Improved dose concept for radioiodine therapy of multifocal and disseminated functional thyroid autonomy

Influence of dose rate on therapeutic success of radioiodine therapy of functional thyroid autonomy

OBJECTIVE

To evaluate the effect of dose rate on the therapeutic success of radioiodine therapy (RIT) by retrospective analysis of two groups of patients with different dose rates during RIT.

METHODS

The average dose rate until deposition of 50% of the total radiation dose achieved (the DL50 value) was calculated. Ninety patients with autonomy of the thyroid receiving radiation doses between 300 and 400 Gy were separated in groups with high DL50 values (the DL50 H group) and low DL50 values (the DL50L group). Before and 4 months after RIT free triiodothyronine (FT3), free thyroxine (FT4), basal thyroid stimulating hormone (TSHB), stimulated TSH (TSHS), thyroidal technetium uptake (TcTU), and the existence of focal abnormalities in thyroid scintigraphy were evaluated.

RESULTS

The DL50 H and DL50L groups showed no differences before RIT in the parameters assessed as well as in total achieved dose and autonomous thyroid volume. After therapy a significant difference was found in TSHB with higher values in DL50 H (1.89+/-1.49 vs. 1.31+/-1.48 mU . l, P<0.05), FT4 showed a trend to lower values (13.8+/-4.0 vs. 16.1+/-6.2 pmol . l, P=0.09).

CONCLUSION

The DL50 value has an influence on therapeutic success of RIT. This influence is only discrete and not likely to produce clinically relevant effects in the practical use of RIT.

The route of administration (oral vs intravenous) does not influence dose or outcome in Graves’ disease and unifocal autonomy

PURPOSE

In a prospective randomised study, we investigated the influence of the route of administration of radioiodide on dosimetry and therapy outcome.

METHODS

Fifty-four patients suffering from Graves' disease (GD) and 60 patients with unifocal autonomy (UA) participated in the study and were randomly treated with either orally or intravenously administered radioiodide. Pretherapeutic dosimetry was based on single uptake measurements with a calibrated uptake probe system. The radioiodine kinetics during hospitalisation was assessed by daily bedside uptake measurements. Therapeutic dose was determined by half-life and thyroid uptake at the time of discharge using the same uptake probe as for the radioiodine test.

RESULTS

No improvement in accuracy of dosimetry was achieved when radioiodide was administered intravenously. Mean therapeutic doses were identical following intravenous or oral administration. Variation in the achieved dose was slightly higher in the patients receiving oral administration, this being attributable to larger deviations in discrete activities of the capsules administered as compared with the values determined by dosimetry. No differences according to treatment modality were found with regard to therapeutic outcome. Eighty-seven patients attended 6-month follow-up after therapy. In the UA group, successful treatment, defined as a normal or elevated TSH level, was observed in 94% of patients after oral administration and in 80% after intravenous administration; corresponding figures in the GD group were 68% and 65%.

CONCLUSION

The causes of individual differences between targeted and therapeutically achieved doses remain undetermined. Variations in the bioavailability of radioiodide or other parameters affecting thyroid status may be involved, and further investigations are needed to clarify this.

Disseminated thyroid autonomy or Graves' disease: reevaluation by a second generation TSH receptor antibody assay

The clinical diagnosis of disseminated autonomy (DISA) can only be established by exclusion of Graves' disease (GD). Both hyperthyroid conditions share the same scintigraphic appearance and can only be distinguished from each other clinically either by the presence or absence of endocrine ophthalmopathy (EO) or thyrotropin (TSH) binding inhibiting immunoglobulins (TBIIs). The purpose of this study was the reevaluation of thyroid autonomies originally classified as DISAs by a second-generation radioreceptor antibody assay (RAA) (DYNOtest TRAKhuman) (B.R.A.H.M.S. Diagnostika, Berlin, Germany). The analysis included 32 patients (female: n = 25, male: n = 7; mean age: 46 +/- 18 years) who were initially diagnosed with DISA. All patients were TSH receptor (TSHR) antibody (TRAb) negative by a conventional radioimmunoassay (RIA) (TSH-REZAK RIA) (Medipan Diagnostica, Selchow, Germany) during their first evaluation. The presence of EO was excluded by clinical signs in all patients. Surgery had been performed prior to our evaluation in 5 patients and after our survey in 1 patient. Four patients had been treated previously with 131I. Ten patients were treated with thionamides during our evaluation, and 13 had not been treated before. One hundred three patients who had either healthy thyroids, nontoxic goiters, or focal autonomies served as controls and were evaluated both by the TSH-REZAK assay and the DYNOtest TRAKhuman assay. Seven of thirty-two (22%) patients originally classified as DISA were TRAb positive in the second-generation assay. In this group, 5 of 7 patients had a total thyroid volume (TTV) <30 mL (positive predictive value [PPW] for TRAb positivity 71%), and 5 of 7 patients had a diffuse goiter (PPW for TRAb-positivity 71%). Six of seven patients were anti-thyroperoxidase (TPO) positive (PPW for TRAb positivity 85%). A hypoechoid pattern on ultrasound was present by visual analysis in 3 of 7 patients (PPW for TRAb positivity 43%). A 100% PPW for TRAb positivity could be obtained if a goiter <30 ml was combined with anti-TPO positivity, but this combination was present in only 4 of 7 (57%) patients. With the second-generation assay, one false positive test result was observed in the control group. Surgery was performed in 6 patients who were TRAb negative in both assays. In all these cases, the histologic findings were compatible with autonomous transformation of the thyroid. Our study demonstrates that a significant number (22%) of patients formerly classified as DISA may actually have GD. However, DISA still exists as a clinical entity, and its pathophysiological link to multifocal and unifocal autonomy should be further investigated.

