Although 123I MIBG imaging is useful in the functional assessment of PPGL with high sensitivity (83–100%) and high specificity (98–100%) in detecting pheochromocytoma, its sensitivity has been downgraded in recent comparative studies, especially in metastatic pheochromocytoma and paraganglioma (mPPGL), SDHx (succinate dehydrogenate) PPGL (particularly SDHB), small tumor and mPPGLs and head and neck paragangliomas (HNPGLs) (Timmers et al. 2024). The gene discussed is SDHB; the disease is head and neck paraganglioma.