Diffuse Metastasis to the Thyroid: Unique Ultrasonographic Finding and Clinical Correlation

Cases of metastases to the thyroidgland seem to be increasing in recent years. The clinical and ultrasonographic findings of diffuse metastases have been sparsely reported. Thirteen cases of diffuse metastases to the thyroidgland were documented by thyroidultrasonography-guided fine needle aspirationcytology between 2004 and 2013. We retrospectively reviewed the patients with diffuse thyroidmetastases. The most common primary site was the lung (n=9), followed by unknown origin cancers (n=2), cholangiocarcinoma (n=1), and penile cancer(n=1). Eleven patients were incidentally found to have thyroidmetastases via surveillance or staging FDG-PET. Other 2 patients were diagnosed during work-up for hypothyroidism and palpable cervical lymph nodes. On ultrasonography, the echogenicityof the enlarged thyroidgland was heterogeneously hypoechoic or isoechoic, and reticular pattern internal hypoechoic lines were observed without increased vascularity found by power Doppler ultrasonography (3 right lobe, 2 left lobe, and 8 both lobes). In the 8 patients who had involvement of both lobes, 3 had hypothyroidism. In conclusion, ultrasonographic finding of diffuse metastasis is a diffusely enlarged heterogeneous thyroidwith reticular pattern internal hypoechoic lines. Thyroid function testingshould be performed in all patients with diffuse thyroidmetastases, especially those with bilateral lobe involvement. Graphical Abstract Keywords: Thyroid, Diffuse, Metastasis, Ultrasonography, Fine Needle AspirationCytology INTRODUCTION The thyroidgland has an abundant arterial supply. Despite this, metastases to the thyroidgland have been reported in only 1.4%-3% of all patients who have surgery for thyroidmalignancy (1, 2, 3). This low incidence is possibly due to a fast arterial flowthrough the thyroid; and high oxygen and iodine content of the thyroidgland may inhibit the settling and growth of metastatic cells (4). In addition, the relatively short life expectancy of patients with advanced non- thyroidmalignancies discourages the evaluation of newly detected thyroidlesions. However, the incidence in autopsy studies has been variable, with rates from 0.5% in an unselected autopsy study to 24% in autopsies of patients with non- thyroidmalignancies (5,6, 7, 8). The incidence in clinical studies has been gradually increasing because of the use of high-resolution ultrasonography (US) and frequent use of fine-needle aspirationcytology (FNAC) in suspected lesions (1, 4, 9, 10, 11). The typical US findings of metastases to the thyroidgland consist of hypoechoic nodules with ill-defined margins and intranodular vascularization; however, diffuse metastases have been sparsely reported. The aim of this study was to determine ultrasonographic findings and clinical characteristics of diffuse metastases to the thyroidgland.