ALARA in rhTSH-stimulated post-surgical thyroid remnant ablation: what is the lowest reasonably achievable activity?

Abstract

For several years now recombinant human TSH (rhTSH) has been approved by the European Medicines Agency (EMEA) for the preparation of differentiated thyroid carcinoma (DTC) patients for radioiodine (I) ablation of thyroid remnants after surgery. Its effectiveness in patient preparation before I ablation has been shown in multiple prospective studies [1–7]. In this editorial we will review the existing literature on the topic of rhTSH-stimulated I ablation with emphasis on factors that may influence the success rate of ablation, such as the administered activity, the size of the postsurgical thyroid remnant and the amount of stable iodine present in patients at the time of ablation. The available studies in the literature differ greatly in the activities of I used for ablation as well as the stages of patients eligible for inclusion, although most studies have used fairly uniform criteria for successful ablation. The available studies with their key criteria and results are summarized in Table 1. In this table, it can be seen that in those studies using 1.85 GBq (50 mCi) I or more, there is little or no reason to doubt the equivalence of rhTSH to levothyroxine (LT4) withdrawal for the preparation of ablation, while rhTSH stimulation significantly decreases the whole-body radiation exposure [8]. The effectiveness of I ablation using rhTSH, which led to its approval by European and US authorities for this indication, is corroborated by a recent retrospective study of 394 patients in which the short-term clinical recurrence did not differ between subjects prepared by rhTSH or LT4 withdrawal [9]. For the studies using 1.11 GBq (30 mCi) I, the results are more differentiated: whereas the studies by Barbaro et al. [6, 7] did not show a significant difference in the rates of successful ablation between patients prepared with rhTSH and LT4 withdrawal, Pacini et al. [5] did find a considerable difference. Some speculation is possible about the reason for the difference found by Pacini et al.; the most likely explanation is that the administration of the ablative I activity on the second day after the last administration of rhTSH is too late. Based on a large international trial [10] it was recommended in the registration that I should be administered on the first day after the last rhTSH injection. The exact activity that needs to be administered in order to achieve successful ablation is still a subject of debate [11]. There are only a few studies comparing the success rate of ablation during classic LT4 withdrawal and none that compare different activities under rhTSH stimulation. Those studies which are available, such as the one by Bal et al. [12], seem to indicate that there is no real increase in ablation success rates for activities exceeding 1.85 GBq (50 mCi), albeit under classic withdrawal conditions. D. Barbaro Endocrinology Unit, General Hospital of Livorno, Livorno, Italy