Renal cell carcinoma is a malignancy that arises from the proximal renal tubule cells of the kidney. The normal function of the kidney is to produce urine through the filtration of blood. If this process gets interrupted by malignancies of the kidney, a patient can show symptoms including hematuria (blood in the urine), urine frequency and/or incontinence, flank pain, and even a palpable mass in the abdomen with large tumors. Patient’s history of symptoms along with physical examination, imaging, and lab work are helpful in the diagnosis of renal cell carcinoma. For confirmation, a patient’s doctor may want to take a biopsy of the tumor to confirm the type of renal cell carcinoma. Imaging such as computed tomography (CT) scan or magnetic resonance imaging (MRI) may be warranted to detect metastases.[7] The most common metastases of renal cell carcinoma include lymph nodes, liver, bone, and brain; however, other metastases such as pancreas and even rarer, thyroid, can occur. The most common genetic aberration seen in clear cell renal cell carcinoma occurs in the tumor suppressor gene VHL, located on chromosome 3.[1,2] A fine needle aspiration biopsy can be used to confirm if metastasis has occurred.[5,6]

The cytologic features of renal cell carcinoma varies widely depending on the subtype and grade of tumor. The most common subtype of renal cell carcinoma is clear cell renal cell carcinoma. Clear cell renal cell carcinoma accounts for about 70% of renal cell carcinomas. Clear cell renal cell carcinomas are highly vascularized; therefore, a biopsy can be very bloody and hypocellular. However, when tumor cells are present, they can be found in sheets or clusters with ill-defined borders and low nuclear-to-cytoplasmic ratio.[3,4] The nuclei can be centric or eccentric with irregular nuclear borders and prominent nucleoli. Depending on the tumor grade, nuclear pleomorphism can be present. The cytoplasm can be wispy, granular, vacuolated, or a mixture. The differential diagnosis includes primary Hürthle cell neoplasm and primary follicular cell neoplasm.

The most common thyroid malignancy is its primary, papillary thyroid carcinoma. Metastatic malignancies to the thyroid are rare, only accounting for up to 4% of malignancies diagnosed in a thyroid.[8,9]  Though uncommon, metastatic malignancies to the thyroid from the kidney, lung, head and neck, and breast.[9] Of these, renal cell carcinoma is the most common metastasis to the thyroid, accounting for up to 50% of metastases to the thyroid.[5] Metastatic renal cell carcinoma to the thyroid may be difficult to diagnose because they appear as benign thyroid nodules on imaging.[10]