On the use of HDR 60Co source with the MammoSite® Radiation Therapy System

This work summarizes Monte Carlo results in order to evaluate the potential of using HDR Co-60 sources in accelerated partial breast irradiation (APBI) with the MammoSite (R) applicator. Simulations have been performed using the MCNP5 Monte Carlo Code, in simple geometries comprised of two concentric spheres; the internal consisting of selected concentrations, C, of a radiographic contrast solution in water (Omnipaque 300 (TM)) to simulate the MammoSite balloon and the external consisting of water to simulate surrounding tissue. The magnitude of the perturbation of delivered dose due to the radiographic contrast medium used in the MammoSite (R) applicator is calculated. At the very close vicinity of the balloon surface, a dose build-up region is observed, which leads to a dose overestimation by the treatment planning system (TPS) which depends on Omnipaque (TM) 300 solution concentration (and is in order of 2.3%, 3.0%, and 4.5%, respectively, at 1 mm away from the balloon - water interface, for C=10%, 15%, and 20%). However, dose overestimation by the TPS is minimal for points lying at the prescription distance (d=1 cm) or beyond, for all simulated concentrations and radii of MammoSite (R) balloon. An analytical estimation of the integral dose outside the CTV in the simple geometries simulated shows that dose to the breast for MammoSite (R) applications is expected to be comparable using HDR Co-60 and Ir-192 sources, and higher than that for Yb-169. The higher enegies of Co-60 sources result to approximately twice radiation protection requirements as compared to Ir-169 sources. However, they allow for more accurate dosimetry calculation with currently used treatment planning algorithms for Co-60 sources, compared to Ir-169.