title: Dosimetric evaluation of the Fletcher-Williamson ovoid for pulsed-dose-rate brachytherapy: a Monte Carlo study.
abstract: We used radiochromic film dosimetry to validate a Monte Carlo (MC) model of a 192Ir pulsed-dose-rate (PDR) source inside a Fletcher-Williamson ovoid. MD-55-2 radiochromic film was placed in a high-impact polystyrene phantom in a plane parallel to and displaced 2.0 cm medially from the long axis of the ovoid. MC N-particle transport code (MCNPX) version 2.4 was used to model the ovoid and the 192Ir source. Energy deposition was calculated using a track-length estimator modified by an energy-dependent heating function, which is a good approximation of the collision kerma. To convert the estimates of the MC dose per simulated particle to clinically relevant absolute dosimetry, additional MC models of an actual and a virtual 192Ir source in dry air were simulated to determine air kerma strength for the penetrating part of the photon spectrum (>11.3 keV). The absolute dose distributions predicted by MCNPX agreed with the film results and were within +/-9.4% (k = 2) and within +/-2% or within a distance to agreement of 2 mm for 94% of the dose grid. Additional MC models characterized the uncertainty resulting from source positioning inside the ovoid. For a worst-case scenario of 1 mm off centre from the nominal source position in the 3 mm diameter ovoid shaft, the average dose deviation over the film plane was +/-5% (1sigma = +/-4%), with maximum deviation near the sharp dose-gradient provided by the shields of -20% to + 26%. A validated MC model is the first requirement to simulate common LDR clinical loadings (5-20 mgRaEq) and, thus, will aid in the transition from the current 137Cs Selectron LDR ICBT to PDR for treatment of gynecologic cancers.
authors.affiliation: Department of Radiation Physics, Unit 1220, The University of Texas M.D. Anderson Cancer Center, 1220 Holcombe Blvd., Houston, TX 77030, USA.
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Price
authors.forename: John L
authors.initials: JL
authors.lastname: Horton
authors.forename: Kent A
authors.initials: KA
authors.lastname: Gifford
authors.forename: Patricia J
authors.initials: PJ
authors.lastname: Eifel
authors.forename: Anuja
authors.initials: A
authors.lastname: Jhingran
authors.forename: Ann A
authors.initials: AA
authors.lastname: Lawyer
authors.forename: Paula A
authors.initials: PA
authors.lastname: Berner
authors.forename: Firas
authors.initials: F
authors.lastname: Mourtada
mesh_terms: D001918:Brachytherapy / Q000379:methods*; D002138:Calibration; D005373:Film Dosimetry / Q000295:instrumentation / Q000379:methods; D006801:Humans; D007496:Iridium Radioisotopes / Q000627:therapeutic use*; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D011137:Polystyrenes / Q000737:chemistry; D011874:Radiometry / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D013696:Temperature; D013997:Time Factors; D014962:X-Ray Film
chemical_list: D007496:Iridium Radioisotopes; D011137:Polystyrenes