MON-335 A Rare Case of Metastasis of Renal Cell Carcinoma to Thyroid Gland presenting as Neck Mass, Four Years After Nephrectomy

Abstract Disclosure: M.B. Herrera Gonzalez: None. C. Cantu Lopez: None. N.G. Baranetsky: None. Background: Metastasis to the thyroid gland from renal cell carcinoma (RCC) is a rare event, typically observed in advanced stages of disease. RCC is known for its tendency to spread to distant organs, with the lungs, bones, and liver being the most common sites of metastasis. Thyroid metastasis, while uncommon, can occur, especially in patients with a history of RCC and advanced disease.Here, we present a case of a patient with thyroid mass due to RCC years after radical nephrectomy and unremarkable follow-up.Case presentation:A 70-year-old female with history of stage 2 RCC, status post-right radical nephrectomy performed four years ago, was referred to the endocrinology clinic due a neck mass. Her cancer history also included right breast ductal carcinoma in situ (DCIS), diagnosed in 2011, and left breast invasive ductal carcinoma (IDC) in 2019, for which she underwent lumpectomy and currently on anastrozole.The patient had been disease-free post-surgery with routine follow-up, including CT scans and renal ultrasounds, whit no signs of recurrence. On examination, she had a palpable left thyroid nodule, approximately 1 cm in size. Laboratory workup revealed a normal TSH level of 1.15 mIU/L (0.45-4.5 mIU/L) and a T4 level of 1.22 ng/dL (0.82-1.77 ng/dL).Thyroid ultrasound demonstrated a complex, cystic nodule on the left side measuring 1.6 cm. A fine needle aspiration (FNA) biopsy revealed atypical cells and classified as Bethesda III (indeterminate). She underwent left thyroidectomy. Pathology confirmed metastatic RCC. Immunohistochemical stains were positive for PAX8, CD10, and CAIX, all markers supportive of RCC, while being negative for TTF1, thyroglobulin, and BRAF V600E mutations. The Ki67 proliferative index was <5%, indicating low mitotic activity. These findings further supported the diagnosis of thyroid metastasis from RCC.Discussion:This case underscores the importance of ongoing surveillance in RCC survivors, even many years after initial treatment, as metastasis can occur long after the primary disease is considered "disease-free." Thyroid metastasis from RCC, while rare, should be considered in the differential diagnosis of thyroid nodules in patients with a history of RCC, mainly when unusual presentation patterns occur. This case contributes to understanding RCC metastasis to the thyroid, emphasizing the need for vigilance in follow-up care and highlighting the slow-growing, indolent nature of some RCC metastases, which may affect both diagnosis and treatment. Presentation: Monday, July 14, 2025