| name | ppgl-123i-mibg-indication |
| description | Determines when to use 123I-MIBG scintigraphy for PPGL diagnosis and whether thyroid blockade is required. Triggers include: need functional imaging to confirm PPGL origin, evaluating for metastatic PPGL or PGL, preparing for 131I-MIBG therapy requires baseline scintigraphy. |
123I-MIBG Scintigraphy Indication for PPGL
STEP 1 — Gather Information
Collect clinical suspicion of PPGL (symptoms, biochemical tests), assess need for functional imaging to confirm PPGL origin, evaluate for metastatic PPGL/PGL, and check if baseline scintigraphy is needed before 131I-MIBG therapy; review medications that may inhibit MIBG uptake (reserpine, tricyclic antidepressants).
If any trigger is present — need functional imaging to confirm PPGL origin, evaluating for metastatic PPGL/PGL, or preparing for 131I-MIBG therapy — proceed to Step 2; otherwise, do not order 123I-MIBG scintigraphy.
STEP 2 — Rule In / Rule Out
Verify that interfering medications (reserpine, tricyclic antidepressants) have been discontinued ≥1 week prior and assess eligibility for thyroid blockade (potassium iodide/Lugol's).
If no contraindications to thyroid blockade and medications are held, proceed to Step 3; otherwise, manage contraindications or consider alternative imaging.
STEP 3 — Classify or Stratify
Determine scintigraphy purpose: diagnostic (to confirm PPGL origin or evaluate metastasis) versus pre‑therapy baseline for 131I-MIBG therapy.
Proceed to Step 4 for both indications.
STEP 4 — Decide
Administer thyroid blockade (potassium iodide 50–150 mg daily or Lugol’s solution 5–10 drops daily) starting 3 days before scintigraphy and continuing through the day of imaging; perform 123I‑MIBG scintigraphy per institutional protocol.
Interpret results for PPGL uptake; false positives/negatives possible — correlate with clinical and biochemical data.
Clinical Guardrails / Mimics / Pitfalls
123I‑MIBG scintigraphy is highly specific but may yield false positives (e.g., uptake in normal adrenal glands, salivary glands, myocardium, neuroendocrine tumors) and false negatives (particularly in metastatic PPGL, SDHx‑variant‑positive cases, or poorly differentiated tumors). Thyroid blockade reduces thyroid radiation exposure and lowers risk of medullary thyroid cancer; do not omit blockade. Discontinue reserpine and tricyclic antidepressants ≥1 week before testing to avoid reduced MIBG uptake. Avoid scintigraphy in patients unable to tolerate thyroid blockade or with known iodine allergy without risk‑benefit assessment.
Concrete Clinical Example
A 45‑year‑old patient with episodic headaches, palpitations, and hypertension has elevated plasma‑free metanephrines. Functional imaging is needed to confirm PPGL origin. After holding tricyclic antidepressants for 1 week and starting potassium iodide 100 mg daily, 123I‑MIBG scintigraphy shows intense adrenal uptake consistent with pheochromocytoma, guiding surgical planning.
Source: Japan Endocrine Society Clinical Practice Guideline for the Diagnosis and Management of Pheochromocytoma and Paraganglioma 2025, Japan Endocrine Society, 2025, DOI: 10.1507/endocrj.EJ25-0165
