On the use of high dose rate Ir192 and Yb169 sources with the MammoSite^® radiation therapy system

Ample literature exists on the dose overestimation by commercially available treatment planning systems in MammoSite^® applications using high dose rate Ir192 sources for partial breast brachytherapy as monotherapy, due to their inability to predict the dose reduction caused by the radiographic contrast solution in the balloon catheter. In this work Monte Carlo simulation is used to verify the dose rate reduction in a balloon breast applicator which does not vary significantly with distance and it is 1.2% at the prescription distance for the reference simulated geometry of 10% diluted radiographic contrast media and 2.5cm balloon radius. Based on these findings and the minimal hardening of the initially emitted photon spectrum for Ir192, a simple analytical method is proposed and shown capable for correcting dosimetry planning in clinical applications. Simulations are also performed to assess the corresponding dose reduction in applications of balloon breast applicators using high dose rate Yb169 sources that have recently become available. Results yield a far more significant and distance dependent dose reduction for Yb169 (on the order of 20% at the prescription distance for the abovementioned reference simulation geometry). This dose reduction cannot be accounted for using simple analytical methods as for Ir192 due to the significant hardening of the initially emitted Yb169 photons within the diluted radiographic contrast media. Combined with results of previous works regarding the effect of altered scatter conditions (relative to treatment planning system assumptions) on breast treatment planning accuracy, which is more pronounced for Yb169 relative to Ir192, these findings call for the amendment of dose treatment planning systems before using Yb169 high dose rate sources in balloon breast applicators.