Residency · Residency · Nuclear Medicine

I-131 MIBG Therapy for Neuroblastoma and Pheochromocytoma

Introduction

I-131 metaiodobenzylguanidine (MIBG) therapy is a targeted radionuclide treatment for tumors of neural crest origin that express the norepinephrine transporter (NET). The FDA approved high-specific-activity (HSA) I-131 MIBG (Azedra/iobenguane I-131) in 2018 for unresectable, locally advanced, or metastatic pheochromocytoma and paraganglioma (PPGL) in patients aged 12 and older. I-131 MIBG remains an important therapeutic modality for relapsed and refractory neuroblastoma in pediatric oncology.

Mechanism of Action

MIBG is a structural analog of norepinephrine and guanethidine. It is actively transported into cells via the norepinephrine transporter (NET), also called the uptake-1 mechanism, and is stored in neurosecretory granules within chromaffin cells and sympathetic neurons. When labeled with I-131, it delivers beta radiation with a maximum energy of 606 keV and a mean tissue range of 0.8 mm directly to tumor cells. I-131 has a half-life of 8.0 days and emits gamma radiation at 364 keV, which allows post-therapy imaging to verify biodistribution.

Diagnostic MIBG Imaging

I-123 MIBG is the preferred diagnostic agent because its 159 keV gamma energy produces superior image quality compared to the 364 keV gamma of I-131. The diagnostic scan confirms MIBG avidity before therapeutic administration. Scoring systems used for neuroblastoma include the Curie score (a semi-quantitative system scored from 0 to 45) and the SIOPEN score. Patients with MIBG-negative tumors are not candidates for I-131 MIBG therapy.

Patient Preparation

Thyroid Blockade

Thyroid blockade is mandatory. Potassium iodide (SSKI) or Lugol solution is administered beginning 24 to 48 hours before I-131 MIBG administration and continued for 10 to 14 days after therapy. This protects the thyroid gland from uptake of free I-131.

Medication Interactions

Several medications must be discontinued before MIBG therapy because they interfere with uptake at the norepinephrine transporter. Tricyclic antidepressants should be stopped 2 weeks prior. Sympathomimetics and decongestants require 48 to 72 hours of washout. Labetalol must be stopped 72 hours before therapy. Cocaine and amphetamines also interfere with uptake. Medications that are safe to continue include ACE inhibitors, calcium channel blockers, and most beta-blockers other than labetalol.

Pheochromocytoma and Paraganglioma

Azedra (HSA I-131 MIBG)

The high-specific-activity preparation has a higher ratio of radiolabeled to unlabeled MIBG, which improves tumor uptake by reducing competition for the norepinephrine transporter from cold (unlabeled) MIBG molecules. The FDA-approved dose is 500 mCi (18.5 GBq) for the first dose and a minimum of 250 mCi for subsequent doses. Two doses are administered at least 90 days apart. The high I-131 activities require inpatient radiation isolation.

Efficacy in PPGL

The pivotal trial demonstrated a 25% overall response rate consisting of partial responses. Durable antihypertensive medication reduction was achieved in 53% of patients, and catecholamine levels responded in many patients. This therapy provides both tumor control and hormonal symptom relief.

Special Considerations for PPGL

Patients with pheochromocytoma are at risk for catecholamine crisis, so alpha-blockade should be established before therapy. Hemodynamic monitoring is required during and after administration. Tumor lysis syndrome is rare but possible in high-volume disease.

Neuroblastoma

Disease Context

Neuroblastoma is the most common extracranial solid tumor of childhood. Approximately 90% of neuroblastomas are MIBG-avid. I-131 MIBG therapy is used primarily for relapsed or refractory high-risk neuroblastoma and is often combined with other therapies including chemotherapy and immunotherapy.

Dosing Approaches

Fixed-dose protocols typically use 12 to 18 mCi/kg (444 to 666 MBq/kg). Dosimetry-guided approaches target a whole-body dose of 2 to 4 Gy. Higher doses may be administered with autologous stem cell rescue to manage the expected myelosuppression. Typical maximum single doses range from 450 to 700 mCi depending on patient weight and the specific protocol.

Efficacy in Neuroblastoma

The overall response rate in relapsed and refractory disease is 30 to 40%. Responses improve when I-131 MIBG is combined with radiosensitizing chemotherapy such as topotecan or vincristine/irinotecan. I-131 MIBG is also used as part of tandem therapy or conditioning regimens before stem cell transplant.

Toxicity

Hematologic (Dose-Limiting)

Thrombocytopenia is the most common and dose-limiting toxicity, with a nadir at 4 to 6 weeks. Neutropenia is significant and may require G-CSF support. Anemia is common. Stem cell support may be required at higher doses. Myelodysplastic syndrome represents a long-term risk with cumulative doses.

Endocrine

Hypothyroidism occurs despite thyroid blockade, and long-term TSH monitoring is essential. Growth hormone deficiency is possible in pediatric patients. Gonadal toxicity with potential long-term fertility effects may also occur.

Other

Nausea and vomiting are usually mild and respond to antiemetics. Hypertension may occur from catecholamine release, particularly in patients with pheochromocytoma. Salivary gland inflammation (sialadenitis) results from I-131 uptake. Radiation cystitis is rare and can be minimized by maintaining hydration and encouraging frequent voiding.

Radiation Safety

Patient Isolation

High I-131 doses require inpatient radiation isolation until the exposure rate falls below NRC release criteria. Typical isolation lasts 3 to 5 days depending on the dose and clearance rate. Specialized lead-lined rooms or rooms with adequate shielding are required. Nursing care must follow time, distance, and shielding principles.

Pediatric-Specific Considerations

Parent and guardian education on radiation safety at home after discharge is essential. School restrictions may apply for 1 to 2 weeks. Urine contamination management is particularly important in non-toilet-trained children. Emotional support for isolated pediatric patients during the isolation period is a critical aspect of care.

Emerging Developments

I-131 MIBG combined with immunotherapy, specifically anti-GD2 antibodies, is under investigation for neuroblastoma. At-211-labeled MIBG, an alpha-emitting analog, is in development. Improved dosimetry using SPECT/CT quantification is advancing the field. Combinations with targeted agents including ALK inhibitors and MEK inhibitors are also being explored.

Clinical Pearls

Thyroid blockade must begin 24 to 48 hours before I-131 MIBG therapy and continue for 10 to 14 days after. Failure to protect the thyroid is a serious protocol violation that can result in preventable hypothyroidism.

High-specific-activity I-131 MIBG (Azedra) improves tumor targeting by reducing competition from unlabeled MIBG at the norepinephrine transporter and is the FDA-approved formulation for pheochromocytoma and paraganglioma.

Thrombocytopenia is the dose-limiting toxicity with a nadir at 4 to 6 weeks. High-dose protocols for neuroblastoma may require autologous stem cell rescue to manage severe myelosuppression.

References

1. Pryma DA, et al. Efficacy and Safety of High-Specific-Activity I-131 MIBG for Pheochromocytoma/Paraganglioma. Journal of Clinical Oncology. 2019;37(34):3172-3179.
2. Defined MIBG Therapy Guidelines for Neuroblastoma. Pediatric Blood & Cancer. 2021;68(5):e28937.
3. Defined SNMMI Procedure Standard for MIBG Therapy. Journal of Nuclear Medicine. 2020;61(6):e1-e13.
4. Defined Radiation Safety Considerations for I-131 MIBG Therapy. Health Physics. 2021;120(4):427-436.

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