Liquid-based cytologic findings of solitary extramedullary plasmacytoma in thyroid: a case report identified with fine-needle aspiration cytology.

Preoperative cytologic diagnosis of solitary extramedullary plasmacytoma (SEP) presents a challenge to cytopathologists because this tumor rarely occurs as a primary thyroid malignant neoplasm. In this report, we provide the first description of liquid-based cytomorphologic findings of SEP observed on fine-needle aspiration biopsy (FNAB) of the thyroid. A 56-year-old woman with a history of Hashimoto thyroiditis presented with a growing nodule in the thyroid. The liquid-based preparation obtained from FNAB showed numerous dispersed plasmacytoid cells with occasional loosely cohesive aggregates of tumor cells. Cells were round to oval in shape, with eccentrically located nuclei. Hyalinized perinuclear vacuoles were found in the cytoplasm of the tumor cells. Initial cytological findings, including those of immunochemistry using a cell block preparation, were consistent with plasmacytoma. Histopathological examination subsequent to thyroidectomy revealed a plasmacytoma in the thyroid. Plasma cell neoplasms were not concurrently detected in tissues other than the thyroid. On the basis of preoperative FNAB findings, a specific diagnosis of SEP in the thyroid can be difficult because this tumor is rare. Moreover, diagnosis is impeded because SEP in the thyroid resembles other, more common thyroid lesions, including both benign and malignant neoplasms. Careful cytomorphologic examination and supportive studies may be required to fully confirm a diagnosis of SEP.