A mathematical model of optimized radioiodine-131 therapy of Graves' hyperthyroidism

A retrospective study with long term follow-up of Graves' disease patients treated with low activities of 131Iodine

BACKGROUND

Previous studies have shown that application of relatively low 131I-Iodine activities can successfully be used to treat patients with Graves' disease (GD). We assessed treatment outcome in the long-term follow-up of our GD patients and influencing factors.

METHODS

We evaluated 521 GD patients in this retrospective clinical single-center study. In all patients we performed scintigraphy and thyroid uptake measurement after 4 and 24 hours using 10 MBq 123I and calculated administered activity using Marinellis' formula. Treatment was done according to national regulations. Minimal routine clinical evaluation of all patients was available after 6 weeks and after 3, 6 and 12 months. Success of treatment was defined as euthyroid state or hypothyroidism 6 months after therapy.

RESULTS

We usually applied relatively low 131I activities. Three hundred seven patients (58.9%) became hypothyroid within 21 years of follow-up. One hundred thirty-nine patients (26.7%) became euthyroid and stayed euthyroid until the end of follow-up. We found a plateau after 7 years of initial therapy with only a few patients becoming hypothyroid after that time and identified 75 patients (14.4%) with persistent hyperthyroidism or recurrence.

CONCLUSIONS

Treatment with relatively low 131I activities produce favorable responses as shown in previous works. We found a high proportion of patients with long-term euthyroid state. Application of low activities reduces radiation burden of patients and, depending on radiation protection legislation, may lead to shortened hospital stay and reduced costs. Therefore, we feel that application of higher activities to treat GD patients as recommended in several current guidelines should be reconsidered.

Radioactive Iodine Therapy of Graves' Disease in Patients Pretreated With Methimazole Without Radioiodine Uptake for Dose Estimation

OBJECTIVE

To assess response predictors to radioactive iodine (RAI) therapy without using thyroid uptake for dose estimate in patients pretreated with methimazole.

METHODS

Retrospective analysis was performed of patients with Graves' disease treated with RAI doses determined without using uptake studies.

RESULTS

In 242 patients (median age, 41.9 years; 66.1% female), initial mean free thyroxine (FT4) level was 4.7 ng/dL with an estimated thyroid size of 49.15 g. Prior to RAI therapy, average methimazole dose was 22.7 mg/day. Mean RAI dose was 737.0 ±199.4 MBq (19.9 ± 5.4 mCi). Two hundred eight patients (85.9%) responded to RAI therapy; 185 (88.9%) became hypothyroid and 23 (11.1%) became euthyroid. The majority (90.4%) responded within 6 months of therapy with a quicker response (13.9 ± 8.3 vs 17.5 ± 13.5 weeks) for those treated with doses per gram of ≥14.8 MBq (0.4 mCi). Thirty-four nonresponders had a higher initial FT4 level and larger thyroid size with a lower RAI dose per gram of thyroid tissue. In multivariate analysis, the independent response predictor to therapy was dose per gram of thyroid tissue of ≥14.8 MBq (0.4 mCi) (hazard ratio, 3.18; 95% CI, 1.1-9.7). Doses per gram of 14.8 to 18.1 MBq (0.4-0.5 mCi) achieved maximal response rate without added advantage of higher doses. Thyroid size prior to RAI therapy, FT4 levels at diagnosis, and age were inversely related to response.

CONCLUSION

RAI therapy for Graves' disease without uptake studies for dose estimates is an effective treatment method. In patients pretreated with methimazole, an RAI dose per gram of thyroid tissue of ≥14.8 MBq (0.4 mCi) showed high response rate. Prospective studies are needed to confirm the viability of this simplified and cost-effective approach.

