Optimized radioiodine therapy of Graves' disease: analysis of the delivered dose and of other possible factors affecting outcome

Abstract

The best approach to radioiodine dose selection in the treatment of Graves' hyperthyroidism remains highly controversial. The formula to calculate the individual dose of 131 I to be delivered has been used for half a century and takes into account the thyroid mass, the effective half-life and the maximum uptake of 131 I. The objective of the present study was to evaluate the accuracy of this formula by determining the relationship between the administered dose of 131 I calculated to deliver a target dose of 50 Gy to the thyroid and the actual exact organ dose. We further analyzed if therapeutic success, defined by euthyroidism following the individually calculated dose, can be predicted by different pretreatment parameters and particularly by organ dose.One hundred patients with a first episode of Graves' disease and who had received optimal thyroid irradiation after precise dosimetry were retrospectively reviewed. The patients were categorized according to their thyroid function (plasma free thyroxine (T 4 ) serum concentration) as eu-, hyperor hypothyroid during and 1 year after treatment. The relationship between the administered dose and organ dose was assessed by simple regression. We compared free T 4 , free tri-iodothyronine, thyroid weight, the number of patients with antithyroperoxidase antibodies and TSH receptor autoantibodies, 24 h urinary iodine excretion, 131 I uptake, and the exact dose of 131 I delivered to the thyroid as pretreatment variables. Although we found a correlation between administered dose (mCi) and organ dose (Gy) (r = 0.3, P = 0.003), the mean coefficient of variation for organ dose was 45%. Individualized radioiodine therapy enabled euthyroidism in 26% of patients and failed in 74% of patients (33% had persistent or recurrent hyperthyroidism and 41% permanent hypothyroidism). 131 I uptake was significantly higher in the hyperthyroidism group in comparison with the euthyroid group. However, organ dose and other pretreatment variables did not differ among the three groups.In conclusion, these results confirm the low performance of individual dosimetry using what are established ratios, since the delivered dose to the gland, although correlated to the intended dose, is highly variable. The finding that other usual pretreatment variables are not different between groups, gives little hope for improving the way of calculating the ideal dose of radioiodine. We suggest to those not yet ready to give a standard or an ablative dose for Graves' hyperthyroidism that they abandon this way to calculate the 131 I dose.

The predictive role of 24h RAIU with respect to the outcome of low fixed dose radioiodine therapy in patients with diffuse toxic goiter

“…13 In their study of radioiodine therapy of diffuse goiter, Catargi et al found that mean uptake of a group of patients that remained hyperthyroid after one year follow-up was significantly higher compared to that of the euthyroid group. 20 The significance of our study is that our patients with diffuse toxic goiter received exactly the same dose. The data, therefore, reflect a homogenous treatment modality.…”

Resumption of methimazole after 131I therapy of hyperthyroid diseases: effect on thyroid function and volume evaluated by a randomized clinical trial

“…Detrimental to any algorithm that includes a 24-h uptake is the great variation in 131 I uptake and turnover over short periods of time (26). In addition, the correlation between the intended and the actual dose delivered to the thyroid is highly variable, with a mean coefficient of variation of 45% (27). It is therefore not surprising that susceptibility of the thyroid to a wide range of 131 I radiation shows huge individual variations (28).…”

The effects of recombinant human thyrotropin, in normal subjects and patients with goitre

“…However, several issues need to be clarified, some of which have already been discussed. It is well known that the kinetics of radioiodine changes markedly during 131 I therapy without the use of rhTSH (Catargi et al, 1999;Traino et al, 2000). It was recently shown that the thyroid RAIU during 131 I therapy, in patients with nodular goitre, was lower than the tracer RAIU, but less pronounced when rhTSH pretreatment was employed (Nieuwlaat et al, 2004).…”