Clear Cell Renal Cell Carcinoma Metastasis to the Thyroid: A Narrative Review of the Literature

Clear cell renal cell carcinoma is the most common histological type of renal cancer, which is a common cancer type usually associated with a long clinical course. During this course, various metastatic sites can be observed. In this review, we have focused on metastases to the thyroid gland. We conducted research in three medical databases, including PubMed, Scopus, and Web of Science, using the same search algorithm. Our inclusion criteria focused on case reports and case series studies since 2011, covering therapeutic strategies for the primary and/or metastatic disease, as well as subsequent follow-up data. Studies with insufficient or uncertain data, or written in a language other than English, were excluded. An analysis of 510 articles from PubMed, 1729 from Scopus, and 649 from Web of Science, after application of inclusion and exclusion criteria, resulted in 77 reports, analyzing 189 patients. A description of the clinical, pathological, ancillary, and follow-up data, in the light of recent therapeutic schemes, was attempted. Our results suggest that metastases' imaging features comprised more commonly a solitary nodule with a median size of 3.5 cm and worrisome features in ultrasonography, such as heterogeneity, hypoechogenicity, partially solid configuration, and variable internal vascularization. Histological and immunohistochemical examination of the lesion is necessary because these findings are not specific. Common non-thyroid metastases are seen in the urogenital system, lungs, and pancreas. We calculated the restricted mean survival from primary diagnosis at 274.6 months (95% CI: 264.3-285.0 months) and the restricted mean survival from thyroid metastases treatment at 93.9 months (95% CI: 65.3-122.4 months). Results regarding how patient characteristics affect these survival numbers were statistically nonsignificant (p > 0.05).