Perirenal ¹⁸F-FDG Uptake in a Patient with a Pheochromocytoma

Abstract

Increased ¹⁸F-fluorodeoxyglucose (FDG) uptake of brown fat on ¹⁸F-FDG positron emission tomography (PET) originating from physiological activation is a common incidental finding and is usually located in the neck, shoulder, and supraclavicular areas. We present a case of an incidental pheochromocytoma showing diffusely increased ¹⁸F-FDG uptake in bilateral perirenal fat tissue as well as supraclavicular and paravertebral fat tissue on ¹⁸F-FDG PET/CT. The patient had no clinical symptoms except hypertension, and a pheochromocytoma was confirmed in a postsurgical specimen. A pheochromocytoma should be considered a cause in cases of increased ¹⁸F-FDG uptake of perirenal brown fat.

Introduction

Brown fat is abundant in infants to regulate body temperature without shivering but diminishes with age. As a result, the proportion of brown fat relative to the total body mass is very small in adults [1, 2]. Cold exposure or food intake can stimulate brown fat, and activated brown fat increases glucose uptake [3]. Uptake of ¹⁸F-fluorodeoxyglucose (FDG) by brown fat on ¹⁸F-FDG positron emission tomography (PET) is a common incidental finding [4] and may mimic tumor uptake [5].

We present a case of intense ¹⁸F-FDG uptake of brown fat in perirenal as well as supraclavicular and paravertebral areas resulting from an incidental pheochromocyotoma.

Case Report

A 53-year-old female was referred to our hospital to evaluate a uterine mass found incidentally during a medical checkup. She was on medication for hypertension and diabetes mellitus. She had no clinically remarkable symptoms or medical history.

An 85-mm-sized lobulated mass involving the uterine body and cervix was observed on pelvic magnetic resonance imaging (MRI) (Fig. 1a). A uterine malignancy could not be excluded completely based on the MRI findings. Therefore, the patient underwent an ¹⁸F-FDG PET/CT scan after a 6-h fast. Her blood glucose level prior to injection of ¹⁸F-FDG was 8.5 mmol/l, and her lean body mass was 35 kg (80 %). At the time of ¹⁸F-FDG injection, the outdoor temperature was 14.7 °C and the room temperature was 26.8 °C. She was provided with a warming blanket in the injection room. The PET/CT scan was performed 60 min after intravenous injection of 370 MBq ¹⁸F-FDG. Non-enhanced CT was performed for attenuation correction, and a subsequent emission scan was performed from the skull base to the proximal thigh. The uterine mass showed homogeneous mild ¹⁸F-FDG uptake with a maximum SUV of 1.9, suggesting a leiomyoma rather than a uterine malignancy (Fig. 1b).

Fig. 1.

T1-weighted transaxial MR image (a) shows an 85-mm-sized mass with low signal intensity involving the uterine body and cervix. Fused transaxial ¹⁸F-FDG PET/CT image (b) shows mild homogeneous uptake into the uterine mass

In addition to the uterine mass, there was symmetric multifocal increased ¹⁸F-FDG uptake with a maximum SUV up to 11.7 in supraclavicular, paravertebral, and perirenal fat tissue (Fig. 2). In addition, a 65-mm-sized heterogeneous hypermetabolic mass with a maximum SUV of 9.3 was detected in the left adrenal gland, which could not be excluded as an adrenal malignancy. To characterize the left adrenal mass, conventional contrast-enhanced abdominopelvic CT was performed, and a 65-mm-sized heterogeneously enhancing mass with an intramural cyst was detected in the left adrenal gland. A 24-h urine sample was collected under suspicion of a pheochromocytoma. The analysis revealed elevated vanillylmandelic acid [65.36 mg/day (normal, < 8 mg/day)], total metanephrine [4.87 mg/day (normal, < 0.8 mg/day)], and epinephrine [3,708.50 μg/day (normal, < 80 μg/day)].

Fig. 2.

