Malignant Struma Ovarii with a Small Occult Carcinoma Hidden within Thyroid-Like Tissue: A case Report in a 58-year-Old Thai Woman

Abstract

A 58-year-old Thai woman with symptomatic ovarian mass is reported herewith to highlight the diagnostic pitfalls of malignant struma ovarii, particularly the follicular variant papillary thyroid carcinoma (FVPTC)—an exceptionally rare subtype. She was found to have bilateral mature cystic teratomas with only focal papillary carcinoma hidden within struma ovarii tissue in the right ovary. The key to reaching the correct diagnosis lies in a meticulous search for a cancerous lesion that could explain the markedly elevated CA-125 level (1,251 IU/mL), which was disproportionate to the tumor size and benign-appearing imaging. The diagnostic challenge in this case is threefold: First, the malignant focus was minute and easily missed during routine sampling. Second, FVPTC exhibits follicular architecture mimicking benign thyroid tissue, where malignant criteria rely solely on subtle nuclear features (ground-glass chromatin, nuclear grooves, and pseudoinclusions) without papillary structures—requiring expertise to recognize. Third, systematic thyroid evaluation was essential to exclude metastatic thyroid carcinoma and confirm primary ovarian origin. An important management dilemma arose: treatment selection between thyroid cancer protocols (radioactive iodine) versus ovarian cancer protocols (chemotherapy). We opted for carboplatin-paclitaxel based on ovarian surface involvement, achieving excellent 4-year disease-free outcome. This case emphasizes critical lessons: exhaustive sampling when CA-125 is inexplicably elevated, mandatory recognition of FVPTC nuclear features, importance of excluding primary thyroid disease, and individualized treatment approach in this rare entity where standard guidelines are lacking.