Malignant struma ovarii: clinicopathological and molecular features in a series of nine cases.

Malignant struma ovarii (MSO) represents a rare form of ovarian tumor, necessitating further refinement in both classification systems and management approaches. In this study, we aimed to delineate the clinicopathological and molecular characteristics of MSO while assessing the utility of the World Health Organization (WHO) 2022 thyroid tumor classification framework for its diagnostic subtyping and clinical management. We analyzed MSO cases through comprehensive histopathology, immunohistochemistry (IHC), and targeted next-generation sequencing (NGS). The nine MSO cases comprised three papillary thyroid carcinomas (PTC; one follicular variant), four follicular thyroid carcinomas (FTC), one oncocytic (Hürthle cell) carcinoma (OCA), and one differentiated high-grade thyroid carcinoma (DHGTC). All tumors were unilateral and FIGO stage I. The patients (median age 59, range 23-73 years) most often presented with an adnexal mass or abdominal pain. Histologically, all cases showed thyroid-type carcinoma arising within a teratomatous ovary. IHC demonstrated universal thyroid differentiation (TTF-1, thyroglobulin, PAX8 +) and typical thyroid carcinoma markers (CK19, HBME1, galectin-3) in most cases, and frequent CD56 loss. Molecular analysis of eight successful sequencing tumors revealed NRAS Q61 or HRAS Q61 mutations in three cases and a rare BRAF exon 11 (p.G469A) mutation in one PTC case. Novel EIF1AX/PTEN/KMT2C/BRCA1 alterations were identified in four cases. No TERT promoter mutations or RET/PTC fusions were detected. Tumors were assigned into RAS-like (5/8) and BRAF-like category (2/8) per WHO 2022 criteria; the remaining one tumor had no specifical mutations or related profiles. Clinically, all patients remain disease-free after surgery (follow-up 4-120 months). In conclusions, MSO shares histomolecular overlap with primary thyroid carcinoma, validating WHO 2022 thyroid tumor classification for risk stratification in struma ovarii. Conservative surgical management achieves excellent outcomes in low-grade cases.