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Background: Poorly differentiated thyroid carcinoma (PDTC) constitutes 1-15% of all thyroid cancers.1 Invasive adrenal metastases secondary to PTDC are exceedingly rare. Clinical Case: A 64-year-old woman with a non-toxic multinodular goiter presented with right upper quadrant abdominal pain and distension for three months. CT imaging revealed a 13.5 cm right suprarenal retroperitoneal mass invading the liver and inferior vena cava (IVC), concerning for adrenocortical carcinoma. She underwent resection of the mass with en block right adrenalectomy, partial hepatectomy, and IVC resection. Pathology demonstrated metastatic thyroid cancer with necrosis of the adrenal gland and IVC. Immunohistochemical staining was positive for PAX8, TTF1, and thyroglobulin (Tg). Completion thyroidectomy revealed an encapsulated 2 cm focus of PDTC with Hurthle cell phenotype in the right thyroid lobe. The mitotic activity was 5/10 per HPF. There were focal areas of tumor necrosis, 3 foci of capsular invasion, and extensive angioinvasion. Surgical margins were free of tumor invasion. Eight resected lymph nodes were negative for malignancy (Stage T1bN0M1; AJCC 8, Stage IVb). Genetic testing was positive for somatic mutations of NRAS, TERT, PTEN, and GNAS with broad copy number loss on chromosome 22q conferring aggressive tumor behavior.3 MRI of the brain and spine ruled out additional metastases. A radioactive iodine (RAI) whole-body scan (WBS) showed residual uptake of 7.6% in the right thyroid bed and a focus of increased uptake at the right sternoclavicular joint. A therapeutic dose of 206 mCi of I-131 was administered. A post-therapy WBS demonstrated focal activity in the right thyroid bed, distal right clavicle, and lower lung lobes. Chest CT and MRI of the right shoulder revealed no structural evidence of metastases corresponding to radiotracer uptake. The stimulated Tg level prior to RAI was 323 ng/mL with a TSH of 66 uU/mL (0.4-4.6 uU/mL). Tg antibodies were undetectable. She was maintained on 150 mcg of levothyroxine with the goal of TSH suppression. Tg levels declined to 4.8 ng/mL at three months, and to 0.3 ng/mL eight months post-RAI. Discussion: PDTC is an aggressive thyroid cancer subtype with distant metastasis reported in 36-85% of cases.2 Distant metastasis is predictive of poorer outcomes, with patients three times more likely to die from the disease than those without metastatic disease.1 Adrenal metastasis of thyroid cancer is rare, and unlike in our patient, usually asymptomatic and frequently detected on a post-therapy scan. Despite a dramatic response to therapy, given the poorly differentiated features of the primary tumor, a whole-body PET-CT is warranted to evaluate for RAI refractory disease. References: 1. Ibrahimpasic T et al. J Clin Endocrinol Metab. 2014;99(4):1245-52. 2. Sanders EM Jr et al. World J Surg. 2007;31(5):934-45. 3. Cheng DT et al. J Mol Diagn. 2015;17(3):251-64.

MON-452 Poorly Differentiated Thyroid Carcinoma Metastatic to the Adrenal Gland