Dose selection for radioiodine therapy of borderline hyperthyroid patients with multifocal and disseminated autonomy on the basis of 99mTc-pertechnetate thyroid uptake

[Guideline for Radioiodine Therapy for Benign Thyroid Diseases (6/2022 - AWMF No. 031-003)]

This version of the guideline for radioiodine therapy of benign thyroid disorders is an update of the version, which was published by the German Society of Nuclear Medicine (Deutsche Gesellschaft für Nuklearmedizin, DGN) in co-ordination with the German Society of Endocrinology (Deutsche Gesellschaft für Endokrinologie, DGE, Sektion Schilddrüse) and the German Society of General- and Visceral-Surgery (Deutsche Gesellschaft für Allgemein- und Viszeralchirurgie, DGAV) in 2015. This guideline was harmonized with the recommendations of the European Association of Nuclear Medicine (EANM). According to the German "Directive on Radiation Protection in Medicine" the physician specialised in nuclear medicine ("Fachkunde in der Therapie mit offenen radioaktiven Stoffen") is responsible for the justification to treat with radioiodine. Therefore, relevant medical indications for radioiodine therapy and alternative therapeutic options are discussed within the guideline. This procedure guideline is developed in the consensus of an expert group. This fulfils the level S1 (first step) within the German classification of Clinical Practice Guidelines.

The EANM guideline on radioiodine therapy of benign thyroid disease

This document provides the new EANM guideline on radioiodine therapy of benign thyroid disease. Its aim is to guide nuclear medicine physicians, endocrinologists, and practitioners in the selection of patients for radioiodine therapy. Its recommendations on patients' preparation, empiric and dosimetric therapeutic approaches, applied radioiodine activity, radiation protection requirements, and patients follow-up after administration of radioiodine therapy are extensively discussed.

Empiric radioiodine for hyperthyroidism: Outcomes, prescribing patterns, and its place in the modern era of theranostics

BACKGROUND

The modern era of radioiodine (I-131) theranostics for metastatic differentiated thyroid cancer requires us to rationalize the role of traditional empiric prescription in nonmalignant thyroid disease. We currently practice empiric I-131 prescription for treatment of hyperthyroidism. This study aims to assess outcomes after treatment of hyperthyroidism by empiric I-131 prescription at our centre, evaluate factors that impact on outcomes and prescribing practice, and gain insight into whether there is a place for theranostically-guided prescription in hyperthyroidism.

PATIENTS AND METHODS

A retrospective review was undertaken of all patients with Graves' disease, toxic multinodular goitre (MNG) and toxic adenoma treated with I-131 between 2016 and 2021. Associations between clinical or scintigraphic variables and remission (euthyroid or hypothyroid) or persistence of hyperthyroidism at follow-up were performed using standard t test as well as Pearson's product correlation.

RESULTS

Of 146 patients with a mean follow-up of 13.6 months, 80.8% achieved remission of hyperthyroidism. This was highest in toxic nodules (90.1%), compared with Graves' disease (73.8%) and toxic MNG (75.5%). In patients with Graves' disease, higher administered activity was associated with remission (p = .035). There was a weak inverse correlation between the Tc-99m pertechnetate uptake vs prescribed activity in Graves' disease (r = -0.33; p = .009). Only one patient (0.7%) had an I-131 induced flare of thyrotoxicosis.

CONCLUSION

Traditional empiric I-131 prescription is a safe and effective treatment of hyperthyroidism and suitable for most patients. However, there may be a role for personalized I-131 prescription by theranostic guidance in selected patients with high thyroid hyperactivity.

