Unusual Pattern of Brown Fat Uptake on ¹⁸F-FDG PET/CT in a Case of Malignant Phaeochromocytoma

A 32-year-old woman presented with backache, sudden onset of excessive sweating, uneasiness, and occasional headache for 3 months and new onset of hypertension. Ultrasonography of the abdomen revealed a right suprarenal mass. Urine metanephrines were 729 ug/24hours and normetanephrines were 1026 ug/24hours. Therefore, the patient was referred to as carcinoma of unknown primary and hence was referred for ¹⁸F-FDG PET/CT.

BAT (brown adipose tissue) is the main thermoregulatory organ of humans and constitutes just 1% of adult body mass. It is under sympathetic control through stimulation of beta 3 adrenoceptors by norepinephrine, for its stress induced thermogenesis and glucose uptake.^[1]

Phaeochromocytoma is a rare catecholamine-secreting tumour arising from chromaffin cells of the adrenal medulla. ^[2,3] Due to the persistent elevation of catecholamines in phaeochromocytoma, there are two pathological changes: upregulation of uncoupling protein-1 (UCP-1) expression leading to hyperplasia and hypertrophy of BAT and trans differentiation of white adipocytes to brown adipocytes, leading to pseudo-tumoral appearance, especially in areas which are in close proximity to phaeochromocytoma.^[4] Suspicious areas of BAT on imaging, including CT scans or PET/CT scans, are identified by low attenuation with Hounsfield unit (HU) of fat density (range: -250 to -50 HU), size greater than 4 mm, and a maximum standardised uptake value of ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) of at least 2.0 g/ mL. Typical locations and bilateral findings support the diagnosis.^[4] Propanolol, being a non-selective beta blocker,decreases its function; thus, even low doses of 20 mg have been shown to reduce brown fat expression, as demonstrated in these images [Fig 1 and 2].^[5,6]

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Fig 1: (a) MIP image of ¹⁸F-FDG PET/CT scan shows intense symmetrical tracer uptake in the cervical, supraclavicular, mediastinal, paravertebral, intercostal, and abdominal (mesentery) region. Increased uptake in the right suprarenal region is noted. A repeat scan was done after 48 hrs with FDG injection done 2 hrs post administration of 20mg oral Propranolol to suppress brown adipose tissue uptake. (b) MIP image of ¹⁸F-FDG PET/CT scan with brown fat suppression shows focal uptake in right suprarenal mass. Uptake is also seen in central thoracic region and bilateral lateral chest wall region corresponding to uptake in dorsal vertebra (D8) and bilateral ribs (right 4th and left 8th ribs). (c) Plain CT image shows a right suprarenal mass. (d) Plain CT images of thorax shows lytic lesion involving D8 vertebra. (e) Plain CT image shows no obvious soft tissue lesion. (f-h) Fused FDG PET/CT images of baseline scan- show intense FDG uptake in right suprarenal mass alongwith FDG uptake without morphologically discernible soft tissue lesion in corresponding CT images in paravertebral, left pararenal region, intercostal spaces and mesentery, suggestive of physiological brown fat uptake. Extensive brown fat interference caused hindrance in metastatic staging. (i-k) Fused images of FDG PET/CT done with fat suppression show focal FDG avidity in right suprarenal mass, D8 vertebral lesion and right 8th rib lesion. No confounding tracer uptake seen in brown fat. MIP: Maximum intensity projection; FDG: Fluorodeoxyglucose; PET/CT: Positron emission tomography/computed tomography; CT: Computed tomography

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Fig 2: Pictograph of the histopathology: (a) Low power view (H&E, 10x); (b) High power view (H&E, 40x) shows large polygonal tumour cells showing abundant fine granular amphophilic cytoplasm with pigmented granules in few of the cells and round to oval hyperchromatic nuclei with moderate anisonucleosis. Dilated capillaries are also seen between tumour cells. Tumour cells show diffuse immunoreactivity for (c) Chromogranin (40x) and for (d) Synaptophysin (40x). Above findings are suggestive of paraganglioma. H&E: Haemotxylin and eosin

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