6477 Surprise Finding for a Thyroid Nodule: Metastatic Renal Cell Carcinoma

Abstract Disclosure: A. Xu: None. E. Hsu: None. Introduction: A patient with an incidental and asymptomatic thyroid nodule underwent fine needle aspiration biopsy (FNA) after the nodule increased in size. Without a definite pathology after three FNA attempts, thyroidectomy was performed, where pathology found metastatic renal cell carcinoma (RCC). Clinical Case: A 64 year old woman had a past medical history of obesity, type 2 diabetes, RCC (stage I: pT1bN0M0) treated with right radical nephrectomy five years prior and thought to be in remission, remote smoking history smoking, and coronary artery disease with coronary artery bypass graft (CABG) two months prior, on anticoagulation. The patient reported a distant history of right thyroid lobectomy over a decade prior for unclear reasons, but she was adamant that the pathology was benign. Two years prior to presentation, CT chest done for RCC surveillance had detected a left thyroid nodule. Ultrasound at the time revealed a 3.6 x 1.9 x 2.3 cm solid, isoechoic nodule in the upper left lobe, which was not taller than wide, and had smooth margins and no echogenic foci. A repeat ultrasound at the time of presentation showed interval growth of the nodule, now measuring 4.3 x 2.1 x 2.9 cm. FNA was unsatisfactory, yielding predominantly blood. Patient felt well and had no compressive neck symptoms. We opted to wait two months and repeat FNA, which resulted as atypia of undetermined significance - Bethesda category III (AUS). We recommended completion thyroidectomy, but patient preferred to avoid surgery. We decided to wait two months and repeat FNA with molecular testing. This third FNA resulted AUS again; however, due to non-medical reasons, molecular testing was not performed. Another ultrasound showed slight growth of the nodule, 5.1 x 2.9 x 2.0 cm, which appeared hypoechoic now, but was still not taller than wide, with smooth margins and no echogenic foci. Completion thyroidectomy was performed, with pathology showing metastatic clear cell renal cell carcinoma. Immunostains were positive for carbonic anhydrase and renal cell carcinoma marker and negative for TTF-1. Whole-body PET-CT showed no hypermetabolic foci, and a repeat PET-CT one year later still showed no evidence of metastatic disease. Other than a hospitalization for COVID-19, patient is doing well and asymptomatic now over a year after her thyroidectomy. Oncology will continue routine surveillance imaging for her metastatic renal cell carcinoma thought to be in remission. Clinical Lessons: This case of metastatic renal cell carcinoma to the thyroid adds to the small body of literature showing the thyroid as a possible site of RCC metastasis. The long time interval between initial RCC presentation and metastasis to the thyroid makes this diagnosis more difficult; however, clinicians should be aware of the possibility, making thyroidectomy a stronger option in cases where FNA cytology is indeterminate. Presentation: 6/2/2024