SAT-326 Three Cases of Malignant Struma Ovarii with Papillary Thyroid Carcinoma in Hamilton, Ontario, Canada

Abstract Disclosure: Y. Yang: None. I. Kak: None. D. Al-Sajee: None. J.E. Young: None. M. Luthra: Boehringer Ingelheim, CPD Network Association, Sanofi, Eli Lilly & Company, Novo Nordisk, Bayer, Inc., Medtronic Diabetes, Pfizer, Inc., Dexcom. T. Abu-Hijleh: None. S.J. Mah: None. D.L. Choi: None. B.N. Alamri: Sanofi, Amryt Pharma. Struma ovarii (SO) is a rare teratoma mainly composed of thyroid tissue, making up <1% of ovarian tumors and <5% of teratomas. Most are benign, but ∼5% are malignant. Non-specific symptoms often delay diagnosis or mimic other ovarian lesions. This series presents three malignant SO cases with papillary thyroid carcinoma (PTC) in Hamilton, Canada. Case 1: A 32-year-old woman presented with pelvic pressure and urinary frequency. MRI revealed a 21.6 cm right ovarian cyst with bilateral hydronephrosis. Right cystectomy pathology showed high-grade follicular variant PTC with a Ki67 index of 12.7%. High-risk features, including >5 mitoses/2 mm² and intraperitoneal rupture, led to total thyroidectomy and completion right salpingo-oophorectomy. Thyroidectomy pathology revealed micro-PTC without lymph node involvement. She received 127.9 mCi of adjuvant radioactive iodine (RAI). Post-treatment whole-body scan showed uptake in the neck and left pelvis. She is managed with pelvic imaging, thyroglobulin monitoring, and TSH suppression. Recent thyroglobulin: 0.4 µg/L. Case 2: A 68-year-old woman with a history of Parkinson’s disease and treated primary hyperparathyroidism. An enlarging left adnexal mass was discovered during workup for recurrent nephrolithiasis, from 4.7 cm to 7 cm over 10 years becoming more complex with heterogeneous fat composition. she underwent total abdominal hysterectomy with bilateral salpingo-oophorectomy (TAH-BSO). Pathology revealed thyroid tissue containing a well-encapsulated 0.4 cm follicular variant of PTC. After multidisciplinary discussion, no further treatment was deemed necessary. Her latest thyroglobulin was 46.9 ng/mL (normal: <48 ng/mL). Interestingly, the patient’s sister was also diagnosed with SO at age 65. Case 3: a 54-year-old woman with toxic thyroid nodules treated with 15 mCi of RAI 8 years prior to presentation, not requiring levothyroxine supplementation. Routine follow up of a left ovarian cyst demonstrated a 7cm multilocular cystic and solid right adnexal mass. She was asymptomatic. She underwent a TAH-BSO with pathology from the left ovary showing a solid trabecular subtype of micro-PTC measuring 0.8cm arising in struma ovarii. There was no angioinvasion or necrosis with 2 mitoses per 2 mm2. Ki67 index was 5-7%. The right ovary was unremarkable. Post-operative thyroglobulin was 10.3 ng/mL (normal: <49.3 ng/mL). Pre-operative CT chest abdomen pelvis did not show metastatic disease. Next steps in management include multidisciplinary discussions to determine the role of thyroidectomy and adjuvant RAI ablation. These cases highlight varied malignant SO management, from aggressive treatment to conservative strategies, emphasizing individualized care. Further research is needed to establish standardized guidelines for managing this rare condition. Presentation: Saturday, July 12, 2025