What is the role of postoperative radioiodine scanning and ablation in the treatment of thyroid cancer?

Updated: May 09, 2018
  • Author: Pramod K Sharma, MD; Chief Editor: Arlen D Meyers, MD, MBA  more...
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Answer

Answer

Because differentiated thyroid tissue and well-differentiated thyroid carcinomas are TSH sensitive and because they take up iodine, radioiodine preferentially targets residual normal or malignant tissue after thyroidectomy. Therefore, radioiodine can be given in diagnostic doses to detect residual normal or neoplastic tissue in the body and in therapeutic doses to ablate this tissue. After thyroidectomy, use of radioiodine scanning and ablation has become commonplace for diagnosing and treating residual thyroid tissue, as well as regional and distant metastases from well-differentiated thyroid carcinomas. Pretherapeutic iodine-uptake scanning is controversial because of its cost and because of concerns about131 I-induced tumor stunning, which may decrease the effectiveness of radioiodine treatment.

After thyroidectomy, patients are given thyroid replacement therapy with T4 (Synthroid) or triiodothyronine (T3, Cytomel).131 I or123 I scanning is performed when the patient is in a hypothyroid state (TSH >30-50). Approximately 4-6 weeks after thyroidectomy, hypothyroid can be induced by discontinuing replacement (T4 for 4 weeks or T3 for 2 weeks) to obtain high serum TSH levels. A diagnostic dose of131 I or123 I is given initially. Whole-body scanning is performed to detect any tissue taking up radioiodine. If any normal thyroid remnant or metastatic disease is detected, a therapeutic dose of131 I is administered to ablate the tissue. Posttreatment scanning should also be performed because it may reveal metastatic disease not otherwise noted.

The role of recombinant human TSH (Thyrogen) in remnant ablation continues to evolve. Thyrogen is approved for postsurgical remnant ablation in Europe but not the United States. Barbaro et al found equivalent results in postsurgical remnant ablation when they compared traditional T4 withdrawal with the discontinuation of T4 1 day before TSH stimulation. Thyrogen stimulation avoids the discomfort of patients having to discontinue thyroid replacement and is especially useful in those unable to tolerate hypothyroidism or to generate a high TSH level.

If a treatment dose of131 I is required, diagnostic thyroid scanning is repeated while the patient is in the hypothyroid state about 6 months after initial treatment. Again, if the diagnostic scan is positive, an additional therapeutic dose is given. This process is repeated until the diagnostic scan is negative.

A promising new development for follow-up thyroid scanning is the use of recombinant human TSH as opposed to withdrawing T4 to increase autogenous TSH levels. This approach avoids the discomfort of having to discontinue thyroid replacement therapy for these scans.

A retrospective analysis of more than 1000 patients with papillary thyroid cancer who underwent total thyroidectomies found that most patients with low-risk local disease and some with high-risk T3 tumors who did not receive radioiodine remnant ablation after surgery had high 5-year recurrence-free survival rates. This suggests that physicians should carefully consider whether the benefits outweigh the risks associated with radioiodine remnant ablation when debating the possibility of employing this technique in individual patients. [17, 18]

A study by Ruel et al indicated that adjuvant radioiodine therapy can improve the overall survival rate in patients with intermediate-risk papillary thyroid cancer. The study, which had a mean 6-year follow-up period, involved 21,870 adult patients who underwent total thyroidectomy for intermediate-risk papillary thyroid cancer, including 15,418 who received adjuvant radioiodine treatment and 6452 who did not. The investigators found that the risk of death was reduced by 29% in the radioiodine patients, with the risk decreased by 36% in those under age 45 years. [19]


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