title: Image-guided in vivo dosimetry for quality assurance of IMRT treatment for prostate cancer.
abstract: OBJECTIVE
  In external beam radiotherapy (EBRT) and especially in intensity-modulated radiotherapy (IMRT), the accuracy of the dose distribution in the patient is of utmost importance. It was investigated whether image guided in vivo dosimetry in the rectum is a reliable method for online dose verification.
  
  
  METHODS
  Twenty-one dose measurements were performed with an ionization chamber in the rectum of 7 patients undergoing IMRT for prostate cancer. The position of the probe was determined with cone beam computed tomography (CBCT). The point of measurement was determined relative to the isocenter and relative to an anatomic reference point. The dose deviations relative to the corresponding doses in the treatment plan were calculated. With an offline CT soft-tissue match, patient positioning after ultrasound was verified.
  
  
  RESULTS
  The mean magnitude +/- standard deviation (SD) of patient positioning errors was 3.0 +/- 2.5 mm, 5.1 +/- 4.9 mm, and 4.3 +/- 2.4 mm in the left-right, anteroposterior and craniocaudal direction. The dose deviations in points at corresponding positions relative to the isocenter were -1.4 +/- 4.9% (mean +/- SD). The mean dose deviation at corresponding anatomic positions was 6.5 +/- 21.6%. In the rare event of insufficient patient positioning, dose deviations could be >30% because of the close proximity of the probe and the posterior dose gradient.
  
  
  CONCLUSIONS
  Image-guided dosimetry in the rectum during IMRT of the prostate is a feasible and reliable direct method for dose verification when probe position is effectively controlled.
authors.affiliation: Department of Radiation Oncology, Mannheim Medical Centre, University of Heidelberg, Mannheim, Germany. hansjoerg.wertz@radonk.ma.uni-heidelberg.de
authors.forename: Hansjoerg
authors.initials: H
authors.lastname: Wertz
authors.forename: Judit
authors.initials: J
authors.lastname: Boda-Heggemann
authors.forename: Cornelia
authors.initials: C
authors.lastname: Walter
authors.forename: Barbara
authors.initials: B
authors.lastname: Dobler
authors.forename: Sabine
authors.initials: S
authors.lastname: Mai
authors.forename: Frederik
authors.initials: F
authors.lastname: Wenz
authors.forename: Frank
authors.initials: F
authors.lastname: Lohr
mesh_terms: D005240:Feasibility Studies; D006801:Humans; D008297:Male; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D011786:Quality Control; D011874:Radiometry / Q000379:methods* / Q000592:standards; D011879:Radiotherapy Dosage; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods / Q000592:standards*; D012007:Rectum / Q000000981:diagnostic imaging; D014057:Tomography, X-Ray Computed; D014463:Ultrasonography