SUN-483 A Retrospective Diagnosis of Malignant Struma Ovarii After Discovery of Pulmonary Metastases

Background: Malignant struma ovarii is a rare ovarian tumor that is histologically identical to differentiated thyroid carcinoma.1 We present a case of a struma ovarii that was recognized as being malignant only after the discovery of pulmonary metastases. Clinical Case: A 29 year old female presented to the hospital with acute right lower abdominal pain, suspicious for ovarian torsion. She underwent urgent right salpingoopherectomy and pathology demonstrated a mature cystic teratoma with benign struma ovarii. Two years later, a CT of the abdomen incidentally revealed bilateral pulmonary nodules. Review of the imaging showed that these pulmonary nodules were also present two years prior, and had since become larger. Video-assisted thoracoscopic surgery was performed and lung biopsy was positive for well-differentiated thyroid carcinoma. The patient then underwent total thyroidectomy which revealed a 0.3 x 0.3 cm infiltrative papillary thyroid cancer, follicular variant, without lymphovascular invasion. Thyroglobulin level decreased from 169 ng/mL pre-operatively to 80 ng/mL post-operatively, but then continued to be variable ranging from 56 to 252 ng/mL (1.6-50 ng/mL). Thyroglobulin antibodies remained negative. Pathology from right ovary was re-reviewed at a second institution and found to be consistent with highly differentiated thyroid carcinoma with characteristic nuclear features of papillary thyroid carcinoma. A diagnostic whole body I-131 scan showed uptake within the thyroid bed, bilateral lung nodules, left distal thigh and right mid thigh. These thigh lesions were not visualized on lower extremity ultrasound. After dosimetry was performed, the patient received radioactive iodine-131 200 mCI. Post-therapy scan six days later demonstrated uptake in the thyroid bed, bilateral lungs and bilateral thighs. About five months later, thyroglobulin level had decreased to 0.4 ng/mL with a suppressed TSH. A repeat CT chest demonstrated that the lung nodules had all decreased in size, largest from 0.5 cm to 0.3 cm. Conclusion: Careful examination of struma ovarii pathology should be performed to evaluate for malignant features since benign appearing histology can present diagnostic difficulty.2 In this case, thyroglobulin level was lower than reported in previous cases; however, sites of metastases were responsive to radioactive iodine therapy indicating well differentiated disease and a favorable prognosis. References: 1. Goffredo P, Sawka AM, Pura J, Adam MA, Roman SA, Sosa JA. Malignant Struma Ovarii: A Population-Level Analysis of a Large Series of 68 Patients. Thyroid. 2015:25(2): 211-216. 2. Roth LM, Miller AW, Talerman A. Typical Thyroid-Type Carcinoma Arising in Struma Ovarii: A Report of 4 Cases and Review of Literature. Int J Gynecol Pathol. 2008:27(4): 496-506.