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Background: Metastatic neoplasms to the thyroid gland are rare and have been observed more in autopsy series than in clinical series. Case: A 71-year old woman presented for thyroid nodule evaluation which was incidentally noted on CT/PET scan. She was diagnosed with renal cell carcinoma (RCC) stage IV (6.4 cm, left nephrectomy) 4 months prior. She had right hemi-thyroidectomy 30 years prior for a benign nodule. She was clinically and biochemically euthyroid with dysphagia and hoarseness in her voice. Thyroid u/s revealed multiple nodules in the left lobe with a dominant 3.6 x 2.9 x 3.6 cm solid, heterogeneous nodule with grade 3 hypervascularity. History positive for 1 cm right renal mass and multiple pulmonary nodules increasing in size thought to be consistent with metastases. The FNA of the dominant nodule was indeterminate, Bethesda III (AUS), GSC suspicious (Affirma, 50% ROM) with negative malignancy classifiers. Patient underwent completion thyroidectomy, and surgical path was consistent multifocal clear cell renal cell carcinoma (CRCC) with the largest focus of 3 cm based on clear cell features and strong positivity for stains: CK OSCAR, RCC, PAX-8 &amp; CD-10. Patient is currently on Pazopanib post thyroid surgery for 18 months, and is stable with no further increase in the size of lung nodules or right renal mass and negative serial PET scans. Clinical lesson: CRCC represents 3-4% of all adult malignancies and 85% of all primary renal tumors. In clinical series, CRCC is the most frequent source of thyroid metastases and represents 12-34% of all secondary thyroid tumors. About 17% of patients with CRCC have metastatic disease at diagnosis. Metastases can be synchronous or metachronous to the primary tumor. Latency from nephrectomy to diagnosis of thyroid mets varies from 2 months-21.9 years. Metastases can be solitary (more common), multiple or diffuse. Radiological findings typically reveal hypoechoic and vascularized mass on ultrasound and cold on thyroid scan. Metastases to thyroid can pose diagnostic problem and be a source of confusion in cytology interpretation. Metastatic CRCC can simulate morphologically primary thyroid neoplasm such as Hurthle cell neoplasm or thyroid carcinomas with clear cell changes. Metastatic carcinoma within the thyroid gland is negative for thyroglobulin, TTF-1, calcitonin unlike primary thyroid cancer. CRCC is usually positive for periodic acid-Schiff and Oil red O, vimentin, and CD10. The mean survival in patients with CRCC who had thyroidectomy alone or with adjuvant treatment was 3 years. Conclusion: Thyroid metastases should be considered in patients with thyroid nodules and positive history of RCC. The preoperative distinction between primary and secondary tumors is difficult. Immunohistochemistry is a useful method for the evaluation of patients with suspected thyroid nodules.

SUN-479 Metastatic Renal Cell Carcinoma (RCC) to the Thyroid: Case Report and Clinical Lesson