What is the protocol for radioiodine (I-131) therapy?

Radioiodine (I-131) Therapy Protocol

For differentiated thyroid cancer, radioiodine therapy should be administered using a risk-stratified approach with doses of 100-200 mCi for high-risk patients, 30-100 mCi for intermediate-risk patients, and 30 mCi for low-risk patients when indicated, with preparation using recombinant human TSH (rhTSH) as the preferred method over thyroid hormone withdrawal. 1

Patient Selection and Risk Stratification

High-Risk Patients (Definitive Indication)

• Patients with distant metastases, gross extrathyroidal extension, or documented lymph node metastases require RAI therapy with 100-200 mCi (3.7-7.4 GBq) with TSH stimulation. 1
• This includes patients with T3-T4 tumors, incomplete resection, or metastatic disease at presentation. 2, 1

Intermediate-Risk Patients (Generally Recommended)

• Administer ≥100 mCi with either rhTSH or thyroid hormone withdrawal for patients with T1 >1cm, T2 tumors, aggressive histology, or vascular invasion. 1
• Consider RAI therapy with 100-150 mCi for patients with stimulated thyroglobulin >10 ng/mL. 2

Low-Risk Patients (Optional, Individualized)

• Use 30-100 mCi with preference for lower doses (30 mCi) with rhTSH if RAI is given. 1
• RAI is not recommended for unifocal T1 tumors <1 cm without high-risk features (no extrathyroidal extension, no lymph node metastases, favorable histology). 2, 1

Preparation Protocol

Recombinant Human TSH (rhTSH/Thyrogen) - Preferred Method

The standard two-dose regimen consists of Thyrogen 0.9 mg IM injection on Day 1 and Day 2, followed by radioiodine administration on Day 3. 1

• This method is equally effective as thyroid hormone withdrawal but with superior patient tolerance, allowing patients to remain on levothyroxine therapy. 2, 1
• Target TSH should be >30 mIU/L before RAI administration. 1
• FDA-approved for remnant ablation using 100 mCi (3700 MBq) of I-131, though lower doses (50 mCi/1850 MBq) are equally effective even with lymph node metastases. 2

Thyroid Hormone Withdrawal (Alternative)

• If high likelihood of therapy exists, thyroid hormone withdrawal may be suggested. 2
• Requires adequate TSH stimulation (>30 mIU/L) before RAI administration. 1

Timing and Administration

• RAI therapy is typically administered 2-12 weeks post-thyroidectomy. 1
• For metastatic disease, radioiodine imaging should be performed every 12 months until no response is seen to RAI treatment in iodine-responsive tumors. 2
• Post-treatment I-131 imaging should be obtained after administration to detect previously undetected metastatic disease (occurs in 6-13% of cases). 1

Special Considerations for Metastatic Disease

CNS Metastases

• Consider neurosurgical resection and/or radioiodine treatment with rhTSH and steroid prophylaxis if radioiodine imaging positive, with consideration of dosimetry to maximize dosing. 2

Bone Metastases

• Radioiodine treatment if radioiodine imaging positive with consideration of dosimetry to maximize dosing and/or radiation therapy. 2
• Consider bisphosphonate therapy and embolization of metastases. 2

Other Sites

• Consider surgical resection and/or RT of selected, enlarging, or symptomatic metastases and/or radioiodine if positive uptake. 2

Post-Treatment Monitoring Protocol

Initial Follow-up (2-3 months)

• Thyroid function tests (FT3, FT4, TSH) to check adequacy of levothyroxine suppressive therapy. 2

6-12 Month Assessment

• Perform rhTSH-stimulated serum thyroglobulin measurement with or without diagnostic whole body scan, along with neck ultrasound. 2, 1
• Thyroglobulin testing becomes a more specific marker for recurrent disease after complete thyroid tissue ablation. 1
• Neck ultrasound is the most effective tool for detecting structural disease in the neck. 1

TSH Suppression Targets

• Maintain TSH levels in the low-normal range (0.5-2 μIU/ml) in patients with excellent response. 1
• Consider mild suppression (0.1-0.5 μIU/ml) in higher-risk patients. 1

Long-term Surveillance

• If I-131 imaging negative and stimulated Tg >2-5 ng/mL, consider additional non-radioiodine imaging (e.g., FDG-PET ± CT if Tg >10 ng/mL). 2

Absolute Contraindications

• Pregnancy is an absolute contraindication due to risk of radiogenic cretinism, as fetal thyroid absorbed doses can reach 5,000 rad/mCi after 10-12 weeks gestation. 3
• Breastfeeding is contraindicated during RAI therapy. 1, 4
• For patients wishing to conceive, at least 6 months of contraception should be observed after I-131 administration. 4

Critical Pitfalls to Avoid

• Avoid administering RAI to very low-risk patients (unifocal T1 <1cm with favorable features), as this represents overtreatment without mortality or morbidity benefit. 2, 1
• Ensure adequate TSH stimulation (>30 mIU/L) before RAI administration to maximize uptake and efficacy. 1
• For patients with thyroid-associated ophthalmopathy, close follow-up is required after I-131 therapy to rule out aggravation of eye disease. 4
• Antithyroid medication should be stopped at least 1 week before radioiodine administration to avoid reduced efficacy. 5