (3D) tumor position is determined during treatment. In this paper, we report on the first use of KIM in a prospective clinical study of prostate cancer patients undergoing intensity modulated arc therapy (IMAT).
  
  
  METHODS
  Ten prostate cancer patients with implanted fiducial markers undergoing conventionally fractionated IMAT (RapidArc) were enrolled in an ethics-approved study of KIM. KIM involves acquiring kV images as the gantry rotates around the patient during treatment. Post-treatment, markers in these images were segmented to obtain 2D positions. From the 2D positions, a maximum likelihood estimation of a probability density function was used to obtain 3D prostate trajectories. The trajectories were analyzed to determine the motion type and the percentage of time the prostate was displaced ≥ 3, 5, 7, and 10 mm. Independent verification of KIM positional accuracy was performed using kV/MV triangulation.
  
  
  RESULTS
  KIM was performed for 268 fractions. Various prostate trajectories were observed (ie, continuous target drift, transient excursion, stable target position, persistent excursion, high-frequency excursions, and erratic behavior). For all patients, 3D displacements of ≥ 3, 5, 7, and 10 mm were observed 5.6%, 2.2%, 0.7% and 0.4% of the time, respectively. The average systematic accuracy of KIM was measured at 0.46 mm.
  
  
  CONCLUSIONS
  KIM for prostate IMAT was successfully implemented clinically for the first time. Key advantages of this method are (1) submillimeter accuracy, (2) widespread applicability, and (3) a low barrier to clinical implementation. A disadvantage is that KIM delivers additional imaging dose to the patient.
authors.affiliation: Radiation Physics Laboratory, Sydney Medical School and Institute of Medical Physics, School of Physics, University of Sydney, New South Wales, Australia.
authors.forename: Jin Aun
authors.initials: JA
authors.lastname: Ng
authors.forename: Jeremy T
authors.initials: JT
authors.lastname: Booth
authors.forename: Per R
authors.initials: PR
authors.lastname: Poulsen
authors.forename: Walther
authors.initials: W
authors.lastname: Fledelius
authors.forename: Esben Schjødt
authors.initials: ES
authors.lastname: Worm
authors.forename: Thomas
authors.initials: T
authors.lastname: Eade
authors.forename: Fiona
authors.initials: F
authors.lastname: Hegi
authors.forename: Andrew
authors.initials: A
authors.lastname: Kneebone
authors.forename: Zdenka
authors.initials: Z
authors.lastname: Kuncic
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Keall
mesh_terms: D019583:Dose Fractionation, Radiation; D057918:Fiducial Markers; D006801:Humans; D021621:Imaging, Three-Dimensional / Q000295:instrumentation / Q000379:methods*; D016013:Likelihood Functions; D008297:Male; D009068:Movement; D010315:Particle Accelerators; D011446:Prospective Studies; D011467:Prostate / Q000000981:diagnostic imaging*; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy*; D011859:Radiography; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D050397:Radiotherapy, Intensity-Modulated / Q000295:instrumentation / Q000379:methods*