Thyroid functional and molecular imaging

Radioiodine uptake (RAIU) test with iodine-123 (Na[¹²³I]I) or iodine-131 (Na[¹³¹I]I) enables accurate evaluation and quantification of iodine uptake and kinetics within thyroid cells. Thyroid Scintigraphy (TS) employing Na[¹²³I]I or ^99mTc-pertechnetate (Na[^99mTc]TcO₄) provides information regarding the function and topographical distribution of thyroid cells activity, including detection and localization of ectopic thyroid tissue. Destructive thyrotoxicosis is characterized by low RAIU with scintigraphically reduced radiotracer activity in the thyroid tissue, while productive thyrotoxicosis (i.e. hyperthyroidism "stricto sensu") is characterized by high RAIU with scintigraphically diffuse (i.e. Graves' Disease, GD and diffuse thyroid autonomy) or focal (i.e. autonomously functioning thyroid nodules, AFTN) overactivity. Accordingly, RAIU and/or TS are widely used to differentiate different causes of thyrotoxicosis. In addition, several radiopharmaceuticals are also available to help differentiate benign from malignant thyroid nodules and inform clinical decision-making: scintigraphic identification of AFTNs obviate fine-needle aspiration (FNA) biopsy, and [^99mTc]Tc-hexakis-(2‑methoxy-2-isobutyl isonitrile ([^99mTc]Tc-MIBI) and/or ¹⁸F-fluoro-d-glucose ([¹⁸F]FDG) may complement the work-up of cytologically indeterminate "cold" nodules for reducing the need for diagnostic lobectomies/thyroidectomies. Finally, RAIU studies are also useful for calculating the administered therapeutic activity of Na[¹³¹I]I to treat hyperthyroidism and euthyroid multinodular goiter. All considered, thyroid molecular imaging allows functional characterization of different thyroid diseases, even before clinical symptoms become manifest, and remains integral to the management of such conditions. Our present paper summarizes basic concepts, clinical applications, and potential developments of thyroid molecular imaging in patients affected by thyrotoxicosis and thyroid nodules.

Molecular Imaging for Thyrotoxicosis and Thyroid Nodules

After exclusion of exogenous iodine overload, radioiodine uptake (RAIU) testing with ¹²³I or ¹³¹I enables the accurate evaluation and quantification of iodine uptake and kinetics within thyroid cells. In addition, scintigraphic evaluation with ¹²³I or ^99mTc-pertechnetate (^99mTc04-) provides the topographic distribution of thyroid cell activity and allows the detection and localization of ectopic thyroid tissue. Destructive thyrotoxicosis is characterized by abolished or reduced uptake whereas productive thyrotoxicosis (i.e., hyperthyroidism "sensu strictu") is characterized by high RAIU with scintigraphically diffuse (i.e., Graves disease and diffuse thyroid autonomy) or focal (i.e., autonomously functioning thyroid nodules [AFTN]) overactivity. Accordingly, RAIU or thyroid scintigraphy are widely used to differentiate different causes of thyrotoxicosis. In addition, several radiopharmaceuticals are also available to help in differentiating benign from malignant thyroid nodules and inform clinical decision making. In fact, AFTNs can be safely excluded from fine-needle aspiration biopsy while either ^99mTc-methoxyisobutylisonitrile (MIBI) and ¹⁸F-FDG may complement the work-up of cytologically indeterminate cold nodules and contribute to reducing the need for diagnostic lobectomies/thyroidectomies. Finally, RAIU studies are also useful for calculating the administered therapeutic activity of ¹³¹I to treat hyperthyroidism and euthyroid multinodular goiter. All considered, thyroid molecular imaging allows us to characterize molecular/functional aspects of different thyroid diseases, even before clinical symptoms become manifest and remains integral to properly managing such conditions. Our present paper summarizes basic concepts, clinical applications, and potential developments of thyroid molecular imaging in patients affected by thyrotoxicosis and thyroid nodules.

