Radioactive iodine

Doses

Three dose ranges for radioactive forms of iodine are used:

• 1.

  For diagnostic purposes usually much less than 1 mCi (37 MBq) is given with thyroid radiation doses of a few rads of ¹²³ I up to 50–200 rads (0.5–2.0 Gray) ( ¹³¹ I).

• 2.

  In the treatment of hyperthyroidism the dose of ¹³¹ I is usually a few millicuries and is either roughly estimated or calculated according to the size of the thyroid gland, the uptake of a tracer dose of iodine, and the type of thyroid disorder (diffuse or nodular); doses range from 80 to 150 microCi (3.0–5.5 MBq) per gram of thyroid tissue [ , ].

• 3.

  In thyroid cancer, ¹³¹ I can be used to eliminate tumor tissue that cannot be removed surgically but still captures iodine; in such circumstances, amounts of 100 mCi (4000 MBq) of ¹³¹ I or more are not unusual; with such high amounts, other tissues besides the thyroid gland can also receive substantial amounts of radiation.

The administration of ¹³¹ I requires safety measures, in order to reduce to a minimum the irradiation of medical personnel and to avoid contamination of rooms and relatives of patients. Capsules containing ¹³¹ I are therefore to be preferred to liquid iodine. At doses above 25 mCi (555 MBq), usually intended only for treatment of patients with thyroid cancer, isolation in a specially constructed room of a service for nuclear medicine is necessary. Waste disposal should also be carefully managed so as to avoid overall contamination [ , ].

General adverse effects and adverse reactions

Radiation thyroiditis is an infrequent complication resulting in swelling and localized pain over the thyroid gland which subsides spontaneously or with anti-inflammatory or glucocorticoid therapy [ ].

Acute exacerbation of hyperthyroidism, resulting particularly in cardiac complications (arrhythmias or decompensation) or even “thyroid storm,” has been reported several times [ ] and should be avoided by treating very severely hyperthyroid patients with antithyroid drugs prior to the administration of ¹³¹ I. A temporary increase in serum triiodothyronine and l-thyroxine without clinical symptoms of exacerbation of hyperthyroidism, however, occurs much more frequently [ ]. In 71 patients given ¹³¹ I for differentiated thyroid carcinoma, short-term adverse effects included gastrointestinal complaints, salivary gland swelling with pain, change in taste and headache [ ].

Radio-labelled lipiodol

See also Iodinated contrast media

Lipiodol is an iodized oil that can be used to outline the ducts of the salivary glands and the lacrimal ducts and intra-arterial lipiodol has been used to enhance the accuracy of computed tomography in hepatic tumors. Lipiodol labelled with ¹³¹ I (Lipiocis) has been used to treat primary liver cancers (hepatocellular carcinoma and cholangiocarcinoma) by radioembolization, the injection of micron-sized embolic particles loaded with a radioisotope delivered by percutaneous intra-arterial injection [ , ]. This can cause transient bowel ischemia with nausea, vomiting and diarrhea.

Following reports of interstitial pneumopathy associated with the use of Lipiocis [ ], CIS Bio (a subsidiary of the Schering group), in agreement with the French agency for medical safety of health products (Agence française de sécurité sanitaire des produits de santé, AFSSAPS), informed health professionals that the incidence appeared to be higher (2%) than was initially observed in clinical trials (0.5%) [ ]. In 13 cases Lipiocis was associated with an interstitial pneumopathy that occurred about 1 month after injection, generally after the second injection. The symptoms included dyspnea, sometimes associated with a dry cough and bilateral crackles. In some cases there were serious complications and there was a high death rate. AFSSAPS recommended that thoracic radiography must be carried out before administering Lipiocis and if respiratory symptoms were observed.

Cardiovascular

In a series of thyrocardiac patients, of those dying primarily from thyrotoxicosis more than 21% did so within 3 weeks of ¹³¹ I treatment [ ], presumably reflecting too sudden a change in metabolic activity for patients with existing cardiac complications.

Respiratory

Acute respiratory embarrassment due to thyroid swelling or subsequently due to cicatrization occurs only rarely.

Two cases of acute airways obstruction following the administration of ¹³¹ I for Graves’ disease have been reported [ ]. In both instances rapid resolution of the respiratory symptoms was obtained with administration of histamine H ₁ receptor antagonists and hydrocortisone.

Nervous system

Although aseptic meningitis has been associated with the use of radioactive iodine, the products used were albumin complexes ( ¹³¹ I-RISA); this complication is almost certainly attributable to the protein content or to pyrogens rather than to the radioactive iodine itself [ ].

Sensory systems

Several papers have reported that radioiodine therapy can lead to worsening of ophthalmopathy, possibly because of the release of thyroid antigens during the inflammatory reaction after ¹³¹ I therapy. The worsening can be prevented by glucocorticoid therapy [ ].