Hyperthyroidism in the personalized medicine era: the rise of mathematical optimization

Thyroid over-activity or hyperthyroidism constitutes a significant morbidity afflicting the world. The current medical practice of dose titration of anti-thyroid drug (ATD) treatment for hyperthyroidism is relatively archaic, being based on arbitrary and time-consuming trending of thyroid function that requires multiple clinic monitoring visits before an optimal dose is found. This prompts a re-examination into more deterministic and efficient treatment approaches in the present personalized medicine era. Our research project seeks to develop a personalized medicine model that facilitates optimal drug dosing via the titration regimen. We analysed 49 patients' data consisting of drug dosage, time period and serum free thyroxine (FT4). Ordinary differential equation modelling was applied to describe the dynamic behaviour of FT4 concentration. With each patient's data, an optimization model was developed to determine parameters of synthesis rate, decay rate and IC₅₀. We derived the closed-form time- and dose-dependent solution which allowed explicit estimates of personalized predicted FT4. Our equation system involving time, drug dosage and FT4 can be solved for any variable provided the values of the other two are known. Compared against actual FT4 data within a tolerance, we demonstrated the feasibility of predicting the FT4 subsequent to any prescribed dose of ATD with favourable accuracy using the initial three to five patient-visits' data respectively. This proposed mathematical model may assist clinicians in rapid determination of optimal ATD doses within allowable prescription limits to achieve any desired FT4 within a specified treatment period to accelerate the attainment of euthyroid targets.

Transient Hypothyroidism after Radioiodine for Graves' Disease: Challenges in Interpreting Thyroid Function Tests

Graves' disease is the most common cause of hyperthyroidism and is often managed with radioactive iodine (RAI) therapy. With current dosing schemes, the vast majority of patients develop permanent post-RAI hypothyroidism and are placed on life-long levothyroxine therapy. This hypothyroidism typically occurs within the first 3 to 6 months after RAI therapy is administered. Indeed, patients are typically told to expect life-long thyroid hormone replacement therapy to be required within this timeframe and many providers expect this post-RAI hypothyroidism to be complete and permanent. There is, however, a small subset of patients in whom a transient post-RAI hypothyroidism develops which, initially, presents exactly as the typical permanent hypothyroidism. In some cases the transient hypothyroidism leads to a period of euthyroidism of variable duration eventually progressing to permanent hypothyroidism. In others, persistent hyperthyroidism requires a second dose of RAI. Failure to appreciate and recognize the possibility of transient post-RAI hypothyroidism can delay optimal and appropriate treatment of the patient. We herein describe five cases of transient post-RAI hypothyroidism which highlight this unusual sequence of events. Increased awareness of this possible outcome after RAI for Graves' disease will help in the timely management of patients.

Rhenium-188 production in hospitals, by w-188/re-188 generator, for easy use in radionuclide therapy

Rhenium-188 (Re-188) is a high energy β-emitting radioisotope obtained from the tungsten-188/rhenium-188 (W-188/Re-188) generator, which has shown utility for a variety of therapeutic applications in nuclear medicine, oncology, and interventional radiology/cardiology. Re-188 decay is accompanied by a 155 keV predominant energy γ-emission, which could be detected by γ-cameras, for imaging, biodistribution, or absorbed radiation dose studies. Its attractive physical properties and its potential low cost associated with a long-lived parent make it an interesting option for clinical use. The setup and daily use of W-188/Re-188 generator in hospital nuclear medicine departments are discussed in detail. The clinical efficacy, for several therapeutic applications, of a variety of Re-188-labeled agents is demonstrated. The high energy of the β-emission of Re-188 is particularly well suited for effective penetration in solid tumours. Its total radiation dose delivered to tissues is comparable to other radionuclides used in therapy. Furthermore, radiation safety and shielding requirements are an important subject of matter. In the case of bone metastases treatment, therapeutic ratios are presented in order to describe the efficacy of Re-188 usage.

Thyroid echogenicity: A clue to precise individual dosimetry in radioiodine therapy of hyperthyroidism

BACKGROUND

Radioiodine therapy is a frequent option in treatment of patients with hyperthyroidism. Despite efforts to plan the thyroid absorbed dose by accounting for the gland size and radioiodine kinetics, the success of radioiodine therapy remains largely unpredictable. The current methods plan the mean thyroid radiation absorbed dose, assuming that it applies to target tissue - the thyroid follicular cells. However, the unique thyroid follicular structure and iodine kinetics may violate this assumption. Upon oral administration and capture by thyroid, the vast majority of time radioiodine spends in organified form in follicular colloidal lumen: the greater the follicle the more radiation is wasted before reaching the target cells.

HYPOTHESIS

The (131)I radiation absorbed dose to thyroid follicular cells is less than the average thyroid dose, the more the greater the follicles. Thyroid echogenicity can be used to assess the amount of colloid in thyroid tissue, which in turn can be used to assess the follicle size and adjust the planned absorbed dose to patient-specific thyroid micro-architecture.

