Metastasis of clear cell renal cell carcinoma to hyalinizing trabecular tumor of the thyroid: A case report.

Renal cell carcinoma (RCC) is the most common malignancy that metastasizes to the thyroid; however, metastasis of RCC to a primary tumor of the thyroid is rare. The present study reports the case of RCC that had metastasized to the primary thyroid tumor; namely, a hyalinizing trabecular tumor (HTT). Notably, the RCC was resected 2 years prior. A 60-year-old female patient was referred to Ningbo Beilun District People's Hospital with radiographic findings indicating thyroid nodules. The patient's previous medical history included a left nephrectomy for the treatment of clear cell (cc)RCC in February 2021. No other distant metastases were identified as of the latest follow-up in April 2023. No abnormalities were observed during thyroid function tests prior to thyroid surgery. The surgical specimen appeared as a multinodular goiter with a solid nodule measuring 55×41×33 mm on the left lobe of the thyroid. Microscopic examination revealed a gray-yellow area inside the capsule of the largest nodule of the left lobe, which was composed of clear cells arranged in a solid pattern. Notably, tumor cells in other areas exhibited trabecular, nested and island patterns. Results of an immunohistochemical examination revealed that the clear cell lesion was negative for thyroid transcription factor-1 and calcitonin, and strongly positive for carbonic anhydrase IX, common acute lymphoblastic leukemia antigen and vimentin. To the best of our knowledge, the present study is the first to report a case of ccRCC that had metastasized to a HTT. A preliminary analysis of the potential mechanisms underlying ccRCC metastasis was performed. Following total thyroidectomy, the patient was treated with levothyroxine, underwent anti-programmed death-1 monoclonal antibody immunotherapy and small molecule tyrosine kinase inhibitor targeted therapy. An enhanced computed tomography scan revealed no evidence of metastatic disease to other organs. As of the latest follow-up, the patient was in a good condition with no sign of metastasis or recurrence.