Influence of breast composition and interseed attenuation in dose calculations for post-implant assessment of permanent breast 103Pd seed implant

The impact of tissue heterogeneity and interseed attenuation is studied in post-implant evaluation of five clinical permanent breast Pd-103 seed implants using the Monte Carlo (MC) dose calculation method. Dose metrics for the target (PTV) as well as an organ at risk (skin) are used to visualize the differences between a TG43-like MC method and more accurate MC methods capable of considering the breast tissue heterogeneity as well as the interseed attenuation. PTV dose is reduced when using a breast tissue model instead of water in MC calculations while the dose to the skin is increased. Furthermore, we investigate the effect of varying the glandular/adipose proportion of the breast tissue on dose distributions. The dose to the PTV (skin) decreases (increases) with the increasing adipose proportion inside the breast. In a complete geometry and compared to a TG43-like situation, the average PTV D-90 reduction varies from 3.9% in a glandular breast to 35.5% when the breast consists entirely of adipose. The skin D-10 increases by 28.2% in an entirely adipose breast. The results of this work show the importance of an accurate and patient-dependent breast tissue model to be used in the dosimetry for this kind of low energy implant.