Maximal Safe Dose Method of I-131 in the Management of Recurrent/Metastatic Differentiated Thyroid Carcinoma

Abstract

 There are several methods of determining I-131 doses for the treatment of recurrent or metastatic differentiated thyroid cancer, including empirical and dosimetric approaches. The most common and simplest method involves the administration of a fixed empirical dose. Doses are determined by disease extent. The merits of using empirical fixed doses are convenience, a long usage history, a reasonably acceptable rate, and a low severe complication rate. However, some investigators found that patients with metastases that persisted after I-131 therapy received significantly lower radiation doses per millicurie of administered I-131. Thus, dosimetric approaches are needed to ensure the proper management of metastatic DTC. To determine the dose in a given case, we need to know how much activity is contained in a lesion. Lesion mass is another parameter that must be determined for calculation. Dose–response relationships differ, which are due to the difficulty in exact measurement of parameters and due to heterogeneity of radioiodine distribution within metastatic lesions. Another approach is to measure maximal safe dose (MSD), which defined the dose delivering 2 Gy (200 rad) to blood (surrogate of bone marrow). MSD is usually higher than the empirical fixed dose, and this therapy is safe in the treatment of residual DTC. This approach provides a better response rate in metastatic DTC than the conventional fixed dose method. Our data suggest that MSD provides an alternative approach when metastases are refractory to conventional fixed dose therapy. A randomized controlled trial is needed to determine the real efficacy of MSD, and comparative studies are required to evaluate treatment regimens and tumor enhancers, such as retinoic acids.