MON-344 Malignant Struma Ovarii: A Rare Case of Papillary Thyroid Carcinoma in the Ovary

Abstract Disclosure: R. Amin: None. K.N. Kobaly: None. A 52-year-old female with metabolic syndrome was incidentally diagnosed with papillary thyroid carcinoma (PTC) arising from a cystic teratoma in her ovary, a condition known as malignant struma ovarii (MSO). The ovarian cyst was first noted thirty years ago, presumed to be a dermoid cyst, and followed conservatively with serial imaging. A recent pelvic ultrasound showed a 5.4 cm predominantly solid, heterogeneous lesion replacing the right ovary. Given a risk of rupture, patient underwent right salpingo-oophorectomy. Pathology revealed PTC, solid variant, arising from a benign cystic teratoma of the right ovary. Immunohistochemical stains were positive for thyroglobulin and thyroid transcription factor-1. She denied any symptoms of hyperthyroidism. Family history was notable for a sister with PTC. Labs showed TSH of 1.57 mIU/L (0.4-4.5 mIU/L), negative thyroglobulin antibodies, and thyroglobulin level of 10.97 ng/mL. CT scans of the chest, abdomen, and pelvis did not reveal metastatic disease, though four stable pulmonary nodules were identified. Thyroid ultrasound noted a right 10x6x7 mm solid, isoechoic nodule, of which fine needle aspiration was benign. Patient underwent total hysterectomy and left salpingo-oophorectomy with pathology showing a superficial focus of low-grade endometroid carcinoma without lymphovascular invasion and negative pelvic washings. She was not recommended for radioactive iodine currently. MSO is a rare and complex clinical entity that requires an individualized approach. With less than 200 reported cases, management is derived from case reports and cervical thyroid cancer guidelines. Initial treatment typically involves surgical resection, but subsequent decisions are influenced by the presence of metastatic disease, the risk of recurrence, and the molecular characteristics. Risk factors for recurrence include the presence of gross extraovarian extension, large tumor size (greater than 4 cm), and BRAF mutation. Patients at low risk of recurrence are observed on T4 therapy to suppress TSH. Conversely, patients at high risk of recurrence may benefit from radioactive iodine therapy (RAI) after near-total thyroidectomy. This enables the malignant cells to uptake the RAI and allows for high-dose treatment. For cases with unclear indications for RAI, thyroid ultrasound and fine needle aspiration of thyroid nodules may help to clarify the appropriate course of action. Molecular testing for BRAF mutations in both the ovarian tumor and the thyroid may provide additional guidance. In summary, the integration of surgical, radioactive, and medical therapies, tailored to the patient’s risk profile, is essential for optimizing outcomes in MSO. A comprehensive approach with careful risk stratification can guide the use of RAI and other interventions, ultimately improving survival and quality of life for patients with this rare and difficult condition. Presentation: Monday, July 14, 2025