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 I 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. 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 I. 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. 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). 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.