EVALUATION

Animal data on intrathyroidal iodine kinetics were considered in conjunction with model predictions that relate the size of thyroid follicles with (131)I irradiation of follicular cells. It turned out that the correction factors in the range 5-40% should be applied to oral activities of radioiodine calculated by the standard method. Next, several histology studies documented that normoechogenic thyroids have relatively large follicles, while hypoechogenic thyroids are mostly cellular, with almost empty, small follicles. All these concur with clinical data that Graves' disease patients with normoechogenic thyroids that received 200Gy in thyroid had comparable outcome to Graves' disease patients with hypoechogenic thyroid that received 100-120Gy in thyroid.

CONCLUSION

Thyroid echogenicity is a probable clue to a better patient-specific dosimetry in radioiodine therapy of hyperthyroidism; direct evidences and precise estimates of benefits over current practices would be provided by controlled clinical trials.

Effects of drugs on the efficacy of radioiodine (|) therapy in hyperthyroid patients

The treatment of hyperthyroidism is targeted at reducing the production of thyroid hormones by inhibiting their synthesis or suppressing their release, as well as by controlling their influence on peripheral tissue (conservative therapy, medical treatment). Radical treatment includes surgical intervention to reduce the volume of thyroid tissue or damage of the mechanisms of thyroid hormone synthesis by radioiodine ((131)|) administration. Radioiodine ((131)|) is a reactor radionuclide, produced as a result of uranium decomposition and emission of β and γ radiation. The therapeutic effects of the isotope are obtained by the emission of β radiation. In the paper, the effects of administered drugs (antithyroid, glucocorticosteroids, lithium carbonate, inorganic iodine, β-blockers) on the final outcome of radioiodine therapy in patients with hyperthyroidism are discussed.

Variability of the administered radioiodine doses for the treatment of hyperthyroidism in Belgium

OBJECTIVE

When using radioiodine for hyperthyroidism there is no consensus regarding the administration of fixed or calculated doses. Guidelines do not specify the preferable approach or the parameters to use to calculate the dose. Therefore, the dose might be quite different with regard to the chosen procedure. This study was undertaken to evaluate the variability of the amount of radioiodine administered in Belgium in various cases of hyperthyroidism.

DESIGN AND PATIENTS

Twenty-one Belgian nuclear medicine physicians received summarized clinical files from 10 patients suffering from overt hyperthyroidism (n = 7) or subclinical hyperthyroidism (n = 3). Five patients had homogeneous goiters, one had multinodular goiter, and four had hot nodule. Participants had to determine the radioiodine dose (millicuries, mCi) they would give in each case.

RESULTS

Proposed doses varied between 2 mCi and 25 mCi. Mean proposed dose for nodular disease was 10.71 mCi; it was 6.79 mCi for homogeneous goiter. For individual cases, a difference between the lowest and the highest dose of more than 17 mCi was observed in more than 50% of the cases.

CONCLUSIONS

We believe that more precise guidelines are mandatory, underlying uncertainties, controversies but recommending however, as minimal and maximal doses to administer, as well as clinical and biological parameters, if any, to be taken into account in order to modulate these doses.

Optimized radioiodine therapy for Graves?? disease: Two MIRD-based models for the computation of patient-specific therapeutic 131I activity

AIM

(131)I therapy is increasingly used for Graves' hyperthyroidism. Debate remains about the best method for calculating the activity to administer, as well as about the potential benefit of such computed activity. Several arguments plead, nevertheless, in favour of a personalized computation, such as inter-individual variations of thyroid volume and biokinetics.

METHODS

A MIRD-based dosimetric approach, with an additional extension that takes into account the variation of thyroid mass during the treatment, has been developed. This approach includes the benefits of a personalized determination of biokinetics. Results were compared with those of six methods widely used in routine practice. Forty-one patients were enrolled (34 women, seven men; mean age +/-SD: 48.11 +/- 6.4 years). (131)I uptakes were measured at 4, 24 and 96 h (36.2 +/- 14.6%, 42.8 +/- 9.7% and 27.6 +/- 6.8%, respectively), following administration of the tracer. The kinetics of iodine in the thyroid were evaluated using a two-compartment model (effective half-life of 5.1 +/- 1.6 days). Computations of activities to deliver the doses prescribed by the physician were done with the eight formalisms.

