Case Report: Decade-delayed thyroid metastasis with cervical lymph node involvement from clear cell renal cell carcinoma: diagnostic pitfalls of cytologic-radiologic discordance.

Background: Renal cell carcinoma (RCC), particularly the clear cell subtype, demonstrates a high metastatic potential and the capacity for late recurrence many years after nephrectomy. Although thyroid metastasis is uncommon, RCC is among the most frequently reported primary malignancies giving rise to secondary thyroid tumors. Distinguishing metastatic RCC from primary thyroid neoplasm is challenging on ultrasound and fine-needle aspiration cytology (FNAC) alone. Case presentation: A 55-year-old man with a history of clear cell RCC treated with radical nephrectomy 10 years earlier presented with a right thyroid nodule. Initial FANC at an outside institution demonstrated atypia of undetermined significance (AUS), and repeat FNAC at our institution yielded oncocytic follicular cells, despite progressively suspicious imaging findings. FNA of a right level III node was negative for malignancy, and lymph node washout thyroglobulin levels were low. The patient ultimately underwent right hemithyroidectomy with central and lateral neck dissection. Histopathology revealed metastatic clear cell RCC involving the thyroid and cervical lymph nodes, confirmed by immunohistochemistry showing positivity for CD10, vimentin, and carbonic anhydrase IX (CAIX), and negativity for thyroglobulin, TTF-1, and calcitonin. Conclusion: This case highlights the diagnostic challenges posed by indeterminate cytology and imaging-pathology discordance in late-presenting RCC thyroid metastasis. In patients with a prior history of RCC, new thyroid nodules or cervical lymphadenopathy should raise suspicion for metastatic disease even many years after initial treatment. Definitive diagnosis relies on histopathology and immunohistochemistry. When cytologic-radiologic discordance persists, early consideration of core needle biopsy is warranted. Management requires a multidisciplinary approach, with surgical resection of isolated metastatic disease integrated within an individualized, sequential systemic therapy strategy.