title: Monte Carlo model for a prototype CT-compatible, anatomically adaptive, shielded intracavitary brachytherapy applicator for the treatment of cervical cancer.
abstract: OBJECTIVE
  Current, clinically applicable intracavitary brachytherapy applicators that utilize shielded ovoids contain a pair of tungsten-alloy shields which serve to reduce dose delivered to the rectum and bladder during source afterloading. After applicator insertion, these fixed shields are not necessarily positioned to provide optimal shielding of these critical structures due to variations in patient anatomies. The authors present a dosimetric evaluation of a novel prototype intracavitary brachytherapy ovoid [anatomically adaptive applicator (A3)], featuring a single shield whose position can be adjusted with two degrees of freedom: Rotation about and translation along the long axis of the ovoid.
  
  
  METHODS
  The dosimetry of the device for a HDR 192Ir was characterized using radiochromic film measurements for various shield orientations. A MCNPX Monte Carlo model was developed of the prototype ovoid and integrated with a previously validated model of a v2 mHDR 192Ir source (Nucletron Co.). The model was validated for three distinct shield orientations using film measurements.
  
  
  RESULTS
  For the most complex case, 91% of the absolute simulated and measured dose points agreed within 2% or 2 mm and 96% agreed within 10% or 2 mm.
  
  
  CONCLUSIONS
  Validation of the Monte Carlo model facilitates future investigations into any dosimetric advantages the use of the A3 may have over the current state of art with respect to optimization and customization of dose delivery as a function of patient anatomical geometries.
authors.affiliation: Department of Radiation Physics, The University of Texas M. D. Anderson Cancer Center, 1220 Holcombe Boulevard, Houston, Texas 77030, USA. mprice@marybird.com
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Price
authors.forename: Kent A
authors.initials: KA
authors.lastname: Gifford
authors.forename: John L
authors.initials: JL
authors.lastname: Horton
authors.forename: Patricia J
authors.initials: PJ
authors.lastname: Eifel
authors.forename: Michael T
authors.initials: MT
authors.lastname: Gillin
authors.forename: Ann A
authors.initials: AA
authors.lastname: Lawyer
authors.forename: Firas
authors.initials: F
authors.lastname: Mourtada
mesh_terms: D001918:Brachytherapy / Q000295:instrumentation* / Q000379:methods; D003198:Computer Simulation; D004867:Equipment Design; D005260:Female; D005373:Film Dosimetry; D006801:Humans; D007496:Iridium Radioisotopes; D008962:Models, Theoretical; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D011829:Radiation Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D012007:Rectum / Q000528:radiation effects; D012399:Rotation; D014057:Tomography, X-Ray Computed / Q000379:methods*; D035501:Uncertainty; D001743:Urinary Bladder / Q000528:radiation effects; D002583:Uterine Cervical Neoplasms / Q000532:radiotherapy*
chemical_list: D007496:Iridium Radioisotopes