[Radioiodine therapy of functional autonomy of the thyroid gland. Treatment results in view of pretreatment scintigraphic diagnosis and early response of triiodothyronine levels to treatment]

AIM

The goal of this retrospective study was to evaluate the results of radioiodine therapy of thyroid autonomy with respect to the underlying scintigraphic pattern, administered I-131 doses and pretherapeutic Tc-99m and I-131 uptake. Furthermore, early post-therapeutic free triiodothyronine (FT3) levels were measured during the first 6 weeks after radioiodine therapy with respect to a FT3-increase.

PATIENTS AND METHODS

Thyroid autonomy in 632 patients was followed for at least 3 months (mean 10.9 months). The patients were divided into several groups based on scintigraphic pattern of unifocal (UA), a multifocal (MFA), a focal/disseminated (FDA) and a disseminated (DA) autonomy. The early effects of therapy on thyroid function were assessed by evaluation of FT3 values of 786 patients during the 1st, 2nd, 3rd, 4th and 6th week after therapy.

RESULTS

Successful elimination of thyroid autonomy as defined by normalization of FT3 and TSH levels was observed in 92.1% (582/632) of the patients. In 2.2% (14/632) of the patients, immunogenic hyperthyroidism following I-131 therapy was observed. There was no significant difference in the success rates in patients with UA and MFA of 94.9% and 96.1%, respectively, using similar target doses based on the amount of autonomous tissue. The success rate in MFA (96.6%) was similar if target dose was calculated based on the volume of the whole thyroid gland. Compared to MFA (96.6%), however, FDA and DA were associated with significantly lower success rate with 82.7% and 75.5%, respectively, although the target doses were not significantly different. Considering the quantitative Tc-99m uptake prior to therapy, there was no significant difference in the success rate for Tc-99m uptake < or = 3% and > 3%, while the success rate in patients with I-131 uptake of < or = 50% was significantly higher (p = 0.032) than in those with an uptake of > 50%. The determination of FT3 levels during the first 6 weeks after radioiodine therapy revealed a dependence of the FT3 decrease and FT3 increase on the scintigraphic pattern and thyroid function. Patients with FDA and DA with hyperthyroidism showed an increase of FT3 (> 2 to 14.7 pmol/l) in 11 to 18% of the cases, during the first 2 weeks after therapy which occurred significantly more frequently in patients with FDA and DA than in UA and MFA. A similar increase in FT3 level in patients with FDA and DA compared to patients with UA and MFA was observed in 317 patients with euthyroid pretherapeutic hormone levels. A decrease in FT3 level following radioiodine therapy was observed significantly more often and earlier in patients with UA and MFA with hyperthyroid values than in patients with FDA and DA during the first 6 posttherapeutic weeks. Concerning patients with euthyroid FT3 levels, the decrease in FT3 level was observed significantly more frequently in patients with UA and MFA only after 6 weeks following radioiodine therapy.

CONCLUSION

In contrast to the multifocal autonomy (MFA), the target dose of 150 to 200 Gy based on total thyroid volume did not result in a comparably high success rate of approximately 95% in disseminated and focal/disseminated types of thyroid autonomy. Therefore, an increase of target dose of 200 to 300 Gy is recommended. The transient FT3 increase particularly observed in FDA and DA in the first weeks following radioiodine therapy makes short-term controls of thyroid function necessary, especially in patients with cardiac risk, in order to initiate necessary therapy.

An update on diagnostic methods in the investigation of diseases of the thyroid

Iodine deficiency and iodine-deficiency disorders continue to be problems in several parts of Europe, requiring further improvements in the techniques employed in thyroid diagnosis, and particularly in the early diagnosis and risk assessment of autonomously functioning thyroid tissue. For the latter purpose, scintigraphy with technetium-99m pertechnetate under exogenous or endogenous thyroid-stimulating hormone (TSH) suppression provides the best results. Significant methodological improvements in laboratory tests have resulted from the application of new luminescent techniques and gene technology to thyroid function tests. Especially TSH measurement using second- or third-generation assays ensures diagnostic accuracy, so that the thyrotropin-releasing hormone (TRH) test is now almost always unnecessary. The differentiation of blocking and stimulating TSH receptor antibodies is relevant when discrepant results are obtained with respect to thyroid function. Determination of glycosaminoglycans in urine may become a helpful tool in the follow-up of endocrine ophthalmopathy. Some new imaging agents have recently been applied in the scintigraphy of thyroid diseases, such as octreotide, or in thyroid diagnosis, such as fluorodeoxyglucose. Both improve the detectability of metastases of thyroid cancer, especially if the radioiodine scan is negative.