Answer: Metastatic renal cell carcinoma

Histology: Grossly, four discrete tumor nodules were identified, ranging in size from 1 cm to 5 cm. The tumor demonstrates a prominent nested pattern of growth. The nests are surrounded by numerous delicate thin-walled vessels. The tumor cells have clear cytoplasm and round nuclei with inconspicuous nucleoli. An immunohistochemical stain for CD10 is positive; while stains for thyroglobulin, TTF-1 and RCC are negative. On careful inspection of the surrounding thyroid parenchyma, several round purple laminated concretions are noted within the stroma and within lymphatic spaces.

Discussion: As discussed in Case 5 above, the thyroid gland is sometimes the site of a solitary metastasis in a patient with an occult primary tumor. By way of contrast to the previous case, this case of metastatic renal cell carcinoma is characterized by multifocality, cytoplasmic clearing, a purely nested architecture, and a prominent network of delicate blood vessels. Immunohistochemical studies are useful in confirming the morphologic impression. Specifically, absence of staining for thyroglobulin and TTF-1 are helpful in eliminating the possibility of a hurthle cell carcinoma. CD 10 has recently been touted as an excellent marker of renal cell carcinoma: about 90% of renal cell carcinomas are immunoreactive. But it is not very specific. A recent study found that the majority of thyroid carcinomas are also immunoreactive, such that a CD 10 staining may not be very useful in this particular differential diagnosis.
The presence of psammoma bodies is quite specific for papillary thyroid carcinoma. The finding of psammoma bodies in a background of otherwise normal thyroid parenchyma is indicative of lymphatic tumor spread and raises the likely possibility of a papillary carcinoma lurking somewhere in the thyroid gland. In the present case, additional sectioning of the thyroid gland disclosed two papillary thyroid carcinomas (not shown).