ISSN 1470-3947 (print) | ISSN 1479-6848 (online)

Endocrine Abstracts (2010) 21 CM4.3

Radioiodine treatment and long-term follow-up of differentiated thyroid cancer

Furio Pacini

Section of Endocrinology and Metabolism, Department of Internal Medicine, Endocrinology and Metabolism and Biochemistry, University of Siena, Siena, Italy.

After total thyroidectomy, patients with differentiated thyroid cancer are treated with 131I activities aimed at ablating any remnant thyroid tissue and potential microscopic residual tumor. This procedure decreases the risk of locoregional recurrence and facilitates the long-term follow-up. In addition the high activity of 131I allows obtaining a highly sensitive post-therapeutic WBS. Radioiodine ablation is recommended in high-risk patients and in low-risk patients, while there is no indication in very low-risk patients (those with unifocal T1 tumors, <1 cm in size, with favorable histology, no extrathyroidal extension and lymph node metastases). Effective thyroid ablation requires stimulation by TSH. The method of choice for preparation to perform a radioiodine ablation is based on the administration of recombinant human TSH (rhTSH) while the patient is on levo-thyroxine (L-T4) therapy. Recent multicenter and prospective studies has demonstrated that this preparation is highly effective and safe and that the rate of successful ablation is similar to that obtained with L-T4 withdrawal when using 3700 MBq (100 mCi) or 1850 MBq (50 mCi) of 131I. Based on these results the use of rhTSH has been approved in Europe by the European Medicine Agency (EMEA) and in USA by the FDA.

After thyroid ablation, aim of follow-up is the early discovery and treatment of persistent or recurrent disease. Two to three months after initial treatment thyroid function tests (FT3, FT4, TSH) should be obtained to check the adequacy of L-T4 suppressive therapy. At 6 to 12 months the follow-up is aimed to ascertain whether the patient is free of disease. This follow-up is based on physical examination, neck ultrasound, rhTSH stimulated serum Tg measurement with or without diagnostic WBS. At this time most (nearly 80%) of the patients will belong to the low-risk categories and will disclose normal neck ultrasound and undetectable (<1.0 ng/ml) stimulated serum Tg in the absence of serum Tg antibodies. Diagnostic WBS does not add any clinical information in this setting and may be omitted. These patients may be considered in complete remission and their rate of subsequent recurrence is very low (<1.0% at 10 years). Patients in remission may be shifted from suppressive to replacement L-T4 therapy, with the goal of maintaining a serum TSH level within the normal range. The subsequent follow-up of these patients should be based on yearly physical examination, serum Tg measurement on replacement L-T4 and neck ultrasound.

The few patients with evidence of persistent disease, or with detectable levels of serum Tg increasing with time, require imaging techniques for the localization of disease and appropriate treatment, including therapeutic doses of 131I. Included in this category are the 5–10% of DTC patients that presented with local or distant metastases at diagnosis and an additional 5–10% that develop recurrent disease during follow-up. When appropriately treated, 2/3 of those patients with local disease and 1/3 of those with distant disease may achieve complete remission. Treatment of loco-regional disease is based on the combination of surgery and radioiodine therapy. External beam radiotherapy may be indicated when complete surgical excision is not possible or when there is no significant radioiodine uptake in the tumor. Distant metastases are more successfully cured if they take up radioiodine, are of small size located in the lungs (not visible at X-rays). Lung macro-nodules may benefit from radioiodine therapy but the definitive cure rate is very low. Bone metastases have the worst prognosis even when aggressively treated by combination of radioiodine therapy and external beam radiotherapy. Whenever radioiodine therapy is not effective and the disease progress, enrollment of the patients in experimental trials with tyrosine kinase inhibitor should be the treatment of choice.

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