Answer: Metastatic renal cll carcinoma involving follicular adenoma

Histology: The thyroid harbors a solitary cellular nodule. The nodule is thinly encapsulated without evidence of transcapsular invasion. The nodule demonstrates a predominant microfollicular growth pattern. In areas, the microfollicles are separated and displaced by nests of cells with clear cytoplasm. These nests are separated by a delicate vascular network.

Discussion: The thyroid gland is a frequent target of metastatic implantation in patients with widely disseminated disease. Less frequently, the thyroid may be the solitary site of implantation in a patient with an occult primary tumor. This occurs most commonly with renal cell carcinoma, colorectal carcinoma, and malignant melanoma. Such cases are often mistaken at the clinical level as primary thyroid neoplasia. Diagnostic error is compounded in those rare instances when the metastatic implant targets a pre-existing thyroid nodule such as a follicular adenoma. The present case represents a renal cell carcinoma metastatic to a follicular adenoma.

Several features are helpful in distinguishing metastatic renal cell carcinoma and from primary thyroid neoplasms with prominent clear cell change (e.g. clear cell variant of Hurthle cell adenoma/carcinoma:
1) a history of renal cell carcinoma or documentation of a renal mass;
2) multifocal thyroid implants;
3) nested growth pattern with sinusoidal vascularity;
4) negative staining for thyroglobulin and TTF-1; and
5) positive staining for RCC.

In the present case, a remote history of renal cell carcinoma was uncovered; and the tumor in the thyroid was found to be immunoreactive for RCC and non-immunoreactive for TTF-1 and thyroglobulin.