Struma Ovarii: Histopathological Mimics, Immunohistochemical Clues, and Diagnostic Pitfalls

Struma ovarii is defined as a monodermal teratoma comprising chiefly thyroid tissue and accounts for approximately 1% of all ovarian tumors.Its histology is highly variable.Cystic, microfollicular, trabecular, oxyphilic, clear-cell, and solid patterns can closely imitate surface epithelial, sex cord-stromal, neuroendocrine, and metastatic tumors, and most misdiagnoses stem from failure to consider struma rather than from its rarity.The reverse is also important: proliferative but benign struma is commonly overdiagnosed as carcinoma, while highly differentiated follicular carcinoma of ovarian origin (HDFCO), a histologically bland struma, is underrecognized, although it tends to spread beyond the ovary.This narrative review organizes the mimics of struma ovarii by histological pattern and summarizes the morphological and immunohistochemical discriminators that resolve them, including thyroglobulin, thyroid transcription factor-1 (TTF-1), paired box gene 8 (PAX8), inhibin, steroidogenic factor-1 (SF-1), forkhead box L2 (FOXL2), and neuroendocrine markers.It also touches upon the principal diagnostic pitfalls: benign struma versus malignant struma, HDFCO, and the so-called peritoneal strumosis, strumal carcinoid, and metastatic cervical thyroid carcinoma, along with frozen-section errors.It summarizes recent molecular data on struma-derived carcinomas, such as B-Raf proto-oncogene, serine/threonine kinase (BRAF), rat sarcoma virus (RAS), and telomerase reverse transcriptase (TERT) promoter alterations, with emphasis on their exploratory nature.A stepwise diagnostic algorithm and a reporting checklist are also proposed