Unilateral Brown Fat Suppression on FDG PET/CT-detecting Sympathetic Denervation

Brown fat is known to be innervated by sympathetic nervous system.[1] Sympathetic fibers from the thoracolumbar region of spinal cord leave the spinal nerves and connect with paravertebral sympathetic chain which can be damaged due many conditions including trauma, malignancies, vascular injury, and iatrogenic injury.[234] The superior cervical ganglion that provides innervations to the head and neck region receives sympathetic fibers from upper thoracic region,[5] and damage to these fibers can result in Horner's syndrome. Few previous studies have shown uptake of FDG PET/CT in the brown fat [67] and have tried to correlate disruption of sympathetic innervations of brown fat using the same. Usually, the brown fat FDG uptake, which is commonly seen in winter season, is bilaterally symmetrical in cervical, interscapular, paravertebral, and axillary region.[6]

We present here a case where 2-deoxy-2-(fluorine-18) FDG positron emission tomography/computed tomography (18-F-FDG PET/CT) showed unilateral uptake corresponding to the brown fat [Figure 1]. This 9-year-old male who initially presented with PNET of the right 3^rd rib was treated with neoadjuvant chemotherapy and 3^rd rib excision followed by adjuvant chemoradiation. He again presented with pleural metastases for which he underwent posterolateral thoracotomy and excision of parietal pleural nodule followed by bone marrow transplant. Two years later, he again presented with lytic expansile at the right 11^th rib lesion with paravertebral soft-tissue component for which he underwent chemoradiation. Follow-up 18F-FDG PET/CT revealed paravertebral soft-tissue thickening near costovertebral junction in the region of right 10^th and 11^th ribs with minimal FDG uptake which show significant reduction in size and metabolic uptake as compared to previous PET/CT. Interestingly, 18F-FDG PET/CT revealed unilateral brown fat metabolic suppression in the form of decreased FDG uptake in the ipsilateral cervical, axillary, and paravertebral brown fat as compared to metabolically active contralateral brown fat which suggested paravertebral sympathetic chain damage during the posterolateral thoracotomy/rib excision surgery or due to infiltration by the paravertebral soft-tissue mass. On detailed clinical enquiry, the patient revealed a history of absent sweating on the ipsilateral side. This finding suggests that unilateral lack of brown fat uptake on 18F-FDG PET/CT can be a pointer of sympathetic denervation and must be brought to the notice of the treating clinician.

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Figure 1 18F FDG PET/CT MIP image (a), Axial CT(b and d), Fused PET/CT (c and e) and Coronal images (f-h) reveals unilateral decreased FDG uptake corresponding to right cervical, axillary and paravertebral brown fat (black arrow) and preserved FDG uptake in brown fat on left side.

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