SUN-318 Metastatic Malignant Struma Ovarii: A Case of Disease Progression Due to Delayed Thyroidectomy

Abstract Disclosure: A. Malipeddi: None. S. Bodepudi: None. L. Jabbar: None. W. Pan: None. Introduction: Malignant struma ovarii (MSO) is a rare ovarian teratoma primarily composed of thyroid tissue, most commonly papillary thyroid carcinoma (PTC). Its rarity complicates diagnosis and management, often delaying intervention. While no standardized guidelines exist, MSO with PTC is typically managed like differentiated thyroid carcinoma. Evidence supports total thyroidectomy (TT) and radioactive iodine (RAI) therapy for better outcomes and surveillance. We present a case illustrating the impact of delayed TT on metastatic MSO prognosis. Case: A 59-year-old female with hypertension underwent bilateral salpingo-oophorectomy (BSO) and hysterectomy at age 57 for postmenopausal bleeding. Pathology revealed MSO with poorly differentiated PTC and mature teratoma. Initial surveillance showed TSH 1.19 μIU/mL (0.45- 4.70), FT4 1.04 ng/dL (0.78- 2.20), and Thyroglobulin 8.8 ng/mL (1.3- 31.8). Neck ultrasound detected a 0.8 cm hypoechoic thyroid nodule and an enlarged lymph node (2.4 × 0.7× 0.9 cm) with benign biopsy findings. CT imaging was unremarkable, and a nuclear medicine scan showed normal thyroid uptake (20.8%). No further intervention was carried out. At 12 months, pelvic pain prompted imaging, revealing peritoneal metastases. PET-CT confirmed FDG-avid nodular densities in the pelvis and posterior peritoneal reflection. A failed CT-guided biopsy necessitated diagnostic laparoscopy, confirming metastatic disease. A multidisciplinary team recommended total thyroidectomy and RAI therapy. At 23 months post-diagnosis, she underwent TT and was started on weight-based levothyroxine, with plans for RAI ablation and endocrinology follow-up. Discussion: MSO with PTC is rare, requiring careful management to reduce recurrence and metastatic risk. Studies suggest TT improves outcomes by enabling TSH suppression during treatment and whole-body iodine-131 scanning for better disease monitoring. Ayhan et al. found significantly higher progression-free survival (94.5% vs. 64.3%, p = 0.05) in patients undergoing TT and RAI. DeSimone et al. reported all recurrences in conservatively managed patients, emphasizing the need for an aggressive approach. Delayed TT may increase recurrence and worsen prognosis. Our case highlights that TT and RAI should be part of immediate follow-up in MSO with poorly differentiated PTC to optimize disease control and long-term surveillance. Presentation: Sunday, July 13, 2025