| Sumario: | Background & Objective: Partial Breast Irradiation (PBI) targets the tissue surrounding the tumor in breast cancer brachytherapy. Dose distribution can be affected by the type of radiation source, tumor depth, which alters backscatter, and by the concentration of contrast agents used for imaging. These factors may modify the dose delivered to the targeted area. Both 192Ir and 60Co sources are commonly used in PBI, and understanding how these factors interact is essential for accurate dose delivery and treatment optimization. Dose modification factors (DMFs) can be used to quantify these effects. The objective of this study is to evaluate the effect of tissue heterogeneity on the dose delivered to the 1-cm tumor margin and to calculate a dose modification factor relative to a homogeneous model. Materials & Methods: Monte Carlo simulations were performed using a 30 cm water-equivalent spherical phantom and a 4 cm diameter MammoSite balloon applicator. HDR 60Co and 192Ir sources were placed inside the balloon. DMF was defined as the ratio of the dose rate at 1 cm from the balloon surface under full-scatter conditions to the dose rate when a limited tissue thickness existed beyond the prescription point. Tissue thicknesses ranged from 0 to 10 cm, and contrast agent concentrations varied from 0% to 25%. Results: Dose reductions of approximately 5% for 192Ir and 1% for 60Co were observed when no tissue was present beyond the prescription depth. At 25% contrast concentration, 192Ir showed an additional 4% reduction, while the dose reduction for 60Co remained below 0.5% across all contrast levels. Conclusion: The study emphasizes the importance of accounting for backscatter loss and contrast-induced attenuation, particularly for 192Ir, to prevent underdosing. Applying DMF is essential for accurate treatment planning in surface-adjacent breast tumors undergoing MammoSite brachytherapy.
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