Answer: Papillary thyroid carcinoma

Histology: The thyroid shows a dense infiltrate of lymphocytes associated with prominent germinal center formation (not shown), which appears reactive in nature. The photomicrographs show a well-circumscribed nodule that appears distinct from the remainder of the thyroid. On low magnification, papillary architecture can be seen within this nodule. On higher magnification, the nuclei are noted to be overlapping and crowded. The nuclei are large, optically clear and have a ground glass appearance. Nuclear pseudoinclusions and nuclear grooves are also observed. Psamomma bodies are identified within the lesion.

Discussion: The infiltrate of lymphocytes with prominent germinal center formation is typical of Hashimoto’s thyroiditis. The well-circumscribed nodule seen adjacent to the thyroid gland shows features classic for papillary thyroid cancer. Those features include papillary architecture and enlargement of the nuclei with clearing of the chromatin. In addition, the nuclei can be seen overlapping each other, along with nuclear grooves and pseudoinclusions. These features are helpful in distinguishing this nodule from follicular adenoma and Grave’s disease, which would have more benign-appearing nuclei.

Patients with Hashimoto’s thyroiditis have an increased risk for papillary carcinoma of the thyroid. These tumors are associated with the RET oncogene. The overall prognosis for patients with papillary cancer is excellent. Extra-thyroidal extension into soft tissues of the neck is seen in approximately twenty-five percent of patients. Involvement of cervical lymph nodes is very common and does not adversely affect the prognosis. Factors that are associated with a worse prognosis are age greater than forty years, extra-thyroidal extension, large tumor size, multicentricity, distant metastasis and presence of anaplastic foci.