Optimization of a breast implant in Brachytherapy PDR: validation with Monte Carlo simulation and measurements with TLDs and GafChromic films.

ABSTRACT

BACKGROUND AND PURPOSE: The calculation of the dose distribution of Brachytherapy breast implant has been carried out in accordance with the Paris System (PS) in the majority of the radiotherapy departments in Europe. PDR (Pulsed Dose Rate) has lead to an improvement of the treatment procedure, optimization tools, however, allow an improvement of the treatment technique. The goal of this study was to perform a dosimetric verification of an optimized seven needles implant and to try to decrease the active length while preserving the same treatment volume. This corresponds to a ratio "treated length/active length" (L(t)/L(a)) that tends towards 1. MATERIAL AND METHODS: A dosimetry phantom was made of polystyrene, capable of receiving the implant, TLDs (LiF100 1mm(3) micro cubes) and films (GafChromic MD 55-2). Dose distributions for one source position and for the implant in conformity with the PS were calculated, utilizing version 14.2 of the Plato TPS (Nucletron); the remote afterloading system was a microSelectron-PDR (Nucletron). MCNP (Monte Carlo N-Particles transport) modeling was used for various configurations to evaluate the influence of the composition of the medium, of the presence of the needles and the lack of scatter. RESULTS: The benefit of the optimization was shown by the determination of a L(t)/L(a) factor of 1.05 instead of 0.7 for the standard PS. The dose distributions calculated by Plato are in agreement with TLD and film measurements for the optimization and the PS (<5%). The TPS results were confirmed by MC calculation as well as by measurements. MC calculations also showed that only the lack of scatter had a significant influence on the dose received by the skin (20%) CONCLUSIONS: The optimization brings a significant benefit in protecting the skin and in homogeneity of the dose distribution in the treated volume. Through MC simulation, this work made it possible to update a parameter significantly influencing dose distribution calculations the lack of scattering.