Liquid‐Based Cytologic Features of a Thyroid <scp>TFE3</scp> ‐Rearranged <scp>PEComa</scp> ‐Like Neoplasm: A Case Report

Perivascular epithelioid cell tumor (PEComa) is a rare mesenchymal neoplasm, and primary involvement of the thyroid gland is exceedingly rare. TFE3-rearranged PEComas constitute a distinct molecular subset characterized by epithelioid morphology and strong nuclear TFE3 expression. Cytologic descriptions of PEComa are limited, and to date, only three cytologic cases of TFE3-rearranged PEComa or PEComa-like neoplasm have been reported in the literature, none of which involved the thyroid gland. We report a case of thyroid TFE3-rearranged PEComa-like neoplasm in a 25-year-old woman who presented with a palpable thyroid nodule. Fine-needle aspiration using liquid-based cytology revealed a hypercellular specimen composed predominantly of dispersed and loosely cohesive epithelioid cells with enlarged round to oval nuclei, fine chromatin, and frequent intranuclear pseudoinclusions, resulting in an initial interpretation suspicious for papillary thyroid carcinoma. Histologic examination demonstrated a well-circumscribed epithelioid neoplasm with pseudoalveolar architecture supported by a delicate arborizing vascular network. Immunohistochemical studies showed tumor cell positivity for TFE3, cathepsin K, desmin (patchy), and vimentin, with negativity for epithelial, thyroid-specific, melanocytic, and other smooth muscle markers, as well as PAX8, CD10, MiTF, and PAS/PAS-D stains, arguing against metastatic renal cell carcinoma and alveolar soft part sarcoma. The patient underwent surgical resection and remains disease-free after 7.5 years of follow-up. This case represents the first description of the liquid-based cytologic features of a thyroid TFE3-rearranged PEComa-like neoplasm. Awareness of this rare entity and its cytologic overlap with papillary thyroid carcinoma is essential to avoid diagnostic pitfalls in thyroid fine-needle aspiration.