I-131-MIBG Therapy

Quick Answer

I-131-MIBG (iobenguane I-131) is a norepinephrine analog taken up via the norepinephrine transporter and stored in catecholamine-secreting tumor cells, where I-131 delivers beta radiation. It treats MIBG-avid, unresectable or metastatic pheochromocytoma and paraganglioma, and is used in relapsed/refractory neuroblastoma. Patients are selected by a diagnostic I-123-MIBG (or I-131-MIBG) scan confirming avidity, and the thyroid is blocked with stable iodine before and after treatment.

Clinical Importance

I-131-MIBG is one of the oldest theranostic pairs (diagnostic I-123-MIBG → therapeutic I-131-MIBG) and remains an important option for MIBG-avid neuroendocrine tumors that are surgically incurable, including catecholamine-driven pheochromocytoma/paraganglioma and high-risk neuroblastoma.

Evidence & Indication

• Pheochromocytoma / paraganglioma: a high-specific-activity I-131-iobenguane formulation (Azedra) was FDA-approved for iobenguane-avid, unresectable, locally advanced or metastatic disease requiring systemic therapy, based on reductions in antihypertensive medication and tumor response.
• Neuroblastoma: used in relapsed/refractory high-risk disease, often in specialized pediatric centers.

Patient Selection

• MIBG-avid disease confirmed on diagnostic I-123-MIBG imaging.
• Adequate marrow and renal function; ability to comply with radiation-safety isolation.
• Review interfering medications that block norepinephrine-transporter uptake (many antihypertensives, tricyclics, sympathomimetics) and hold per protocol.

Preparation

• Thyroid blockade with stable iodine (e.g. potassium iodide/SSKI) starting before and continuing after therapy to protect the thyroid from free radioiodine.
• Hold interfering drugs.
• Hydration and antiemetics.

Workflow

Confirm avidity → thyroid blockade → slow IV administration in a shielded/isolation setting → inpatient radiation isolation until release criteria met → post-therapy scan.

Dose

Therapeutic activity depends on formulation and protocol (fixed-activity or dosimetry-guided, weight-based in children). Multiple cycles may be used. Follow the specific product labeling and institutional dosimetry protocol.

Monitoring

CBC (delayed and sometimes prolonged myelosuppression, especially thrombocytopenia), thyroid function (risk of hypothyroidism despite blockade), blood pressure/catecholamine symptoms.

Radiation Safety

I-131 is a penetrating gamma emitter — therapy usually requires inpatient radiation isolation with release governed by measured dose rate/retained activity per regulatory criteria. Detailed post-discharge precautions are provided.

Expected Outcomes

Symptom control (including catecholamine excess), reduced antihypertensive requirements, and tumor responses in avid disease.

Complications & Toxicities

• Myelosuppression — can be prolonged; thrombocytopenia is often dose-limiting.
• Hypothyroidism — monitor long-term.
• Catecholamine release / hypertensive effects around administration.
• Secondary malignancy risk with cumulative radiation.

The theranostic gate & preparation detail

Confirm MIBG avidity on diagnostic I-123-MIBG before offering I-131-MIBG therapy — it is the theranostic gate, and non-avid disease will not respond.

Preparation centers on two things: thyroid blockade with stable iodine (potassium iodide/SSKI) started before and continued after treatment to protect the thyroid from free radioiodine, and a careful medication review — many drugs block norepinephrine-transporter uptake (labetalol and other antihypertensives, tricyclics, sympathomimetics, some opioids) and must be held per protocol or the therapy is wasted.

Isolation & radiation safety

Because I-131 emits a penetrating 364 keV gamma, high-activity therapy usually requires inpatient radiation isolation with release governed by measured dose rate or retained activity per regulatory criteria, followed by detailed home precautions. This contrasts with the outpatient delivery of pure/low-gamma beta emitters like Lu-177 and Ra-223, and it is a major logistical difference in planning MIBG therapy.

Response Assessment

MIBG scan avidity, catecholamine/metanephrine levels, blood pressure control, and conventional imaging.

Self-Check

Key References

• Pryma DA, et al. Efficacy and safety of high-specific-activity I-131-MIBG (iobenguane) in metastatic pheochromocytoma/paraganglioma. J Nucl Med.
• SNMMI/EANM guidance on I-131-MIBG therapy.