Tumor-to-tumor metastasis: metastatic clear cell renal cell carcinoma within an encapsulated oncocytic papillary thyroid carcinoma – a case report

Abstract Background Tumor-to-tumor metastasis (TTM) is a rare but important entity in which a metastasis from one neoplasm is deposited within a histologically distinct second neoplasm. Renal cell carcinoma (RCC) is both the most common malignancy to metastasize to the thyroid and the most frequent donor in thyroid TTM; however, deposition within an oncocytic papillary thyroid carcinoma has rarely been documented. Case presentation A 51-year-old man underwent left radical nephrectomy for clear cell RCC (WHO/ISUP grade 2/4, pT1bN0). A left thyroid nodule detected incidentally during staging was benign on initial fine-needle aspiration (Hürthle cell hyperplasia). During surveillance the nodule enlarged; repeat aspiration showed atypical follicular cells, and core-needle biopsy demonstrated an oncocytic follicular neoplasm. Left thyroid lobectomy revealed a 0.8-cm focus of metastatic clear cell RCC embedded within a 1.9-cm encapsulated oncocytic papillary thyroid carcinoma. Immunohistochemistry confirmed two distinct populations: the thyroid carcinoma was positive for thyroglobulin and thyroid transcription factor-1 (TTF-1) and negative for carbonic anhydrase IX (CA IX), whereas the metastatic cells showed the reverse pattern. The patient was referred for oncologic follow-up. Conclusions TTM of clear cell RCC to a thyroid carcinoma is exceptionally rare. In a patient with a history of RCC, an enlarging thyroid nodule with evolving cytology should raise suspicion of metastasis; complete surgical excision with a targeted immunohistochemical panel (thyroglobulin/TTF-1 versus CA IX) is essential for accurate diagnosis.