Abstract
Current treatment planning systems (TPSs) for partial breast irradiation using the MammoSite brachytherapy applicator (Cytyc Corporation, Marlborough, MA) often neglect the effect of inhomogeneity, leading to potential inaccuracies in dose distributions. Previous publications either have studied only a planar dose perturbation along the bisector of the source or have paid little attention to the anisotropy effect of the system. In the present study, we investigated the attenuation‐corrected radial dose and anisotropy functions in a form parallel to the updated American Association of Physicists in Medicine TG‐43 formalism. This work quantitatively delineates the inaccuracies in dose distributions in three‐dimensional space. Monte Carlo N‐particle transport code simulations in coupled photon–electron transport were used to quantify the changes in dose deposition and distribution caused by the increased attenuation coefficient of iodine‐based contrast solution. The source geometry was that of the VariSource wire model VS2000 (Varian Medical Systems, Palo Alto, CA). The 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 6 cm were simulated. Dose rates at the typical prescription line (1 cm away from the balloon surface) were determined for various polar angles. The computations showed that the dose rate reduction throughout the entire region of interest ranged from 0.64% for the smallest balloon diameter and contrast concentration to 6.17% for the largest balloon diameter and contrast concentration. The corrected radial dose function has a predominant influence on dose reduction, but the corrected anisotropy functions explain only the effect at the MammoSite system poles. By applying the corrected radial dose and anisotropy functions to TPSs, the attenuation effect can be reduced to the minimum.PACS number: 87.53.‐j
Highlights
The MammoSite Radiation Therapy System (Cytec Corporation, Marlborough, MA) is a new, minimally invasive method of delivering internal radiation therapy following lumpectomy for breast cancer.[1,2,3,4,5,6,7,8,9] Therapy is given on an outpatient basis and can be completed in 5 days.140 Zhang et al.: Three-dimensional quantitative dose reduction analysis...essential part of the procedure
To incorporate contrast attenuation of this kind into the treatment planning systems (TPSs), we suggest using attenuation-corrected radial dose functions and anisotropy functions in a form that parallels the American Association of Physicists in Medicine (AAPM) TG-43U1 formalism.[15]
Because the mass attenuation coefficient is large for low energies and high atomic number media, the dose reduction attributable to contrast attenuation cannot be ignored
Summary
140 Zhang et al.: Three-dimensional quantitative dose reduction analysis. Small perturbations of the source position can result in significant variations in dose at the prescription line. The radiopaque contrast used inside the balloon enhances image quality and contributes to treatment planning and quality assurance of the procedure.[10]. In the United States, 192Ir is being widely used as a HDR source. It emits a wide spectrum (0.11 MeV – 1.378 MeV), including many low-energy components.[11] Because of the photoelectric effect, low-energy photons are preferentially absorbed by high-Z media. Because contrast materials typically contain elements with high atomic numbers—for example, iodine (Z = 53), the balloon content cannot be considered tissue- or water-equivalent
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