Discussion

Clear cell renal cell carcinoma comprises 75%-80% of all RCCs and is strongly associated with a variety of deletions on chromosome 3p, the site for VHL gene. Size is not a determinant of malignancy, but the frequency of metastases does correlate with increasing size of the primary tumor. Cytomorphologic features of clear cell renal cell carcinoma include: large cohesive cell groups, abundant wispy cytoplasm with ill-defined edges, cytoplasmic vacuoles, large, round, eccentrically placed nucleus, nucleoli (vary in size depending on WHO/ISUP grade), and blood.

Despite its robust vascularity, metastases to the thyroid gland only account for a small proportion of benign and malignant thyroid entities diagnosed. The possibility of metastasis should be considered whenever the cytologic picture does not conform with common thyroid neoplasms or the patient has a history of cancer elsewhere in the body.

Definitive diagnosis of metastatic RCC to the thyroid can only be confirmed by immunohistochemical staining and histologic examination. FNA with ultrasound guidance is the most common approach. FNA sensitivity and specificity for diagnosing metastases to the thyroid have been reported to be as high as 94% and 100%.

When a metastasis is suspected, comparing the aspirated specimen to previous histologic or cytologic material is very helpful, as is immunohistochemistry. Immunohistochemical staining for markers specific to RCC are utilized including CD10, vimentin, cytokeratin 7, and renal cell carcinoma marker. It is also helpful to use staining markers for thyroglobulin, calcitonin, and thyroid transcription factor 1 (TTF-1), which are specific to primary thyroid carcinomas and negative with metastatic RCC.

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