A 3-D Quantitative Dose Reduction Analysis in MammoSite Balloon by Monte Carlo Calculations

Abstract

Abstract:  Current treatment planning systems (TPS) for partial breast irradiation (PBI) using the MammoSite^® brachytherapy applicator often neglects the effect of inhomogeneity, leading to potential inaccuracies in dose distributions. Previous publications have studied either only a planar dose perturbation along the bisector of the source or pay little attention to the anisotropy effect of the system. In this study we investigate the attenuation-corrected radial dose and anisotropy functions in a form parallel to the updated AAPM TG-43U1 formalism. This will delineate quantitatively the inaccuracies in dose distributions in three-dimensional space. The changes in dose deposition and distribution caused by the increased attenuation coefficient of iodine-based contrast solution are quantified using MCNP Monte Carlo simulations in coupled photon/electron transport. The source geometry was that of the VariSource wire (model VS2000). Concentration of the iodine-based solution was varied from 5% to 25% by volume, a range recommended by the balloon’s manufacturer. Balloon diameters of 4, 5 and 6cm were simulated. Dose rates at the typical prescription line of 1cm away from the balloon surface were determined for different polar angles. According to the computations verified by Mosfet measurements in water phntom, the dose rate reduction throughout the entire region of interest ranged from 0.64% for the smallest balloon diameter and contrast concentration to a maximum of 6.17% for the largest balloon and concentration.