A prospective comparative study of two methods of individual calculation of ¹³¹I activity in the treatment of hyperthyroidism

Radioiodine (¹³¹I) is an established modality of definitive treatment of hyperthyroidism. In spite of the vast experience available, there are still several aspects to be clarified, such as whether fixed or calculated doses should be used. The aim of this study was to assess whether efficacy of this treatment could be improved by implementing a simple dosimetric calculation method including ultrasonographic estimation of thyroid volume and a single measurement of 24-hour ¹³¹I thyroid uptake.

A prospective non-inferiority study was designed to compare two procedures to calculate radioiodine activity: the «semi-fixed» dose method (A), and the «calculated» dose method (B). The first consisted of activity escalation (185MBq steps) based on etiology of hyperthyroidism, ¹³¹I uptake, and treatment objective. The second method was based on the «dosimetric compromise» concept, considering 24-hour uptake and thyroid volume as the only factors and using a standard half-life of 5.5 days. The target absorbed dose was 150Gy, but after a preliminary analysis (first 100 cases) it was increased to 200Gy in diffuse toxic goiters (DTGs).

A total of 212 patients were included. MethodB was at least as effective in terms of final and functional outcome, with a trend to more success and less hypothyroidism. In addition, activities administered were significantly lower.

In radioiodine therapy of hyperthyroidism, a simple dosimetric method that provided results at least equal to those of a fixed dose-based method, with lower administered activities, could be implemented.