RESULTS

There was no statistical difference between results of the two MIRD-based formalisms (227 +/- 148 MBq and 213 +/- 124 MBq), which were also not significantly different from those obtained with the majority of the other methods (from 128 +/- 95 MBq to 275 +/- 223 MBq). However, a large intra-individual difference up to a factor of 2 between two given methods was found.

CONCLUSION

The formalism developed appears to be a good compromise between all the common formalisms already used in many institutions. Furthermore, it allows the exposures of target volumes and non-target volumes to be planned individually and practical individual radiation protection recommendations to be implemented.

Long-term follow-up study of radioiodine treatment of hyperthyroidism

OBJECTIVE

To determine the cumulative incidence of hypothyroidism during long-term follow-up in patients treated for hyperthyroidism by radioactive iodine (131)I (RAI) therapy, the significance of clinical factors in predicting the development of hypothyroidism, and the outcome after a fixed 7 mCi (259 MBq) dose of RAI.

DESIGN

Prospective cohort study of patients treated for hyperthyroidism by RAI.

PATIENTS AND MEASUREMENTS

Since 1965, details on 2043 patients treated by RAI therapy in Tampere University Hospital were entered into a computerized register. Following RAI treatment, thyroid status was monitored every 1-3 months during the first year, and subsequently at 1-3-year intervals until June 2002 or until the patient died or moved out of the Tampere University Hospital district. results The cumulative incidence of hypothyroidism in patients with Graves' disease and toxic multinodular goitre at 1, 10 and 25 years was 24%vs. 4%, 59%vs. 15% and 82%vs. 32%, respectively. In a Cox regression model, previous partial thyroidectomy [risk ratio (RR) = 1.63 in patients with Graves' disease and RR = 1.59 in those with toxic multinodular goitre] and age at the first RAI treatment (RR = 0.998 and RR = 0.996 per year) were statistically significantly associated with the development of hypothyroidism both in patients with Graves' disease and in those with toxic multinodular goitre. Antithyroid medication preceding RAI therapy (RR = 0.47) decreased and female gender (RR = 1.53) increased the risk of hypothyroidism only in patients with Graves' disease. Administration of a single dose of RAI resulted in the control of hyperthyroidism in 75% of patients, while two to six RAI treatments were needed in 25% of patients to achieve either a hypothyroid or a euthyroid state in both groups. None of the clinical factors studied was associated with the remission rate either in patients with Graves' disease or in those with toxic multinodular goitre. The remission rate did not differ between the patients who received a dose of RAI calculated according to the uptake of RAI and thyroid size and those who received an empirical dose of RAI. The fixed 7 mCi (259 MBq) dose of RAI cured 80% of patients.

CONCLUSION

RAI treatment is effective in treating hyperthyroidism in patients with Graves' disease, but hypothyroidism will develop in 82% of patients in 25 years. Because the development of hypothyroidism seems to be inevitable and unpredictable by any clinical factors, the objective of RAI treatment should be to minimize the persistence of hyperthyroidism with the simplest possible form of treatment. We recommend a fixed 7 mCi dose of RAI to be used as the first empirical dose in the treatment of hyperthyroidism, at least in Graves' disease.

Minimizing radiation exposure from patients treated with iodine-131 for hyperthyroidism using a lead collar: a simple and effective approach

OBJECTIVE

To construct a lead collar useful for shielding the electromagnetic radiation from the thyroid of patients treated with radioiodine (131)I for hyperthyroidism, thereby reducing the radiation emitted from these patients.

MATERIALS AND METHODS

A specially designed cervical lead collar was used to shield radiation exposure from 'hot thyroids' of 20 patients treated with iodine-131 for hyperthyroidism. The collar was made of lead strips stacked together around a plastic neck support fastened around the patient's neck. Measurements of the radiation exposure rate were obtained at increasing distances from the patients (50, 100, 200 cm), with and without the lead collar, at various times after (131)I treatment.

RESULTS

Radiation exposure measurements over 3 weeks showed relatively high exposure rates in the first week after treatment. The radiation exposure from the patients wearing the lead collar was half that found in the same patients not wearing the collar.

CONCLUSION

The use of the lead collar significantly reduced the radiation exposure of patients' surroundings. It was particularly useful in patients who could not abide strictly to the standard restrictive protocols after treatment with (131)I.