Analysis of Residence Time, Effective Half-Life, and Internal Dosimetry Before Radioiodine Therapy

Radioiodine therapy has been widely used for ablation of remnant tissue after surgical treatment of differentiated thyroid carcinoma (DTC). Internal dosimetry provides a new approach to choosing the administered activity—an approach that considers the distribution and retention of ¹³¹I individually per patient. This study used clinical techniques of internal dosimetry to assess the accumulated activity, internal bone marrow dosimetry, and effective half-life in patients undergoing treatment for DTC. Methods: This was a quantitative, retrospective study analyzing diagnostic documents and images. The internal dosimetry method calculated the dose absorbed by the bone marrow per administered activity of ¹³¹I. Calculation of the absorbed dose took into account the accumulated activity, which was obtained through measurements of whole-body images acquired at 4 intervals over 5 d. Results: The median dose absorbed by the bone marrow per administered activity was 0.117 mGy/MBq (range, 0.043–0.152 mGy/MBq). The median whole-body residence time was 22.0 h (range, 12.6–39.4 h). The median effective half-life was 15.6 h (range, 7.6–28.2 h). Conclusion: Internal dosimetry provides information relevant to safe dose limits for DTC radioiodine therapy, especially in advanced cases of the disease for which greater activities may be necessary.