Radioiodine therapy of thyroid autonomy

Radioiodine therapy (RIT) of thyroid functional autonomy (TFA) is rapidly evolving, though it has been recognized for decades as a very effective treatment of toxic nodular varieties. Indeed, TFA is a frequent cause of persistent subclinical hyperthyroidism, which should be regarded as a new metabolic syndrome, with well-established adverse cardio-vascular consequences. Sensitive TSH assays and multiparametric ultrasounds are not accurate enough to reliably diagnose TFA and identify its main variants, unifocal, multifocal (UFA/MFA) and disseminated autonomy (DISA). Modern diagnostic tools are extensively presented and rely upon Thyroid Scan imaging and quantification. A new relationship allows predicting at baseline, an excess of 123I uptake as compared to the TSH stimulation in compensated TFA. Suppressed TS are useful with either isotope, otherwise. Diagnosis of the DISA variant is presented as compared to Graves' disease. Dosimetry has some specificity in TFA work-up. Indeed, the spatial distribution of the dose is as important as the mean value itself and can be eventually controlled by adjusting the TSH level with the smart use of LT3 or antithyroid drug therapy (ATD). A review of the different ways to determine the target mass from anatomical to functional approaches is presented. Main clinical and dosimetric published results of RIT are summarized according to clinical goals. Endogenous TSH stimulation using an ATD preparation has promising results in reducing big autonomously functioning goiters. Finally, we report preliminary successful results of preventive RIT using short term LT3 suppression in compensated TFA, with low administered activities and low rate of hypothyroidism.

A thermoluminescent method for the evaluation of the 131I effective half-life in the thyroid when treating Graves' disease

When planning treatment for Graves' disease with ¹³¹I, the effective half-life (T_eff) should be estimated individually as it depends on biological characteristics such as iodine uptake and excretion, which differ from an individual to another (Berg et al. 1996). All the methods to quantify T_eff described in the literature are quite complex and are difficult to be used in clinical routine. With the aim of optimizing this process, a simplified method is proposed here to evaluate T_eff of ¹³¹I during treatment of Graves' disease. The present study suggests improving the method of determining T_eff based on thermoluminescence dosimetry. This involves implementing a new method and includes reduction of TLD (Thermoluminescent Dosimeter) measurements. The proposed method was validated on patients with Graves' disease. The radiation dose delivered to the patients was determined using the MIRD (Medical Internal Radiation Dosimetry) formalism. The relative difference between T_eff obtained based on seven measurement intervals at [0-24 h, 24-48 h, 48-72 h, 72-96 h, 96-120 h, 120-144 h, 144-168 h] and based on three measurement intervals at [0-24 h, 72-96 h, 144-168 h] and [0-24 h, 120-144 h, 144-168 h] was 1.9% and 3.81%, respectively. Comparison of doses obtained based on a general T_eff and on a personalized T_eff gave a statistically significant difference with a correlation coefficient R²of 0.44. The T_eff obtained from just three measurements was found to be sufficiently accurate and easily applicable. The results obtained demonstrate the need to determine and use personalized T_eff values instead of using a fixed value of 7 days.

[Thyroid medicine for ENT physicians]

Diseases of the thyroid gland are frequent incidental findings during ultrasound examination of the neck. They affect nearly one third of the normal population. Treatment is not always indicated; however, laboratory diagnostic measures must be initiated to specify the disease. The primary indications for consulting a thyroid specialist are thyroid nodules, goiters, autonomy of the thyroid gland, autoimmune diseases, Graves' disease, and Hashimoto thyroiditis. The aim of this review is to provide an overview of the most important thyroid diseases and their treatment options.

Impact of different approaches to calculation of treatment activities on achieved doses in radioiodine therapy of benign thyroid diseases

Purpose

Radioiodine has been used for the treatment of benign thyroid diseases for over 70 years. However, internationally, there is no common standard for pretherapeutic dosimetry to optimally define the individual therapy activity. Here, we analyze how absorbed tissue doses are influenced by different approaches to pretherapeutic activity calculation of varying complexity.

