What is the typical regimen for thyroid ablation radiology?

Thyroid Ablation Radiology: Typical Regimen

For radioiodine ablation of differentiated thyroid cancer, the typical regimen involves 30-50 mCi (1110-1850 MBq) of I-131 for low-to-intermediate risk patients prepared with recombinant human TSH (rhTSH) while continuing levothyroxine therapy, with higher doses of 100-200 mCi (3700-7400 MBq) reserved for high-risk patients or those with known metastatic disease. 1

Risk-Stratified Approach to RAI Dosing

The decision to administer radioiodine and the dose selection depends critically on risk stratification:

Very Low-Risk Patients (No RAI Indicated)

• Unifocal tumors <1 cm with no extrathyroidal extension or lymph node metastases 1
• Complete surgical resection with favorable histology 1
• These patients should NOT receive radioiodine ablation 1, 2

Low-to-Intermediate Risk Patients (30-100 mCi)

• Primary tumors >1 cm but <4 cm without aggressive features 1
• Microscopic invasion into perithyroidal soft tissues 1
• Cervical lymph node metastases or vascular invasion 1
• Standard dose: 30-50 mCi (1110-1850 MBq) 1
• Evidence demonstrates that 50 mCi is as effective as 100 mCi in this population, even with lymph node metastases, while reducing whole-body radiation exposure 1

High-Risk Patients (≥100 mCi)

• Macroscopic extrathyroidal extension 1
• Incomplete tumor resection 1
• Primary tumor >2 cm with aggressive features 1
• Dose: 100-150 mCi (3700-5550 MBq) 1, 2

Metastatic Disease (100-200 mCi)

• Known distant metastases require higher activities 1
• Dose: 100-200 mCi (3700-7400 MBq) 1, 2
• Administered every 6 months for 2 years if RAI-avid, then less frequently 1

Preparation Protocol

TSH Stimulation (Critical for Efficacy)

Recombinant human TSH (rhTSH) is the method of choice for preparation 1:

• Patient remains on levothyroxine (LT4) therapy throughout 1
• Demonstrates equal efficacy to thyroid hormone withdrawal but with better quality of life 1
• Reduces radiation exposure to extrathyroidal compartments 3
• FDA and EMEA approved for ablation preparation 1

Thyroid hormone withdrawal is an alternative when rhTSH is unavailable or for metastatic disease 1, 3:

• Discontinue LT4 for 3-4 weeks to achieve TSH >30 mIU/L 3
• Associated with hypothyroid morbidity and reduced quality of life 3

Low-Iodine Diet

• Implement 2-3 weeks before RAI administration 3
• Enhances radioiodine uptake by depleting body iodine stores 3

Administration Details

Standard Fixed-Dose Approach

The most common clinical approach uses fixed activities rather than dosimetry 3, 4:

• Ablation: 30-100 mCi 3, 4
• Adjuvant therapy: 30-150 mCi 4
• Treatment of disease: 100-200 mCi 4

Dosimetric Approach

• Reserved for metastatic disease when feasible 3
• May use higher activities (4-11 GBq) based on individual calculations 3
• No clear survival benefit over standard fixed dosing for most patients 1

Post-Administration Monitoring

Immediate Post-Therapy (First Week)

• Post-therapeutic whole-body scan obtained 3-7 days after administration 1
• Provides highly sensitive detection of residual tissue and occult metastases 1

Radiation Safety Precautions

Based on actual biokinetic measurements, precautions are less stringent than older models suggested 5:

For 30-50 mCi ablation dose:

• Sleep apart from pregnant women/children for 3 days 5
• Avoid close contact with children for 10-16 days depending on age 6
• Return to work after 2-3 days 6

For 100-200 mCi therapeutic dose:

• Sleep apart from pregnant women/children for 3-5 days 6, 5
• Avoid close contact with children for 4-5 days 6
• More stringent precautions for ablation patients due to slower clearance 6

Short-Term Follow-Up (2-3 Months)

• Thyroid function tests (FT3, FT4, TSH) to verify adequate LT4 suppressive therapy 1
• Target TSH <0.1 mIU/L for high-risk patients 1

Intermediate Follow-Up (6-12 Months)

• Physical examination and neck ultrasound 1
• rhTSH-stimulated serum thyroglobulin (Tg) measurement 1
• Diagnostic whole-body scan may be omitted if Tg <1.0 ng/ml and ultrasound normal 1

Critical Pitfalls to Avoid

Do not administer RAI to very low-risk patients - This provides no benefit and exposes patients to unnecessary radiation 1, 2. The 2015 ATA guidelines specifically advise against systematic RAI use in unifocal tumors <1 cm without adverse features 1.

Do not use excessive doses for ablation - Studies demonstrate 50 mCi is as effective as 100 mCi for remnant ablation, even with lymph node metastases 1. Using lower effective doses reduces whole-body radiation exposure and potential long-term complications.

Do not continue RAI indefinitely for metastatic disease - Disease is considered RAI-refractory if structural progression occurs within 6-12 months after administration or after cumulative dose of 600 mCi 1. Continuing beyond this point is unlikely to provide benefit and increases toxicity risk.

Ensure adequate TSH stimulation - Inadequate TSH elevation (<30 mIU/L) significantly reduces RAI uptake and treatment efficacy 3. When using rhTSH, follow approved protocols; when using withdrawal, verify TSH levels before administration.