FDG PET in Pheochromocytoma:Current status

In urologic practice, diagnosis of pheochromocytoma typically begins with the evaluation of an adrenal mass as a catecholamine-hypersecreting lesion. According to current Endocrine Society guidelines, CT is the first-choice imaging due to its excellent spatial resolution of the thorax, abdomen, and pelvis.(1) Functional imaging is usually not needed for small, sporadic, unilateral adrenal tumors with an adrenergic biochemical profile.

In a meta-analysis, ⁶⁸Ga-DOTA-SSA PET/CT had the highest detection rate for PPGL (93%) compared to ¹⁸F-FDOPA (80%), ¹⁸F-FDG (74%), and ¹²³/¹³¹I-MIBG (38%).(2) However, high physiological adrenal uptake may limit its ability to detect small PHEOs in hereditary cases. Comparative studies show ¹⁸F-FDOPA PET/CT is better at detecting small PCCs in hereditary cases.

However, these newer tracers are costlier and less available; making FDG PET/CT a more practical option. FDG PET/CT has shown far superior sensitivity over ¹²³I-MIBG & ¹⁸F-FDOPA PET/CT in detecting metastatic PPGLs. In aggressive SDHx-mutated tumors, dedifferentiation leads to a loss of SSTR expression, resulting in reduced DOTATATE uptake. At the same time, increased aerobic glycolysis increases FDG uptake, making FDG PET more effective for detecting these tumors. However, its sensitivity falls to about 40% in MEN2 (RET mutation), but extra-adrenal or metastatic disease is uncommon in these cases.

Both American and European guidelines endorse ¹⁸F-FDG PET/CT for metastatic PPGLs and SDHB mutations. Though ⁶⁸Ga-DOTA-SSA PET/CT was favoured in earlier meta-analyses, they included only eight quantitative studies, mostly retrospective.(2) In a prospective NIH study of 100 surgeries, preoperative ¹⁸F-FDG PET/CT detected additional lesions in 15 cases compared to CT, MRI, and MIBG, in patients with or without known mutations.(3) This study also detected additional lesions in patients outside standard functional imaging indications.

In conclusion, anatomical imaging alone is inadequate for PPGL management. Functional imaging enhances diagnostic accuracy, guides surgical planning, and identifies occult metastatic disease. ¹⁸F-FDG PET/CT is the most practical choice due to its availability, sensitivity in metastasis, and compatibility with existing clinical workflows.

		Radionuclide Tracers
Tracer	Mechanism	Best for
123I-MIBG SPECT/CT	Norepinephrine transporters (NET)	Sporadic PCC,eligibility for 131I-MIBG therapy
¹⁸F-FDG	Glucose uptake (Warburg effect)	Aggressive, SDHB-related, dedifferentiated PCCs
⁶⁸GaDOTATATE/DOTATOC/DOTATATE/DOTANOC	Somatostatin receptor	Well-differentiated tumors, head/neck PGLs
¹⁸F-FDOPA	Amino acid uptake and decarboxylation	VHL, RET, sporadic PCCs

by Dr Avishek Mandal, VMMC & SJH, New Delhi