where T_eff, T_biol and T_phys are the effective, biological and physical half-lives. The effective half-life is the important parameter when estimating radiation dose as it determines the duration and rate of delivery of radiation dose.

Radionuclide therapy has been in use since the 1940s and some treatments have been relatively unchanged for several decades. However, more recently, there has been much research effort dedicated to improving strategies for treatment, developing new radiopharmaceuticals and looking at alternative radionuclides to take advantage of different properties. This is ongoing, but has resulted in introductions such as iodine-131 labelled m-IBG in the 1980s, samarium-153 EDTMP in the 1990s, and now radiolabelled monoclonal antibodies for non-Hodgkin’s lymphoma (NHL).

The subject is growing and this text has been limited to products with licences or marketing authorizations, but it should be noted that there are many interesting areas of development, such as the use of yttrium-90-labelled somatostatin analogues for neuroendocrine tumours [1–3].

Please note that where a product is unique to a particular company, the trade name has been given.

Iodine-131 in the treatment of thyroid disease

Iodine-131 (¹³¹I) is the radionuclide most widely used therapeutically. Iodine is readily concentrated in the thyroid gland and using ¹³¹I, emitting beta particles with a maximum range of 3 mm in tissue, allows a high radiation dose to be delivered to the thyroid and a low dose to the rest of the body. It is most commonly used in the form of sodium iodide[¹³¹I] for treatment of benign thyroid disease (thyrotoxicosis and non-toxic goiter) and in thyroid carcinoma. It has no role in medullary thyroid cancer. There are specific guidelines [4–6] from the Royal College of Physicians (RCP) in the UK, as well as from Europe and the USA for using radioiodine in the management of hyperthyroidism and of thyroid cancer. The characteristics of ¹³¹I are shown in Table 7.1. As specified by the European Association of Nuclear Medicine (EANM) and the Society of Nuclear Medicine (SNM) [5,6], it is essential that, before any treatment, all thyroid hormones, iodine-containing preparations and supplements and any other medications that could suppress thyroid uptake are discontinued for a sufficient length of time. Almost all thyroid treatments are given orally, as a capsule or as a liquid.

Table 7.1 Properties of the radionuclides used in therapy

Thyrotoxicosis

In thyrotoxicosis, or hyperthyroidism, the thyroid gland is over-producing thyroid hormones. The possible approaches to radionuclide therapy have included giving sufficient radioiodine to render the patient hypothyroid and giving low activities of ¹³¹I in combination with anti-thyroid drugs. However, the RCP recommend that the aim of treatment should be to render the patient euthyroid, while accepting that there will be a moderate rate of hypothyroidism [4]. For a standard case of hyperthyroidism, the RCP suggest a guide activity of 400 to 550 MBq at first presentation. An alternative is to use pretreatment thyroid uptake measurements, with tracer activities of ¹³¹I, to calculate the activity to be administered to deliver a prescribed radiation dose. Such calculations require knowledge of the thyroid mass, the percentage uptake and the rate of clearance from the gland, requiring repeated measurements over a period of several days. Some centres may use measured uptake and thyroid mass but assume a standard turnover rate. There have been studies looking at the effectiveness of different treatment schedules and corresponding rates of hypothyroidism [7, 8]. Most treatments in the UK are administered to outpatients, although this will depend on the amount of activity prescribed, the patient’s home circumstances and national regulations.

Thyroid tumours

There is also a role for ¹³¹I in the treatment of well-differentiated thyroid cancer, when it is administered both for the ablation of thyroid remnant after surgery and for the treatment of metastases. Following total thyroidectomy, the aim of remnant ablation is to destroy any remaining normal thyroid tissue and any microscopic deposits of thyroid carcinoma [9]. The RCP guidelines [4] state that the usual activity administered for ablation is 3.7 GBq but that some centres may use a lower activity (1.1 GBq) and, as for thyrotoxicosis, some use a dosimetric assessment of uptake and clearance in order to prescribe an activity. By destroying any remaining thyroid tissue, the theory is that the only remaining source of thyroglobulin production is any remaining malignant cells, thus making the measurements of thyroglobulin level a sensitive test of any local recurrence or metastatic disease. Metastatic lesions have a lower avidity for iodine than normal thyroid tissue and it is customary to administer higher activities, for instance 7 GBq or more. Treatments for thyroid cancer require an in-patient stay, until the level of radioactivity has fallen sufficiently for safe discharge as outlined in the Medical and Dental Guidance Notes (MDGN) [10]. Gamma camera images, using the 364 keV photons of ¹³¹I, may be obtained after treatment to confirm uptake in residual thyroid, recurrence or metastases. Scanning protocols may also be used after surgery and before ablation. It may also be used for instance, to determine the completeness of ablation as part of a patient’s treatment. Iodine-123 may provide a suitable alternative, with better characteristics for imaging with a gamma camera. The first report of the use of ¹³¹I in treating metastatic thyroid cancer was in 1946 [11]. Even 50 years after that first report, much is being discussed and written about optimization of these treatments.