Ablation with radioiodine should be used selectively. In low-risk patients (younger than 40 years, with a tumor less than 1 to 1.5 cm, without extracapsular extension or vascular invasion, submitted to total thyroidectomy) (75), the long-term prognosis after surgery alone is so favorable that I-131 ablation is not recommended in general (76,77).

Ablation with I-131 is performed when the patient has a tumor with the potential for recurrence, because it decreases both recurrence and death rates (50). Retrospective data from multiple studies show that radioiodine ablation is associated with a 50% reduction in locoregional relapse and the long-term disease-specific mortality is probably reduced in primary tumors that are at least 1 to 1.5 cm in diameter, are multicentric, or have soft-tissue invasion at presentation (40,77,78). Although debate about ablating the thyroid bed with I-131 after total thyroidectomy continues (9,79), there are some compelling reasons to do this (78): (i) to destroy any residual microscopic foci of residual disease, (ii) to increase the sensitivity of subsequent I-131 scanning for detection of recurrent or metastatic disease by eliminating uptake by residual normal tissue, (iii) high circulating TSH levels, necessary to enhance tumor I-131 uptake, cannot be achieved with a large thyroid remnant, and (iv) to improve the value of measurements of serum Tg as a tumor marker.

Adjuvant radioiodine should be administered to all patients with differentiated carcinoma, when the primary tumor is likely to relapse or cause death, and to patients who have evidence of extrathyroidal disease, either by direct invasion outside of the gland or locoregional metastases. In patients with residual disease following surgery, including extracervical metastases, therapy with I-131 should be performed.

Therapy with I-131 is performed by either administering an empiric fixed dose or using dosimetry-guided activities. Quantitative dosimetry methods are used to determine the activity for the therapeutic dose, based on the individual patient's radioiodine pharmacokinetics. Some authors advocate this approach (80) because radiation exposure from arbitrarily fixed doses of I-131 can vary considerably. Using dosimetry methods, the therapeutic dose may be adjusted to compensate for patient-to-patient variability in the rate of iodine clearance (67,81). If the calculated dose to the tumor is less than 35 Gy, it is unlikely that the cancer will respond to radio-iodine therapy (82). Radioiodine activities that deliver 500 to 600 Gy to the residual normal tissue and 40 to 50 Gy to metastatic foci are likely to be effective. Dosimetry for diffuse lung metastases may be difficult to estimate because of the need to estimate tumor size.

Although the clinical merits of dosimetry-guided radioiodine therapy have been demonstrated, most centers have adapted the fixed-dose technique using 3.7 to 7.4 GBq (100 to 200 mCi) I-131 owing to the technical and logistic difficulties of dosimetry studies. Tumor/remnant tissue volume estimates are often the greatest challenge for individual assessment of radioiodine dosages based on dosim-etry. Patients who show I-131 uptake and have no metastatic disease are treated with 2960 to 3700 MBq (80 to 100 mCi). Lymph-node metastases that are not large enough to excise are treated with approximately 5550 MBq (150 mCi).

In recent years, lower activities of radioiodine (e.g., 1200 MBq) have been used, mainly in the United States, allowing outpatient treatment. The literature reports conflicting results with this practice. Some studies show a similar efficacy for both types of doses, principally with lower radioiodine uptake and small remnants of residual thyroid tissue (83-85). There are however other reports suggesting less efficacy for lower doses (86,87). Nonetheless, this practice is not widely implemented, and further studies are required before such low dose regimens can be generally recommended (88). More recently, in the United States, newer guidelines from the Nuclear Regulatory Commission provide greater flexibility in the dosing and management of outpatients treated with I-131. They also allow outpatient treatment with higher doses of radio-iodine (89).

The efficacy of radioiodine depends on patient preparation, tumor-specific characteristics, sites of disease, and administered radioiodine activity. Iodide uptake by thyroid tissue is stimulated by TSH and is suppressed by increased endogenous iodide stores. Following thyroidectomy, the patient's thyroid hormone levels must decline sufficiently to allow the TSH concentration to rise to above 25 to 30 mU/L. This period of hormone withdrawal typically lasts four to five weeks. Patients should avoid foods with high iodine content for at least one week prior treatment (90).

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