Cytopathology of primary sclerosing mucoepidermoid carcinoma with eosinophilia of the thyroid: a multi-institutional case series and review of literature.

Introduction: Sclerosing Mucoepidermoid Carcinoma with Eosinophilia (SMECE) of the thyroid is an extremely rare tumor that exhibits unique histologic characteristics and is nearly always associated with lymphocytic thyroiditis (LT). However, the cytomorphologic and clinicopathologic characteristics of SMECE have only been described in rare case reports.
Materials and Methods: Authors' institution laboratory information systems were searched for records of SMECE between 2012 and 2023. Literature review was performed using keywords "Sclerosing mucoepidermoid carcinoma with eosinophilia", "thyroid", and "cytopathology" to search through institution electronic library databases for relevant articles.
Results: A total of 19 cases were identified, 3 unpublished in the authors' archives and 16 in the literature which had fine needle aspiration (FNA) material or cytologic features available for review, and were comprised of 3 males and 16 females. The common cytomorphologic characteristics of SMECE included fragments or loose clusters of intermediate-type epidermoid cells in a background of prominent LT and eosinophils. Overt keratinization, mucinous cells, and extracellular mucin were not commonly encountered, resulting in diagnostic challenges, especially if eosinophils associated with epithelial cell clusters were rare. The cases were reported as "Nondiagnostic" (1 case), "Atypia of Undetermined Significance" (4 cases), "Suspicious for Malignancy" (3 case), or "Malignant" (11 cases).
Conclusions: The clinical course of SMECE of the thyroid varied and distinct cytomorphologic characteristics in a subset of patients who experienced aggressive disease raises the possibility of different prognostic grades. Cases with keratinized squamous cells and necrosis mimic anaplastic (undifferentiated) thyroid carcinoma, but the clinical history and radiologic findings can be helpful to exclude this diagnosis.
(Copyright © 2023 American Society of Cytopathology. Published by Elsevier Inc. All rights reserved.)