The Biochemical–Imaging Connection: Urinary Noradrenaline and Fluorodeoxyglucose-Positron Emission Tomography in Unresectable or Metastatic Pheochromocytomas and Paragangliomas

<b>Background/Objectives</b>: Pheochromocytomas and paragangliomas (PPGLs) are rare tumors of neural crest origin that secrete varying levels of catecholamines. [¹⁸F]Fluorodeoxyglucose-positron emission tomography (FDG-PET) is a valuable tool for the detection of metastases and the prediction of prognoses. However, varying FDG avidities in PPGLs raise concerns regarding cost-effectiveness and unnecessary radiation exposure. Catecholamine secretion patterns are associated with metastasis and clinical outcomes. This study aimed to explore the relationships among FDG avidity, catecholamine levels, and clinical factors in patients with PPGLs. <b>Methods</b>: This retrospective study included 25 patients with unresectable or metastatic PPGLs scheduled for [¹³¹I]metaiodobenzylguanidine therapy with FDG-PET data available within 40 days of urine catecholamine measurements. FDG avidity was assessed using semiquantitative parameters such as the maximum standardized uptake value (SUVmax), total metabolic tumor volume (MTV), and total lesion glycolysis (TLG). Urine catecholamine levels were quantified. Logistic regression and Spearman's correlation were performed to evaluate the relationship between FDG parameters and urinary catecholamine levels. <b>Results</b>: Urinary noradrenaline levels were significantly higher in patients with FDG-avid lesions than in those without (726.25 μg/day vs. 166.3 μg/day, <i>p</i> = 0.001). Noradrenaline levels showed significant positive correlations with SUVmax, MTV, and TLG (ρ = 0.527, 0.541, and 0.557, respectively; all <i>p</i> < 0.01). Urinary noradrenaline levels predicted FDG avidity with an AUC of 0.849; a cutoff value of 647.5 μg/day achieved 55.6% sensitivity and 100% specificity. <b>Conclusions</b>: Urinary noradrenaline levels were significantly associated with FDG avidity in PPGLs, suggesting their potential utility in predicting FDG-PET outcomes. Therefore, FDG-PET may be unnecessary in PPGL patients with low urinary noradrenaline levels. These findings may help optimize imaging strategies for patients with PPGLs.