en Clinical Medicine Clinical Medicine مقاله پژوهشی Original Research Article <p style="text-align: justify;"><span style="font-size:16px;"><span style="font-family:Times New Roman;"><b><span style="background:#2d7f8f"><span style="color:white">Background & Objective:</span></span><b> </b></b><span style="color:black">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.</span><span style="color:black"></span><br> <b><span style="background:#2d7f8f"><span style="color:white"><span style="letter-spacing:-.1pt">&nbsp;Materials & Methods:</span></span></span></b> <span style="color:black"><span style="letter-spacing:-.1pt">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%.</span><span style="letter-spacing:-.1pt"></span></span><br> <b><span lang="EN-IN"><span style="background:#2d7f8f"><span style="color:white">Results: </span></span></span>&nbsp;</b><span style="color:black"><span style="letter-spacing:-.1pt">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.</span><span style="letter-spacing:-.1pt"></span></span><br> <b><span style="background:#2d7f8f"><span style="color:white">Conclusion: </span></span>&nbsp;</b><span style="color:black">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.</span></span></span></p> MammoSite Brachytherapy, Partial Breast Irradiation, Dose Modification Factor, Contrast Agent, Monte Carlo Simulatio 307 314 http://journal.zums.ac.ir/browse.php?a_code=A-10-7160-1&slc_lang=en&sid=1 Atefeh Samadani atefehsamadani75@gmail.com 5200319475328460087460 0009-0003-5118-9771 No Department of Energy Engineering and Physics, Amirkabir University, Tehran, Iran Mojtaba Shamsaei pysham@aut.ac.ir 5200319475328460087461 5200319475328460087461 Yes Department of Energy Engineering and Physics, Amirkabir University, Tehran, Iran Ali Mousavi Zarandi moussavi.zarandi@gmail.com 5200319475328460087462 5200319475328460087462 No Department of Energy Engineering and Physics, Amirkabir University, Tehran, Iran