title: Source position verification and dosimetry in HDR brachytherapy using an EPID.
abstract: OBJECTIVE
  Accurate treatment delivery in high dose rate (HDR) brachytherapy requires correct source dwell positions and dwell times to be administered relative to each other and to the surrounding anatomy. Treatment delivery inaccuracies predominantly occur for two reasons: (i) anatomical movement or (ii) as a result of human errors that are usually related to incorrect implementation of the planned treatment. Electronic portal imaging devices (EPIDs) were originally developed for patient position verification in external beam radiotherapy and their application has been extended to provide dosimetric information. The authors have characterized the response of an EPID for use with an (192)Ir brachytherapy source to demonstrate its use as a verification device, providing both source position and dosimetric information.
  
  
  METHODS
  Characterization of the EPID response using an (192)Ir brachytherapy source included investigations of reproducibility, linearity with dose rate, photon energy dependence, and charge build-up effects associated with exposure time and image acquisition time. Source position resolution in three dimensions was determined. To illustrate treatment verification, a simple treatment plan was delivered to a phantom and the measured EPID dose distribution compared with the planned dose.
  
  
  RESULTS
  The mean absolute source position error in the plane parallel to the EPID, for dwells measured at 50, 100, and 150 mm source to detector distances (SDD), was determined to be 0.26 mm. The resolution of the z coordinate (perpendicular distance from detector plane) is SDD dependent with 95% confidence intervals of ± 0.1, ± 0.5, and ± 2.0 mm at SDDs of 50, 100, and 150 mm, respectively. The response of the EPID is highly linear to dose rate. The EPID exhibits an over-response to low energy incident photons and this nonlinearity is incorporated into the dose calibration procedure. A distance (spectral) dependent dose rate calibration procedure has been developed. The difference between measured and planned dose is less than 2% for 98.0% of pixels in a two-dimensional plane at an SDD of 100 mm.
  
  
  CONCLUSIONS
  Our application of EPID dosimetry to HDR brachytherapy provides a quality assurance measure of the geometrical distribution of the delivered dose as well as the source positions, which is not possible with any current HDR brachytherapy verification system.
authors: William Buckland Radiation Oncology, Alfred Health, Melbourne 3004, Australia and School of Applied Sciences and Health Innovations Research Institute, RMIT University, Melbourne 3001, Australia.
authors: R L
authors: RL
authors: Smith
authors: M L
authors: ML
authors: Taylor
authors: L N
authors: LN
authors: McDermott
authors: A
authors: A
authors: Haworth
authors: J L
authors: JL
authors: Millar
authors: R D
authors: RD
authors: Franich
mesh_terms: D000465:Algorithms; D001918:Brachytherapy / Q000379:methods*; D002138:Calibration; D057785:Catheters; D004867:Equipment Design; D006801:Humans; D007496:Iridium Radioisotopes / Q000737:chemistry; D019300:Medical Errors / Q000517:prevention & control; D009068:Movement; D011786:Quality Control; D011874:Radiometry / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D015203:Reproducibility of Results
chemical_list: D007496:Iridium Radioisotopes