What is radioactive iodine (RAI) therapy for thyroid cancer?

From the Guidelines

Radioactive iodine (RAI) therapy for thyroid cancer involves the administration of iodine-131 (I-131) to selectively target and destroy thyroid tissue, including cancer cells, with a typical dose ranging from 100 to 200 millicuries (mCi) 1.

• The treatment is usually given orally as a single dose, and patients with distant metastases should receive this dose after TSH stimulation 1.
• Between treatments, suppressive doses of levothyroxine are given to maintain serum TSH levels below 0.1 mIU/ml (unless there are specific contraindications) 1.
• Treatment duration is typically short-term, with patients usually being hospitalized for a short period to minimize radiation exposure to others.
• Follow-up care involves monitoring for potential side effects, such as thyroiditis, and assessing treatment efficacy through serial thyroid function tests and imaging studies.
• RAI-refractory disease is considered when distant metastases lose their ability to concentrate RAI or structural progression occurs within 6–12 months after RAI administration 1.
• The goal of RAI therapy is to achieve a cure, which is possible in approximately one out of three patients with distant metastases, usually young individuals with well-differentiated thyroid cancer and small non-FDG-avid metastatic lesions 1.

From the FDA Drug Label

Most patients with thyroid disease present with a thyroid nodule. ... Surgical removal is the treatment of choice for thyroid carcinomas At 2 to 4 months post surgery, a whole-body I-131 scan is performed following cessation of thyroxin drug for 4-6 weeks prior to the scan, thereby causing hypothyroidism; TSH then rises and stimulates iodide uptake. Patients with significant I-131 uptake are given ablative (100 - 150 mCi) doses of I-131 For minimal uptake, doses of 30-50 mCi I-131 are given on an outpatient basis. Patients with extra thyroidal uptake from metastatic disease may be given larger doses of greater than 100 mCi. For thyroid carcinoma, 50 mCi is the usual dose for ablation of normal thyroid tissue, and 100 to 150 mCi is the usual subsequent therapeutic dose.

Radioactive Iodine (RAI) Therapy for thyroid cancer involves the administration of I-131 to destroy thyroid tissue, particularly after surgical removal of the thyroid carcinoma. The dose of I-131 varies depending on the uptake and the presence of metastatic disease, ranging from 30-50 mCi for minimal uptake to 100-150 mCi for ablative doses 2 2.

From the Research

Overview of Radioactive Iodine (RAI) Therapy

• RAI therapy with iodine-131 is used in select cases of differentiated thyroid cancer (DTC) for remnant ablation, adjuvant therapy, or treatment of known persistent disease 3.
• The therapy involves the use of radioactive iodine to destroy thyroid tissue, including cancer cells, by releasing beta particles that cause DNA damage 4.

Aims and Dosage of RAI Therapy

• The three basic aims of RAI therapy in well-differentiated thyroid tumors are ablation of the remnant, adjuvant therapy, and disease management 4.
• The dosage for ablation is 30-100 mCi, 30-150 mCi for adjuvant therapy, and 100-200 mCi for treatment 4.
• The dose of RAI is usually chosen empirically based on the risk assessment of tumor recurrence and other factors, with most medical societies recommending a dose of 1.11 GBq (30 mCi) I-131 for remnant ablation 3.

Risks and Benefits of RAI Therapy

• There is an ongoing debate about the risk of developing secondary primary malignancies (SPM) due to RAI therapy, with several multicenter studies and meta-analyses showing varying results 3.
• Some studies have shown no increased incidence of SPM, while others have indicated that cumulative RAI doses exceeding 5.6-7.4 GBq (150-200 mCi) may represent an increased risk for developing SPM 3.
• RAI therapy has been shown to be effective in achieving complete removal of the disease and increasing survival in patients with DTC 4.

Targeted Therapy and RAI Therapy

• Targeted therapy (TTx) may be used to resensitize tumors to RAI in patients with radioactive iodine-refractory (RAIR) DTC, allowing for subsequent I-131 administration and resulting in meaningful responses 5.
• TTx has been shown to downregulate MAPK signaling and sensitize tumors to RAI, with durable disease control and partial response observed in some patients 5.

Association of RAI Treatment with Cancer Mortality

• RAI treatment has been associated with elevated risks of total and site-specific cancer death among patients with hyperthyroidism, with greater organ-absorbed doses appearing to be modestly positively associated with risk of death from solid cancer, including breast cancer 6.
• The risk of cancer mortality associated with RAI treatment is a concern, and additional studies are needed to understand the risks and advantages of all major treatment options available to patients with hyperthyroidism 6.