title: Dose perturbations due to contrast medium and air in mammosite treatment: an experimental and Monte Carlo study.
abstract: In the management of early breast cancer, a partial breast irradiation technique called MammoSite (Proxima Therapeutic Inc., Alpharetta, GA) has been advocated in recent years. In MammoSite, a balloon implanted at the surgical cavity during tumor excision is filled with a radio-opaque solution, and radiation is delivered via a high dose rate brachytherapy source situated at the center of the balloon. Frequently air may be introduced during placement of the balloon and/or injection of the contrast solution into the balloon. The purpose of this work is to quantify as well as to understand dose perturbations due to the presence of a high-Z contrast medium and/or an air bubble with measurements and Monte Carlo calculations. In addition, the measured dose distribution is compared with that obtained from a commercial treatment planning system (Nucletron PLATO system). For a balloon diameter of 42 mm, the dose variation as a function of distance from the balloon surface is measured for various concentrations of a radio-opaque solution (in the range 5%-25% by volume) with a small volume parallel plate ion chamber and a micro-diode detector placed perpendicular to the balloon axis. Monte Carlo simulations are performed to provide a basic understanding of the interaction mechanism and the magnitude of dose perturbation at the interface near balloon surface. Our results show that the radio-opaque concentration produces dose perturbation up to 6%. The dose perturbation occurs mostly within the distances <1 mm from the balloon surface. The Plato system that does not include heterogeneity correction may be sufficient for dose planning at distances > or = 10 mm from the balloon surface for the iodine concentrations used in the MammoSite procedures. The dose enhancement effect near the balloon surface (<1 mm) due to the higher iodine concentration is not correctly predicted by the Plato system. The dose near the balloon surface may be increased by 0.5% per cm3 of air. Monte Carlo simulation suggests that the interface effect (enhanced dose near surface) is primarily due to Compton electrons of short range (<0.5 mm). For more accurate dosimetry in MammoSite delivery, the dose perturbation due to the presence of a radio-opaque contrast medium and air bubbles should be considered in a brachytherapy planning system.
authors.affiliation: Arizona Oncology Associates, 2625 N. Craycroft Road, Suite 100, Tucson, Arizona 85712, USA. cheewai.cheng@usoncology.com
authors.forename: C W
authors.initials: CW
authors.lastname: Cheng
authors.forename: R
authors.initials: R
authors.lastname: Mitra
authors.forename: X Allen
authors.initials: XA
authors.lastname: Li
authors.forename: Indra J
authors.initials: IJ
authors.lastname: Das
mesh_terms: D016477:Artifacts; D001918:Brachytherapy / Q000379:methods*; D002404:Catheterization / Q000379:methods*; D003198:Computer Simulation; D003287:Contrast Media / Q000009:adverse effects*; D005260:Female; D006801:Humans; D008954:Models, Biological; D015233:Models, Statistical; D009010:Monte Carlo Method; D011832:Radiation Injuries / Q000209:etiology* / Q000517:prevention & control*; D011874:Radiometry / Q000379:methods*; D011879:Radiotherapy Dosage; D012062:Relative Biological Effectiveness; D012542:Scattering, Radiation
chemical_list: D003287:Contrast Media