Extramedullary plasmacytoma of the thyroid: a case report and literature review.

[BACKGROUND] Thyroid extramedullary plasmacytoma (EMP) is an exceedingly rare malignancy, often coexisting with Hashimoto's thyroiditis (HT). Due to its nonspecific clinical presentation, it is frequently misdiagnosed. Here, we presented a complete diagnostic workup of a primary thyroid EMP patient, including imaging and comprehensive pathology. Furthermore, we have provided an in-depth literature review of this rare entity.

[CASE PRESENTATION] This is a case of a 70-year-old woman who presented with a neck mass. A fine-needle aspiration before the surgery suggested HT, and intraoperative frozen section rapid pathology suspected medullary thyroid carcinoma. What surprised us was that postoperative immunohistochemistry confirmed the diagnosis of EMP. The patient had favorable outcomes at 6 months after completing the treatment course, which included surgical resection.

[CONCLUSION] Thyroid EMP is highly associated with HT, and many researchers hypothesize that its development may be linked to chronic antigenic stimulation within the inflammatory microenvironment of HT. Cytopathological diagnosis of thyroid EMP can mimic medullary thyroid carcinoma or lymphoma. Therefore, immunohistochemical analysis is mandatory for accurate differentiation. Generally, the prognosis of thyroid EMP is favorable, with recurrence and metastasis being relatively uncommon.