Methods

Pretherapeutic determination of treatment activity was retrospectively recalculated in 666 patients who had undergone radioiodine therapy for benign thyroid diseases (Graves’ disease, non-toxic goiter, and uni- and multinodular goiter). Approaches considering none, some, or all of a set of individual factors, including target volume, maximum radioiodine uptake, and effective half-life, were applied. Assuming individually stable radioiodine kinetics, which had been monitored twice a day under therapy, hypothetically achieved tissue doses based on hypothetically administered activities resulting from the different methods of activity calculation were compared to intended target doses.

Results

The Marinelli formula yields the smallest deviations of hypothetically achieved doses from intended target doses. Approaches taking individual target volume into consideration perform better than fixed therapy activities, which lead to high variances in achieved doses and high deviations of hypothetically achieved doses from intended target doses.

Conclusion

Elaborate pretherapeutic dose planning, taking individual radioiodine uptake, half-life, and target volume into consideration, should be used whenever possible. The use of disease-specific fixed activities cannot be recommended. Deviations of achieved tissue doses from target doses can already be significantly lowered by application of volume-adapted treatment activities if more elaborate means are not available.

[Radioiodine therapy for benign thyroid diseases (version 5). German Guideline]

The version 5 of the guideline for radioiodine therapy of benign thyroid disorders is an update of the version 4, which was published by the German Society of Nuclear Medicine (Deutsche Gesellschaft für Nuklearmedizin, DGN) in co-ordination with the German Society of Endocrinology (Deutsche Gesellschaft für Endokrinologie, DGE, Sektion Schilddrüse) and the German Society of General- and Visceral-Surgery (Deutsche Gesellschaft für Allgemein- und Viszeralchirurgie, DGAV) in 2007. This guideline was harmonized with the recommendations of the European Association of Nuclear Medicine (EANM). According to the German "Directive on Radiation Protection in Medicine" the physician specialised in nuclear medicine ("Fachkunde in der Therapie mit offenen radioaktiven Stoffen") is responsible for the justfication to treat with radioiodine. Therefore, relevant medical indications for radioiodine therapy and alternative therapeutic options are discussed within the guideline. This procedure guideline is developed in the consensus of a representative expert group. This fulfils the level S1 (first step) within the German classification of Clinical Practice Guidelines.

Imaging the thyroid in children

Color Doppler Ultrasounds (CDU) and Thyroid Scanning (TS) have much improved in recent years and offer a likely diagnosis of the disorder and its main subtypes. This especially applies when diagnosing permanent or transient causes of congenital hypothyroidism (CH), where dual imaging has proven to be more informative than single scanning. Though both isotopes have acceptable performances, the use of (123)I appears more advisable, since it more accurately identifies the various aetiologies of CH and probably has better dosimetric characteristics than (99m)Tc. Detailed dual imaging patterns are presented in connection with most of the underlying mechanisms explaining CH, thyroid dysgenesis (75%) and dyshormonogenesis (20%). Imaging of thyroid autoimmunity, of immunogenic thyrotoxicosis and of thyroid autonomy, is helped by CDU but most often requires a quantified (123)I-TS (molecular imaging). We finally show the interest of CDU to sort suspicious nodule and present the new TIRADS scoring system.

Radioiodine therapy for Graves disease: thyroid absorbed dose of 300 Gy-tuning the target for therapy planning

PURPOSES

Based on the committed thyroid absorbed dose, the aim was to compare the efficiency of I therapy against Graves disease (GD) within 1 year after treatment and, by exploring the dose-response relationship, indicate an absorbed dose to be targeted into patient therapeutic planning.

METHODS

Thyroid-absorbed doses were calculated to 196 patients with GD by applying Medical Internal Radiation Dose formalism and taking into account administered I activity, thyroid radioiodine uptake, effective half-life, and gland tissue mass. Statistical analysis was applied to assess the relationship between absorbed doses and the patient's clinical response.

