Analysis of residence time, effective half-life and internal dosimetry prior to radioiodine therapy

Abstract

Radioiodine therapy has been widely used for ablation of remnant tissue after a surgical procedure for treatment of differentiated thyroid carcinoma (DTC). The use of internal dosimetry procedures provides a new approach in choosing the activity to be administered considering the distribution and retention of ¹³¹I individually per patient. This study aims to assess the accumulated activity, internal bone marrow dosimetry and effective half-life of cases undergoing treatment for DTC. This is a quantitative retrospective study with analysis of diagnostic documents and images. The internal dosimetry method used consists of calculating the dose absorbed by the bone marrow per administered activity of ¹³¹I. The calculation of the absorbed dose takes into account the accumulated activity obtained through measurements of whole-body images acquired at 4 intervals over a period of 5 days. Dosimetry presented the values of absorbed dose per administered activity, with a mean of 0.101 mGy/MBq (min.: 0.042 - max.: 0.151). The mean whole-body residence time is equal to 23.1 hours (min: 12.6 – max: 39.4). Effective half-life equal to 16.0 hours (min.: 7.6 and max.: 28.2). Internal dosimetry provides information relevant to safe dose limits for application to DTC radioiodine therapy, especially in advanced cases of the disease where the use of greater activities may be necessary.