PSUN378 A Turducken Phenomenon: A Case of Papillary Thyroid Carcinoma Arising Within Struma Ovarii Associated With a Large Mucinous Cystadenoma

INTRODUCTION: Struma Ovarii (SO) is one of the types of mature teratoma accounting for up to 3% of all ovarian tumors, with predominant thyroid tissue (>50%). Most cases are reported in middle-aged women. We are presenting a case of asymptomatic papillary thyroid cancer (PTC) presented in SO that was incidentally found and treated successfully CASE PRESENTATION: A 72-year-old Hispanic female with a history of hysterectomy, tobacco smoking was found to have a suspicious lung nodule on screening with CT scan of Chest. A subsequent PET/CT Scan revealed incidental findings of left adnexal lesion of 12×12 cm with no FDG uptake and had a minimal uptake in lung nodule favoring a hamartoma. At the gynecology clinic, patient denied having vaginal bleeding, bowel dysfunction, abdominal pain or weight loss. She did report urinary incontinence. Pelvic exam demonstrated a mobile left adnexal mass without tenderness. MRI of Pelvis revealed a 15 cm multi-cystic, left ovarian mass compressing the urinary bladder without any lymphadenopathy. Tumor markers of CEA and CA-125 were normal. The patient underwent bilateral salpingo-oophorectomy with extensive adhesolysis. Entire ovarian mass was removed intact. Surgical Pathology revealed a 1.2 cm Unifocal PTC arising in 3 cm SO associated with mucinous cystadenoma without lympho-vascular invasion and extra-ovarian extension. No malignant cells were identified in peritoneal washing. This was staged according to AJCC 8th edition as pT1aNxMna. Immunostaining of TTF-1 and TG were positive, which additionally supported above diagnosis. Mutational testing of BRAF, K-RAS and N-RAS were negative. Post-operatively, at the Endocrine clinic, her TSH and Free T4 were normal. No nodules were detected on the thyroid ultrasound. She was deemed low risk for recurrence and no further treatment was advised except serial monitoring of Thyroglobulin levels and goal TSH < 2.5 mIU/L. DISCUSSION: In general, SO can be diagnosed only on a histopathological specimen and no specific radiological or biomarkers are available. Most patients are euthyroid and remain asymptomatic like our patient. Thyrotoxicosis occurs in only 5–15% of patients. SO is mostly benign. Its malignant transformation occurs in only 5% of all cases. The most widespread malignant forms of SO are papillary and follicular carcinomas. There are no well-established guidelines for optimal management of malignant SO. After undergoing oophorectomy, the decision to perform a prophylactic thyroidectomy and administration of 131I-Radioiodine is typically reserved for presence of metastatic disease or high risk of recurrence (>4 cm tumor size, gross extra-ovarian extension, BRAF positivity or histology other than papillary type). Presentation: Sunday, June 12, 2022 12:30 p.m. - 2:30 p.m.