RESULTS

Overall, successful therapy was achieved in 167 patients, whereas in 29 the disease persisted, even though 64.8% and 89.3% of all the treated patients had received, respectively, thyroid absorbed dose and activity superior to 300 Gy and 11.1 MBq/g (300 μCi/g) of thyroid tissue. Among those in whom the disease persisted, 24 (83%) had a 6- to 24-hour I uptake ratio equal or superior to 0.9, whereas only 5 (17%) presented a lower ratio. According to statistical analysis, there was no difference in cure rate between the groups that received 300 Gy or less and that which received more (84.1% vs 85.8%, P = 0.8336).

CONCLUSIONS

A thyroid absorbed dose of 300 Gy is plausible as a targeted therapeutic dose in GD therapy planning, because statistical analysis has proven there to be no advantage in treating patients with doses above this level. On the other hand, numerous efforts should be made to develop an optimized and easily applicable protocol of patient-specific dosimetry and to provide data that show its clinical impact on patient management.

Radioiodine therapy in benign thyroid diseases: effects, side effects, and factors affecting therapeutic outcome

Radioiodine ((131)I) therapy of benign thyroid diseases was introduced 70 yr ago, and the patients treated since then are probably numbered in the millions. Fifty to 90% of hyperthyroid patients are cured within 1 yr after (131)I therapy. With longer follow-up, permanent hypothyroidism seems inevitable in Graves' disease, whereas this risk is much lower when treating toxic nodular goiter. The side effect causing most concern is the potential induction of ophthalmopathy in predisposed individuals. The response to (131)I therapy is to some extent related to the radiation dose. However, calculation of an exact thyroid dose is error-prone due to imprecise measurement of the (131)I biokinetics, and the importance of internal dosimetric factors, such as the thyroid follicle size, is probably underestimated. Besides these obstacles, several potential confounders interfere with the efficacy of (131)I therapy, and they may even interact mutually and counteract each other. Numerous studies have evaluated the effect of (131)I therapy, but results have been conflicting due to differences in design, sample size, patient selection, and dose calculation. It seems clear that no single factor reliably predicts the outcome from (131)I therapy. The individual radiosensitivity, still poorly defined and impossible to quantify, may be a major determinant of the outcome from (131)I therapy. Above all, the impact of (131)I therapy relies on the iodine-concentrating ability of the thyroid gland. The thyroid (131)I uptake (or retention) can be stimulated in several ways, including dietary iodine restriction and use of lithium. In particular, recombinant human thyrotropin has gained interest because this compound significantly amplifies the effect of (131)I therapy in patients with nontoxic nodular goiter.

Development of hypothyroidism during long-term follow-up of patients with toxic nodular goitre after radioiodine therapy

OBJECTIVE

To investigate the cure rate and incidence of hypothyroidism of radioiodine treatment with a calculated dose regimen and an intended thyroid dose of 150 Gy in patients with toxic nodular goitre during long-term follow-up.

PATIENTS

A total of 265 consecutive patients with toxic nodular goitre were treated between March 2003 and August 2004 at our institute and followed up for a maximum of 8 years. Preliminary radioiodine testing with volumetric measurement of the thyroid by ultrasound as well as individual thyroidal radioiodine uptake and half-life measurements were performed before radioiodine therapy. The estimated radiation dose to the thyroid was 150 Gy.

MEASUREMENTS

Follow-up controls with respect to success of therapy and development of hypothyroidism were performed 3 months, 1 and up to 8 years after radioiodine treatment. The relation of the achieved thyroid dose to the success rate of treatment and to the incidence of hypothyroidism was analysed.

RESULTS

The cure rates were 85% at 3 months, 98% at 1 year and 98% at the end of follow-up. Above an achieved thyroid dose of more than 120 Gy, there was no significant association between the dose achieved in the thyroid and the cure rate on follow-up. The incidences of hypothyroidism at 3 months, at 1 year and at the end of follow-up were 32%, 55% and 73%, respectively.

CONCLUSIONS

Radioiodine treatment with a calculated dose regimen is a highly effective treatment option in patients with toxic goitre with an overall success rate of 98%. However, radioiodine treatment with an intended thyroid dose of 150 Gy leads to a high incidence of hypothyroidism on long-term follow-up. This finding supports the suggestion that in future intended thyroid doses could be lowered in patients treated with a calculated dose regimen for toxic nodular goitre.

