Advanced Clinical Practice in Nuclear Medicine

1. Advanced Clinical Practice in Nuclear Medicine Dr.Sayedabbas NMT 364 Lecture 3

2. Radioiodine Therapy for Benign Thyroid Disease

3. Historical Perspective • Diseases of the thyroid constitute the most common form of endocrine disorders and, over the past 60 years, nuclear medicine has contributed significantly to the management of thyroid patients, both in terms of diagnosis and treatment. • Radionuclide therapy for both benign disease (thyrotoxicosis and goitre) and thyroid cancer has served as a model for the development of other radionuclide therapies in recent years.

4. The presence of iodine in the thyroid gland was reported in 1895 by Baumann. • The first reports of radionuclide therapy for thyrotoxicosis used iodine-130, and iodine-131 rapidly became the favoured radionuclide with β- and γ-ray emissions and a half-life suitable for therapy.

5. Although there have been many years of clinical experience in the use of 131I radioiodine for the treatment of benign thyroid disease, there are few prospective studies published comparing the efficacy of 131I radioiodine and other treatment modalities such as antithyroid drugs or surgery.

6. The development of guidelines for the management of thyrotoxicosis and the use of 131I radioiodine has resulted in some harmonisation of practice although there are still significant differences in the protocols used in Europe compared with the USA.

7. Iodine-131 • The physical properties of 131I are listed in Table.

8. Iodine-131 has a physical half-life of 8.04 days, which is well-suited to the biological half-life of iodine in patients with differentiated thyroid cancer. • The medium-energy beta-particle emission (Emax 0.61 mev) with a path length of about 0.5 mm in tissue, ensures an intracellular radiation dose following the cellular internalization of 131I. • The gamma emissions of 131I have both benefits and disadvantages.

9. Gamma-ray emissions facilitate gamma-camera imaging, which enables tracer doses of 131I to be used diagnostically and for dosimetry calculations, and also permits post-therapy imaging to confirm uptake of the therapy dose in all known tumor sites. • The high-energy gamma emissions, however, contribute to the unwanted whole-body radiation load associated with radionuclide therapy and also to the radiation protection problems for the staff and the patients’ relatives.

10. Pathology of the thyroid gland • Like all endocrine organs, the thyroid gland may over- or underproduce hormones or be subject to malignant change. • Iodine-131 may be used to treat the various forms of thyroid hormone overproduction, and also for the treatment of differentiated thyroid cancer.

11. 99mTc thyroid scan of a patient with toxic diffuse goiter (Graves’ disease) showing diffusely increased uptake and visualizing the pyramidal lobe.

12. 99mTc thyroid scan of a patient with a multinodular goiter showing non-homogeneous uptake and developing autonomousnodules at the left lower pole.

13. Patients may also develop a single nodule, which on isotope imaging is demonstrated to show increased uptake with suppression of the remainder of the gland. • Uninodular toxic goitre is diagnosed using conventional biochemical tests and radionuclide imaging with technetium-99m (99mTc) or iodine-123 (123I).

14. 99mTc Thyroid scan in a patient with biochemical evidence of thyrotoxicosis. Upper right zoomed image demonstrates a toxic nodule in the lower pole of the right lobe of thyroid with suppression of the remainder of the gland.

15. Diagnosis of thyrotoxicosis • The diagnosis of thyrotoxicosis is made on the basis of history, clinical examination and investigations. • The investigations include free T4, free T3 and ultra-sensitive TSH investigations. • A 99mTc or an 123I thyroid scan will differentiate toxic diffuse goiter from toxic nodular goiter. • Thyroid stimulating immunoglobulin measurement, if raised, will strongly support a diagnosis of toxic diffuse goiter (Graves’ disease), as does the presence of dysthyroid eye disease.

16. RADIOIODINE THERAPY FOR THYROTOXICOSIS Selection of patients TOXIC DIFFUSE GOITER (GRAVES’ DISEASE) • As has already been stated, 131I therapy usage is considered in patients who have completed a conventional 12- to 24-month course of anti-thyroid medication and whose thyroid function tests have again become elevated, or alternatively in patients who fail to respond to anti-thyroid medication. • Patients who are poorly compliant in taking anti-thyroid medication should also be considered for radioiodine therapy.

17. As diffuse goiter (Graves’ disease) frequently affects young and middle-aged women the issue of using 131I in women of childbearing years has been an issue of discussion. • In the previous decades, such women were not selected for 131I therapy because of the potential risk to the offspring. • However, to date, no evidence has been presented to suggest damage to the offspring of patients treated with 131I despite large patient population studies.

18. The use of 131I in young patients remains controversial. • In the USA, treatment of patients under the age of 18 with 131I is now taking place routinely. • In Europe, however, the concerns about radioiodine treatment in young patients with radiosensitive tissues has led to a general limitation of 131I use to adults. • However, it is important to recognize and consider the risks of thyroid surgery in children.

19. Thank you