title: Monte Carlo calculations of the dose distribution around a commercial gynecologic tandem applicator.
abstract: OBJECTIVE
  Dose rate distributions around Fletcher Suit Delclos (FSD) tandem applicators used for intracavitary brachytherapy are usually calculated by assuming each source is a point source and summing the contributions from each of the sources. Consequently, interpellet attenuation and scattering are ignored. Additional error may be introduced because the applicator walls and tip screw are not considered. The focus of this study was a Monte Carlo simulation of a Selectron tandem, verification of the calculations, and presentation of the implications of the point-source approximation for treatment planning.
  
  
  METHODS
  MCNPX 2.4.k was used to calculate dose rate distributions around straight and curved tandems. The Monte Carlo calculations were verified with radiochromic film.
  
  
  RESULTS
  MCNPX calculated dose to within +/-2% or +/-2 mm for 97% of the points on the film parallel to the long axis and 98% on a film perpendicular to the long axis of the straight portion of the tandem. The point source approximation overestimated dose by as much as 33% superior to the tip of the tandem as compared to MCNPX. The point source approximation overestimated dose when photons passed through multiple pellets by as much as 18% as compared to MCNPX. Laterally, the dose distribution was not affected greatly.
  
  
  CONCLUSIONS
  Interpellet attenuation was a dominant factor in determining the distribution along the length of the pellet train. MCNPX calculated doses accurately when the pellets and applicator walls were included in the geometry. The point source approximation is adequate lateral to the tandem. The point source approximation does not calculate dose accurately superior to the tandem or when photons pass through multiple pellets.
authors.affiliation: Department of Radiation Physics, The University of Texas M. D. Anderson Cancer Center, Houston 77030, USA. kagifford@mdanderson.org
authors.forename: Kent A
authors.initials: KA
authors.lastname: Gifford
authors.forename: Firas
authors.initials: F
authors.lastname: Mourtada
authors.forename: Sang H
authors.initials: SH
authors.lastname: Cho
authors.forename: Ann
authors.initials: A
authors.lastname: Lawyer
authors.forename: John L
authors.initials: JL
authors.lastname: Horton
mesh_terms: D001918:Brachytherapy / Q000295:instrumentation* / Q000379:methods; D019583:Dose Fractionation, Radiation; D004307:Dose-Response Relationship, Radiation; D005260:Female; D005833:Genital Neoplasms, Female / Q000473:pathology / Q000532:radiotherapy*; D006801:Humans; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation* / Q000379:methods; D012680:Sensitivity and Specificity