GATE based Monte Carlo simulation of planar scintigraphy to estimate the nodular dose in radioiodine therapy for autonomous thyroid adenoma

The recommended target dose in radioiodine therapy of solitary hyperfunctioning thyroid nodules is 300-400Gy and therefore higher than in other radiotherapies. This is due to the fact that an unknown, yet significant portion of the activity is stored in extranodular areas but is neglected in the calculatory dosimetry. We investigate the feasibility of determining the ratio of nodular and extranodular activity concentrations (uptakes) from post-therapeutically acquired planar scintigrams with Monte Carlo simulations in GATE. The geometry of a gamma camera with a high energy collimator was emulated in GATE (Version 5). A geometrical thyroid-neck phantom (GP) and the ICRP reference voxel phantoms "Adult Female" (AF, 16ml thyroid) and "Adult Male" (AM, 19ml thyroid) were used as source regions. Nodules of 1ml and 3ml volume were placed in the phantoms. For each phantom and each nodule 200 scintigraphic acquisitions were simulated. Uptake ratios of nodule and rest of thyroid ranging from 1 to 20 could be created by summation. Quantitative image analysis was performed by investigating the number of simulated counts in regions of interest (ROIs). ROIs were created by perpendicular projection of the phantom onto the camera plane to avoid a user dependant bias. The ratio of count densities in ROIs over the nodule and over the contralateral lobe, which should be least affected by nodular activity, was taken to be the best available measure for the uptake ratios. However, the predefined uptake ratios are underestimated by these count density ratios: For an uptake ratio of 20 the count ratios range from 4.5 (AF, 1ml nodule) to 15.3 (AM, 3ml nodule). Furthermore, the contralateral ROI is more strongly affected by nodular activity than expected: For an uptake ratio of 20 between nodule and rest of thyroid up to 29% of total counts in the ROI over the contralateral lobe are caused by decays in the nodule (AF 3 ml). In the case of the 1ml nodules this effect is smaller: 9-11% (AF) respectively 7-8% (AM). For each phantom, the dependency of count density ratios upon uptake ratios can be modeled well by both linear and quadratic regression (quadratic: r(2)>0.99), yielding sets of parameters which in reverse allow the computation of uptake ratios (and thus dose) from count density ratios. A single regression model obtained by fitting the data of all simulations simultaneously did not provide satisfactory results except for GP, while underestimating the true uptake ratios in AF and overestimating them in AM. The scintigraphic count density ratios depend upon the uptake ratios between nodule and rest of thyroid, upon their volumes, and their respective position in a non-trivial way. Further investigations are required to derive a comprehensive rule to calculate the uptake or dose ratios based on post-therapeutic scintigraphy.

Radioiodine therapy in hyperthyroid disease: poorer outcome in patients with high 24 hours radioiodine uptake

PURPOSE

To evaluate the importance of 24 h radioiodine uptake (24 h RIU) for the outcome of radioiodine treatment of hyperthyroidism.

METHODS

Retrospective analysis of 72 patients who underwent radioiodine treatment for toxic goiter at our outpatient clinic [29 diffuse goiters (DG), 30 toxic multinodular goiters (TMG) and 13 toxic adenomas (TA)]. Thyroid status was determined by TSH, fT3 and fT4 levels, and outcome was rendered successful when hyperthyroidism was absent. Relation between low 24 h RIU (below median) or high 24 h RIU (above or equal to median) and outcome was evaluated.

RESULTS

Of patients with DG and low 24 h RIU, 15% remained hyperthyroid, as opposed to 56% of patients with DG and high 24 h RIU (P<0.05). Of patients with TMG and low 24 h RIU, none remained hyperthyroid, as opposed to 44% of patients with TMG and high 24 h RIU (P<0.01). Of patients with TA and low 24 h RIU, none remained hyperthyroid, as opposed to 43% of patients with TA and high 24 h RIU (NS, P = 0.19).

