Unusual Adrenal Gland Metastasis in a Patient with Follicular Carcinoma of the Thyroid Evidenced by 18F-FDG PET/CT and Confirmed by Biopsy

Follicular thyroid carcinoma (FTC) is a relatively rare form of differentiated thyroid carcinoma, but it is important to recognise that FTC may be very aggressive, may not respond to radioiodine treatment and may cause life-threatening complications [1, 2]. The use of 18F-fluorodeoxyglucose positron emission tomography/computed tomography (18F-FDG PET/CT) has rapidly grown in the latest years in patients with differentiated thyroid carcinoma (DTC) with a negative radio-iodine scan despite clearly elevated levels of thyroglobulin [3]. The biological pattern of DTC, which loses its thyroid-specific iodide-trapping properties and becomes FDG-avid, reflects enhanced aggressiveness and is related to a worse prognosis than FDG-negative tumours [4]. In addition, there is evidence that several factors associated with FDG positivity (tumour size, lymph node metastasis, and glucose transporter expression and differentiation) are related to a poor prognosis in patients with FTC [5]. A 78-year-old woman with a history of FTC came under our observation to undergo an 18F-FDG PET/CT scan for the first time for restaging purpose. Total thyroidectomy had been performed 19 years previously and radio-iodine treatment reached a total activity level of 1.8 Ci (66.6 GBq) over 10 years. The aggressive treatment did not prevent the development of a widespread metastatic disease involving lymph nodes and bones. The most painful bone metastases were treated with external beam radiation therapy, with effective relief of the symptoms. A suppressive therapy with L-thyroxine was started. Thyroglobulin levels were checked yearly, and were relatively high but under control (maximum value, 32.3 ng/ml). However, the thyroglobulin levels suddenly rose up to 117.5 ng/ml 6 months previously. A 131I whole-body scan was requested but the result was negative (Fig. 1). Therefore the patient was referred to our department to undergo an 18FDG-PET/CT scan. Fig. 1 131I whole-body scan images Sixty minutes following intravenous injection of 138 MBq of 18F-FDG, low-dose CT and PET images from the skull base to mid-thigh were acquired using a hybrid Biograph mCT TOF PET/CT scanner (Siemens Medical Solutions, Erlangen,Germany). Images revealed FDG-avid lesions in the sternum (SUVmax 3.5), L1 (SUVmax 8) and in the lower lobe of the right lung (SUVmax 6.3) (Fig. 2a). Moreover, there was evidence of another pathological uptake (SUVmax 7.3) in the left adrenal gland (Fig. 2b-d). The adrenal lesion was surgically removed and pathological examination revealed metastasis from the FTC (Fig. 3). Fig. 2 a Maximum intensity projection (MIP) image; b transaxial PET image; c transaxial CT image; d transaxial fused PET/CT image. Arrows FDG-avid lesions Fig. 3 Haematoxylin and eosin stained section of the surgically removed adrenal lesion The patient was then referred to the clinical oncologist to evaluate possible further therapies. The adrenal gland is a very uncommon site of metastatic spread from thyroid carcinoma. Only a few cases have been reported in the literature, describing adrenal metastases in patients affected by papillary thyroid carcinoma [6–8] or anaplastic thyroid carcinoma [9]. Although a case of iodine-avid adrenal lesion has been described previously [10], to our knowledge, this is the first case report of an adrenal gland metastasis imaged by 18F-FDG PET/CT in a patient affected by FTC with high thyroglobulin levels and negative 131I whole-body scan.