Study on the dose modification factor of strut adjusted volume implant (SAVI) with a ¹⁶⁹Yb source using MCNP4C

The SAVI has gained widespread use for accelerated partial breast irradiation (APBI) brachytherapy. Treatments with SAVI produce inherent heterogeneities including variable backscatter due to proximity to the tissue-air interface and variable cavity contents, causing inaccuracy in the dose calculation. In this study, a model of SAVI with sources of ¹⁶⁹Yb developed recently was defined and simulations with MCNP4C code of Monte Carlo were performed through different scenarios to assess the effects of these heterogeneities on the dose distribution. The results showed that the dose delivered to target volume may be lower than the planned dose by up to 9-16%. Therefore, the therapy with¹⁶⁹Yb must be viewed with caution and a correction factor must be applied in treatment planning systems. It was also observed that the presence of air cavity changed the relative dose 1% for a symmetric plan compared to a water cavity and up to 5% for an asymmetric plan. It was indicated that the dose modification factor (DMF) did not differ in any significant way to that of the symmetric plan. The effect of the composition on the DMF was negligible because the natures of water and breast tissue were approximately similar. All the obtained results indicated that should ¹⁶⁹Yb sources be used with SAVI applications, some amendments of treatment planning systems would be employed.