title: Phantom and in-vivo measurements of dose exposure by image-guided radiotherapy (IGRT): MV portal images vs. kV portal images vs. cone-beam CT.
abstract: OBJECTIVE
  Positioning verification is usually performed with treatment beam (MV) portal images (PI) using an electronic portal imaging device (EPID). A new alternative is the use of a low energy photon source (kV) and an additional EPID mounted to the accelerator gantry. This system may be used for PI or--with rotating gantry--as cone-beam CT (CBCT). The dose delivered to the patient by different imaging processes was measured.
  
  
  METHODS
  A total of 15 in-vivo dose measurements were done in five patients receiving prostate IMRT. For anterior-posterior (AP) and lateral PI with MV and kV photons measurement points were inside the rectum and at the patient's skin. Dose for CBCT was measured in the rectum. Additional measurements for CBCT were done in a cylindrical CT-dose-index (CTDI) phantom to determine peripheral, central and weighted CTDI.
  
  
  RESULTS
  The dose for AP MV PI was 57.8 mGy at the surface and 33.9 mGy in the rectum, for lateral MV PI 69.4 mGy and 31.7 mGy, respectively (5 MU/exposure). The dose for AP kV PI was 0.8 mGy at the surface and 0.2 mGy in the rectum, for lateral PI 1.1 mGy and 0. 1 mGy, respectively. For a CBCT the rectal dose was 17.2 mGy. The peripheral CTDI was 23.6 mGy and the center dose was 10.2 mGy, resulting in a weighted CTDI of 19.1 mGy in the phantom and an estimated surface dose of < or =28 mGy.
  
  
  CONCLUSIONS
  Even taking into account an RBE (Relative Biological Effectiveness) of 2 for kV vs. MV radiation, for kV PI the delivered dose is lower and image quality is better than for MV PI. CBCT provides a 3D-image dataset and dose exposure for one scan is lower than for two MV PI, thus rendering frequent volume imaging during a fractionated course of radiotherapy possible.
authors.affiliation: Department of Radiation Oncology, University of Heidelberg, Mannheim Medical Center, Theodor-Kutzer-Ufer 1-3, Mannheim, Germany. co_walter@web.de
authors.forename: Cornelia
authors.initials: C
authors.lastname: Walter
authors.forename: Judit
authors.initials: J
authors.lastname: Boda-Heggemann
authors.forename: Hansjörg
authors.initials: H
authors.lastname: Wertz
authors.forename: Iris
authors.initials: I
authors.lastname: Loeb
authors.forename: Angelika
authors.initials: A
authors.lastname: Rahn
authors.forename: Frank
authors.initials: F
authors.lastname: Lohr
authors.forename: Frederik
authors.initials: F
authors.lastname: Wenz
mesh_terms: D054893:Cone-Beam Computed Tomography / Q000379:methods*; D006801:Humans; D008297:Male; D019047:Phantoms, Imaging; D011471:Prostatic Neoplasms / Q000532:radiotherapy; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D012062:Relative Biological Effectiveness