Eosinophilic replacement infiltrates in cystic Hashimoto's thyroiditis: a potential diagnostic pitfall

A 50-year-old woman underwent a fine-needle aspiration biopsy for progressive enlargement of the left thyroid lobe which was cystic and solid on ultrasound exam. The smears contained innumerable eosinophilic leukocytes along with lymphocytes, Hurthle cells, cells from a papillary thyroid carcinoma (PTC), and atypical glandular and squamous cells. The cytologic interpretation was Hashimoto's thyroiditis (HT), suspicious for epithelial neoplasm. The associated diagnostic comment stated concern for a sclerosing mucoepidermoid carcinoma with eosinophilia (SMECE) arising in a PTC. Thyroidectomy demonstrated a PTC, HT with multiple lymphoepithelial cysts, and extensive multifocal infiltrates of eosinophils, generally confined to the cyst walls. As the cytologic findings mimicked a SMECE, we report these specimens as a most unusual diagnostic pitfall.