| Summary: | Introduction: Some aspects of the treatment protocol for breast cancer during pregnancy (PrBC) have not been thoroughly studied. This study provides clarity regarding the safety of the use of <sup>125</sup>I-seeds as a localization technique for breast-conserving surgery in patients with PrBC. Methods: To calculate the exposure to the fetus of one <sup>125</sup>I-seed implanted in a breast tumor, we developed a model accounting for the decaying <sup>125</sup>I-source, time to surgery, and the declining distance between the <sup>125</sup>I-seed and the fetus. The primary outcome was the maximum cumulative fetal dose of radiation at consecutive gestational ages (GA). Results: The cumulative fetal dose remains below 1 mSv if a single <sup>125</sup>I-seed is implanted at a GA of 26 weeks. After a GA of 26 weeks, the fetal dose can be at a maximum of 11.6 mSv. If surgery takes place within two weeks of implantation from a GA of 26 weeks, and one week above a GA of 32 weeks, the dose remains below 1 mSv. Conclusion: The use of <sup>125</sup>I-seeds is safe in PrBC. The maximum fetal exposure remains well below the threshold of 100 mSv, and therefore, does not lead to an increased risk of fetal tissue damage. Still, we propose keeping the fetal dose as low as possible, preferably below 1 mSv.
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