CONCLUSION

In patients with hyperthyroid disease treated with radioiodine the outcome is poorer for patients with high 24 h RIU compared with low 24 h RIU measured prior to treatment when the radioiodine dose is calculated on the basis of 24 h RIU.

Dose selection for radioiodine therapy of borderline hyperthyroid patients according to thyroid uptake of 99mTc-pertechnetate: applicability to unifocal thyroid autonomy?

PURPOSE

The aim of this study was to evaluate the feasibility of applying a previously described dose strategy based on (99m)Tc-pertechnetate thyroid uptake under thyrotropin suppression (TcTU(s)) to radioiodine therapy for unifocal thyroid autonomy.

METHODS

A total of 425 consecutive patients (302 females, 123 males; age 63.1+/-10.3 years) with unifocal thyroid autonomy were treated at three different centres with (131)I, using Marinelli's formula for calculation of three different absorbed dose schedules: 100-300 Gy to the total thyroid volume according to the pre-treatment TcTU(s) (n=146), 300 Gy to the nodule volume (n=137) and 400 Gy to the nodule volume (n=142).

RESULTS

Successful elimination of functional thyroid autonomy with either euthyroidism or hypothyroidism occurred at a mean of 12 months after radioiodine therapy in 94.5% of patients receiving 100-300 Gy to the thyroid volume, in 89.8% of patients receiving 300 Gy to the nodule volume and in 94.4% receiving 400 Gy to the nodule volume. Reduction in thyroid volume was highest for the 100-300 Gy per thyroid and 400 Gy per nodule strategies (36+/-19% and 38+/-20%, respectively) and significantly lower for the 300 Gy per nodule strategy (28+/-16%; p<0.01).

CONCLUSION

A dose strategy based on the TcTU(s) can be used independently of the scintigraphic pattern of functional autonomous tissue in the thyroid.

Potential third-party radiation exposure from outpatients treated with I131 for hyperthyroidism

Thirty-three hyperthyroid patients treated with radioiodine (mean administered activity 414 MBq, range 163-555) were studied to determine if pretreatment dosimetry could be used to give radiation protection advice that could assure compliance with the effective dose constraints suggested by the European Commission. Effective doses to travelers, co-workers, and sleeping partners were estimated by integrating the effective dose rate-versus-time curve obtained by fitting the dose rates measured several times after radioiodine administration to a biexponential function. The mean estimated effective doses to travelers, co-workers, and sleeping partners were 0.11 mSv (0.05-0.24), 0.24 mSv (0.07-0.52), and 1.8 mSv (0.6-4.1), respectively. The best correlation was found between effective dose (D) in mSv and maximum activity (AUmax) in MBq taken up in the thyroid: Dtraveler=0.0005 * (AUmax) +0.04 (r=0.88,p< 0.01); Dco-worker=0.0013 * (A Umax) +0.03 (r=0.89,p < 0.01); Dsleeping partners=0.0105 * (AUmax)+0.16 (r=0.93,p < 0.01). Private/public transports are always allowed. For the co-workers the effective dose constraint of 0.3 mSv is met without restrictions and with 3 days off work if AUmax is lower or higher than 185 MBq, respectively. For the sleeping partners the effective dose constraint of 3 mSv is met without restriction and with 4 nights separate sleeping arrangements if AUmax is lower or higher than 185 MBq, respectively. The potential for contamination by the patients was determined from perspiration samples taken from the patient's hands, forehead, and neck and in saliva at 4, 24, and 48 h after radioiodine treatment. The mean highest 131I activity levels for hands, forehead, neck, and saliva were 4.1 Bq/cm2, 1.9 Bq/cm2, 0.9 Bq/cm2, and 796 kBq/g, respectively. The results indicate that there is minimal risk of contamination from these patients.