Estimation of the delay for resumption of breastfeeding after 131-iodine therapy by milk radioactivity measurement.

Guidelines recommend stopping breastfeeding before radioactive iodine (131I) therapy, yet the optimal timing for safe resumption remains unclear. We report a detailed dosimetric case study of a 29-year-old woman treated with 3,700 MBq 131I for papillary thyroid carcinoma who wished to resume breastfeeding. Weekly breast milk samples were analyzed using calibrated gamma counting until radioactivity fell below a conservative safety threshold. Results showed substantial iodine excretion in milk (∼15% of administered activity) with biphasic clearance. Estimated infant radiation exposure varied dramatically depending on resumption timing, ranging from 0.62 to 688 mSv. A strict activity-based threshold suggested delaying breastfeeding for 51 days, whereas a dosimetric approach limiting infant exposure to 1 mSv supported resumption as early as 9-10 days post-therapy. These findings highlight the limitations of fixed recommendations and support an individualized, measurement-based strategy to optimize mother and child safety.