Dosimetric evaluation of the Fletcher-Williamson ovoid for pulsed-dose-rate brachytherapy: a Monte Carlo study

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% (1σ = ±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.