Dose For Toxic Nodular Goiter

In addition to GD, thyrotoxicosis can also result from a single hyperfunctioning nodule, or multiple hyperfunctioning nodules (i.e., toxic multinodular goiter). Although antithyroid drugs can ameliorate hyperthyroidism, definitive treatment is more commonly accomplished with RAI or surgery. Less commonly, percutaneous ethanol injection has also been used for large solitary nodules (16,17). Compared with treatment with RAI, hypothyroidism is a more common sequel of surgery (18). The choice of surgery versus radiation for nodular goiters is beyond the scope of this chapter, although surgery should be considered strongly in patients with goiters causing significant airway obstruction or an increased risk of harboring thyroid cancer. Treatment decisions for these patients should be made in consultation with a surgeon with expertise in thyroid surgery.

Most patients with toxic nodular goiters will remain hyperthyroid until definitively treated. Occasionally, central necrosis may occur in a single hyperfunc-tioning nodule with spontaneous resolution of hyperthyroidism, although this should not be anticipated in lieu of more definitive treatment. An initial course of antithyroid drugs may be considered in order to render the patient euthyroid before surgical or radioiodine treatment.

Nodular goiters are believed to be more radio resistant than the diffuse goiter of GD. Large doses, between 150 and 300 Gy, have frequently been used for toxic adenomas (19). A calculated administered activity of 7.4 MBq (200 mCi) per gram to the nodule, corrected for 24-hour uptake, has been used successfully. Standardized administered activities at appropriate doses (e.g., 740-1110 MBq) may also prove effective. With administered activities of less than 370 MBq (10 milliCuries), treatment failures are common (20).

Radiation exposure to normal thyroid tissue in the setting of solitary toxic nodules has never been shown to increase the incidence of thyroid cancer. This is likely because uptake in the normal thyroid tissue is suppressed. Nevertheless, suppressed thyroid tissue may still receive a dose as high as 23 Gy (19). However, hypothyroidism following RAI treatment does seem to occur less frequently for solitary hyperfunctioning nodules compared with GD or multinodular goiter. In order to minimize the risk of hypothyroidism following treatment, a suppressed thyroid-stimulating hormone (TSH) level should be present, and a thyroid scan should be performed to exclude significant extranodular uptake (21). A reduction in the nodule size can be expected following RAI treatment (22).

For toxic multinodular goiters, doses of 150 Gy may be adequate to resolve hyperthyroidism. Administered activities between 3.7 and 7.4 MBq (100-200 mCi) per gram have been shown to be effective (18). Fixed administered activities (e.g., 1110 MBq) have also been used. Not uncommonly, patients with toxic multinodular goiters may have large glands and 24-hour RAI uptake measurements that are not significantly elevated. This may necessitate the administration of relatively large amounts of radioactivity. In the United States, higher administered activities may be used for nonhospitalized patients, if it can be documented that radiation exposure to the public is not likely to exceed 5 mSv (0.5 rem) (23).

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