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.