Maximum-intensity projection image of ¹⁸F-FDG PET (a) shows symmetrical uptake in perirenal (long arrows) and paravertebral areas (short arrows) as well as intense uptake into the left adrenal gland (arrowhead). Fused transaxial ¹⁸F-FDG PET/CT image (b) shows intense uptake into the left adrenal gland (arrowhead) and both perirenal fat areas (long arrows). Transaxial contrast-enhanced CT image (c) shows a 65-mm-sized heterogeneously enhanced mass with an intramural cyst in the left adrenal gland (arrowhead)

The patient underwent a left adrenalectomy and total hysterectomy on the same day. The left adrenal mass was histologically confirmed as a pheochromocytoma, and the uterine mass was confirmed as a uterine leiomyoma. In addition, brown fat was identified in the perirenal tissue adjacent to the left adrenal mass (Fig. 3).

Fig. 3.

Photomicrograph of the surgical specimen shows the pheochromocytoma and brown fat tissue. The pheochromocytoma is composed of large eosinophilic cells arranged in an alveolar and trabecular pattern with capillaries in between (long arrow). Brown fat cells have centrally located nuclei and multivacuolated cytoplasm (short arrows) (hematoxylin-eosin stain, original magnification, ×400)

Discussion

Increased ¹⁸F-FDG uptake originating from physiological activation of brown fat is usually located in the neck, shoulder, and supraclavicular areas [6]. In the present case, diffuse ¹⁸F-FDG uptake was identified in the bilateral perirenal area and was probably related to the pheochromocytoma.

Physiological activation of brown fat due to cold exposure or food intake has been reported in adults [3]. Brown fat is rich in adrenergic receptors, and catecholamines increase glucose uptake by sympathetic stimulation of brown fat [7]. A relationship between plasma catecholamine levels and brown fat activity has been reported [8]. In the present case, extensive ¹⁸F-FDG uptake into brown fat was postulated to have originated from sympathetic stimulation by the pheochromocytoma.

The differential diagnosis of ¹⁸F-FDG uptake of brown fat includes a tumor, lymph node, and muscle [5, 9]. The advent of integrated PET/CT has enabled identifying brown fat uptake. Intense FDG uptake corresponding to no soft tissue other than fat can be considered brown fat [10]. The typical location and distribution of brown fat also helps differentiate it from other entities. Brown fat is commonly found in the neck, shoulder, and supraclavicular areas with a bilateral symmetric distribution and it may be found in the mediastinal, paravertebral, axillary, and perirenal areas [6]. In our case, high ¹⁸F-FDG uptake of perirenal brown fat may have obscured the left adrenal tumor on ¹⁸F-FDG PET/CT.

Pheochromocytoma is an uncommon characteristic catecholamine-secreting neoplasm derived from chromaffin cells [11]. Its annual prevalence is about 1–2 per 100,000 adults [12]. Excessive release of catecholamines can cause clinical symptoms such as hypertension, headache, palpitations, and diaphoresis [11]. In our case, the hypertension may have been related to the pheochromocytoma. About 10 % of pheochromocytomas are incidentally discovered on CT or MRI [13]. A biochemical diagnosis of pheochromocytoma is established by increased levels of catecholamines and their metabolites in a 24-h urine specimen [14].

Pheochromocytoma has been reported to be a cause of ¹⁸F-FDG uptake into brown fat, and extensive ¹⁸F-FDG uptake into perirenal brown fat has also been reported [15–18]. To our knowledge, however, there is no report about the difference in SUV between brown fat with pheochromocytoma and brown fat without pheochromocytoma. In our case, low body weight, female sex, and low outdoor temperature could be prevalent factors causing increased ¹⁸F-FDG uptake of brown fat [1]. Nevertheless, because the incidence of increased ¹⁸F-FDG uptake into perirenal brown fat is much lower than that into supradiaphragmatic brown fat [18], it is highly possible that combined pheochromocytoma induced intense ¹⁸F-FDG uptake into perirenal brown fat in our case.

In summary, we report a case of intense ¹⁸F-FDG uptake in bilateral perirenal as well as supraclavicular and paravertebral fat tissue resulting from an incidental pheochromocytoma. A pheochromocytoma should be considered a cause of increased ¹⁸F-FDG uptake into perirenal brown fat.