Answer: Hurthle cell carcinoma, widely invasive type

Histology: The neck CT demonstrates a large mass centered in the left lobe of the thyroid gland and compressing the trachea. The resection specimen (not shown) consists of an en bloc resection of thyroid, larynx and trachea. The tumor is highly infiltrative. Sheets and nodules of tumor extend between plates of tracheal cartilage and into the submucosa of the upper respiratory tract. They do not induce much of a desmoplastic stromal reaction. Vascular and perineural invasion is identified. The tumor cells have abundant granular cytoplasm, round to oval nuclei, and prominent nucleoli. They do not demonstrate appreciable mitotic activity or pleomorphism. Metastatic carcinoma is identified in several regional lymph nodes.

Discussion: Hurthle cell carcinomas are follicular thyroid tumors comprised predominantly or exclusively of oncocytes. Like other follicular carcinomas, diagnosis is based on tumor invasion. As is emphasized in the present case, tumor invasion can occur in the absence of significant cytologic atypia such that hurthle cell adenomas and carcinomas are not easily distinguished at the cellular level. Hurthle cell carcinomas can be further subdivided into minimally invasive (encapsulated) and widely invasive types. The widely invasive type demonstrates massive destruction of the tumor capsule with extensive invasion of the thyroid parenchyma. Moreover, these widely invasive tumors have a propensity for spread into the perithyroidal soft tissues of the neck. Compared to follicular carcinomas, regional lymph node metastases occur with increased frequency. Oncocytic carcinomas of thyroid and salivary gland origin are not easily distinguished morphologically, but the clinical and imaging findings should fully clarify this distinction. 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. Most problematic is the distinction of metastatic renal cell carcinoma from primary Hurthle cell carcinoma. Some similarities and differences are shown below.

Table 3. Primary Hurthle cell carcinoma vs metastatic renal cell carcinoma

Widely invasive hurthle cell carcinomas are aggressive tumors that are associated with poor patient outcomes. These tumors generally do not uptake radioactive iodine, rendering this therapeutic tool ineffectual.