What are the treatment protocols for a patient requiring Sodium 131 (Radioiodine) therapy?

Treatment Protocols for Radioiodine (I-131) Therapy

For differentiated thyroid cancer, administer 1850-3700 MBq (50-100 mCi) of I-131 for post-surgical remnant ablation after preparation with recombinant human TSH (rhTSH), with the lower dose being equally effective and reducing whole-body radiation exposure. 1

Primary Indications and Disease-Specific Protocols

Differentiated Thyroid Cancer

Post-Surgical Ablation:

• Administer 1850 MBq (50 mCi) I-131 as the standard dose for remnant ablation in patients prepared with rhTSH, as this is equally effective as the traditional 3700 MBq (100 mCi) dose, even with lymph node metastases present 1
• The higher dose of 3700 MBq (100 mCi) remains FDA-approved but offers no additional benefit while increasing radiation exposure 1, 2
• Radioiodine ablation is indicated for high-risk and low-risk patients but NOT for very low-risk patients (unifocal T1 tumors <1 cm with favorable histology, no extrathyroidal extension or lymph node metastases) 1

Preparation Method:

• Use recombinant human TSH (rhTSH) while maintaining levothyroxine therapy as the method of choice, avoiding the morbidity of thyroid hormone withdrawal 1
• This preparation achieves equivalent ablation success rates compared to levothyroxine withdrawal 1
• rhTSH was approved by EMEA (2005) and FDA (2007) specifically for this indication 1

Metastatic Disease:

• For patients with distant metastases, doses range from 3700-7400 MBq (100-200 mCi) 3
• Treatment requires demonstration of radioiodine uptake on diagnostic imaging at all tumor sites 1

Malignant Pheochromocytoma/Paraganglioma

I-131-MIBG Therapy:

• Administer 5.5-38 GBq (150-1000 mCi) of I-131-MIBG at 3-6 month intervals for patients with good uptake on diagnostic 123-I-MIBG scans 1
• In UK protocols, typical activities range 7.4-11.2 GBq per treatment cycle 1
• Consider I-131-MIBG as first-line therapy for patients with: 1
  • Good 123-I-MIBG uptake on diagnostic scans
  • Unresectable, progressive disease
  • Symptomatic disease not amenable to locoregional control
  • High tumor burden with low number of bone metastases

Patient Selection Criteria:

• Approximately 50% of patients are eligible based on diagnostic scan uptake 1
• Objective responses occur in 22-47% of cases 1
• Grade 3-4 toxicity reported in 16-83% of patients 1

Neuroendocrine Tumors (Other)

I-131-MIBG Protocol:

• Activities of 7.4-11.2 GBq administered at 3-6 month intervals 1
• Up to 80% of functioning malignant NETs concentrate MIBG, though intensity may be insufficient for therapy 1
• Response rates of 40-60% after repeated therapy cycles 1
• Partial objective responses in 10-15% by WHO criteria 1

Pre-Treatment Requirements

Mandatory Assessments:

• Confirm adequate bone marrow reserve (especially critical for I-131-MIBG therapy) 1
• Verify adequate renal function 1
• Ensure patient is continent and self-caring to minimize nursing staff radiation exposure 1
• Measure dose with suitable radioactive calibration system immediately prior to administration 2

Thyroid Blockade (for I-131-MIBG):

• Administer potassium iodide/iodate to prevent thyroidal uptake of free radioiodide 1

Fertility Counseling:

• Provide complete oncofertility counseling to all reproductive-age patients before initiating I-131 treatment 3
• Offer sperm cryopreservation to male patients as standard of care 3
• For females, discuss oocyte/embryo cryopreservation, ovarian tissue cryopreservation, or ovarian transposition 3
• A single 3700 MBq dose may cause temporary fertility effects but permanent infertility is uncommon 3

Administration Protocols

Facility Requirements:

• I-131-MIBG therapy requires dedicated, shielded isolation facility for radiation protection 1
• Capsules can be utilized up to 30 hours after calibration time, then must be discarded per safety procedures 2
• User must wear waterproof gloves when handling capsules or containers 2

Radiation Safety:

• Annual whole body dose limits: 1 mSv (100 mrem) for public, 50 mSv (5000 mrem) for radiation workers 4
• Implement appropriate shielding, distance, and time reduction strategies 5

Post-Treatment Management

Thyroid Cancer Follow-Up:

• Measure thyroid function tests (FT3, FT4, TSH) at 2-3 months to verify adequate levothyroxine suppressive therapy 1
• At 6-12 months, perform: 1
  • Physical examination
  • Neck ultrasound
  • rhTSH-stimulated serum thyroglobulin measurement
  • Diagnostic whole body scan (WBS) may be omitted if ultrasound normal and stimulated Tg <1.0 ng/mL

TSH Suppression Targets (Intermediate-Risk Patients):

• Maintain TSH 0.1-0.5 mIU/mL for patients with detectable thyroglobulin without structural disease, requiring approximately 2.1 mcg/kg/day levothyroxine 6
• For excellent responders (undetectable Tg, negative imaging), target TSH 0.5-2 mIU/mL 6
• For structural incomplete response, maintain TSH <0.1 mIU/mL 6

Toxicity Monitoring:

• I-131-MIBG: Monitor for myelosuppression 4-6 weeks post-therapy 1
• Myelotoxicity is cumulative and may be dose-limiting after repeated cycles 1
• More severe in patients with bone marrow infiltration or prior chemotherapy 1

Critical Caveats

Contraindications:

• Pregnancy and breastfeeding are absolute contraindications 1
• Patients with biliaryenteric anastomoses after pancreatic surgery require 3 months of rotating oral antibiotics after hepatic ablation procedures 1

Organ-Specific Risks:

• Significant risks involve eye/nasolacrimal, salivary, pulmonary, gastrointestinal, hematopoietic systems, and gonads 7
• Secondary primary malignancies are a recognized long-term risk 7
• TSH suppression below 0.1 mIU/mL increases risks of atrial fibrillation, bone loss, and cardiovascular events 6

Multidisciplinary Approach:

• All cytoreductive ablation cases should be discussed at specialist hepatobiliary MDT meetings 1
• Treatment choices for metastatic pheochromocytoma/paraganglioma should be discussed case-by-case in multidisciplinary specialized settings 1