Isolated Thyroid Metastasis from High-Grade Clear Cell Renal Cell Carcinoma After Radical Nephrectomy: A Case Report

Background and Clinical Significance: Thyroid metastasis from clear cell renal cell carcinoma (ccRCC) is uncommon and may occur after radical nephrectomy despite no evidence of residual or metastatic disease at that time. Diagnosis can be challenging because imaging and fine-needle aspiration (FNA) may mimic benign or primary thyroid lesions. Case Presentation: A 55-year-old man underwent left radical nephrectomy for WHO/ISUP grade 3 ccRCC with focal grade 4 differentiation. Two years later, surveillance ultrasonography detected a suspicious lesion involving the thyroid isthmus and left lobe. FNA was interpreted as Bethesda II (benign), but metastatic ccRCC was subsequently confirmed after total thyroidectomy. Histopathology showed focal vascular and capsular invasion. Tumor reached the margin of the main thyroid specimen, whereas separately submitted perithyroidal soft tissue had tumor-free margins, resulting in a final R0 resection. The tumor was positive for CK8/18, PAX8, EMA, vimentin, and CD10 and negative for TTF-1, thyroglobulin, calcitonin, and CD5. Postoperative 18F-FDG PET/CT showed no additional metastatic disease. Conclusions: Benign thyroid cytology should be interpreted cautiously in patients with previous ccRCC when clinical or imaging findings remain suspicious. Diagnosis requires integration of oncological history, imaging, histopathology, and immunohistochemistry. Surgical resection may provide local disease control in selected patients, with surveillance.