SAT-138 An Interesting Case of Papillary Thyroid Carcinoma of Mature Cystic Teratoma in a Young Female

Abstract Disclosure: A.E. De Rosairo: None. D. Patel: None. T. Rave: None. S. Tariq: None. D.H. Sacoto: None. R. Belokovskaya: None. A.A. Franco Akel: None. Introduction: Mature cystic teratomas (MCT) are the most common ovarian tumors in women of reproductive age. The malignant transformation of MCT is a rare entity. Struma ovarii (SO) is a rare monodermal variant of ovarian teratoma that contains predominantly thyroid tissue (>50%). Malignant struma ovarii (MSO) typically has histological changes of differentiated thyroid carcinoma, most commonly papillary thyroid carcinoma (PTC). We present a case of a 29-year-old female who was diagnosed with PTC of right ovarian MCT. Case: A 29-year-old female GP1001 with past medical history of a left ovarian cyst status post cystectomy (8.0 x 7.7 x 7.0 cm with benign pathology) 5 years prior to presentation and a right ovarian cyst measuring 2.6 x 2.5 x 3.1 cm 22 months prior presented due to persistent pelvic pain. A repeat pelvic ultrasound noted an increase in the right ovarian cyst size to 9.7 x 10.2 x 7.5 cm and underwent right cystectomy. Pathology showed PTC, usual type, arising in association with MCT and was positive for TTF-1, CK19 and HBME-1. A thyroid ultrasound was unremarkable. She was lost to follow up; however, two years later returned for followup. Labs revealed TSH 1.90 uIU/mL [0.27 - 4.20uIU/mL], thyroglobulin 102 ng/mL [1.60 - 59.90 ng/mL] and thyroglobulin antibodies <20.0 IU/mL [<40.0 IU/mL]. A thyroid ultrasound showed 0.5 x 0.3 x 0.5 cm solid, hypoechoic, wider than tall, nodule in the left lower pole/isthmus junction. A transvaginal pelvic ultrasound noted the right ovary measuring 2.4 x 1.7 x 1.9 cm with 2 echogenic foci measuring up to 5 mm may represent complex/hemorrhagic cyst, dermoid cyst; and the left ovary measuring 2 cm x 1.5 cm x 2 cm with a 1.8 cm cyst. A PET scan showed no abnormal hypermetabolic activity. A fine needle aspiration of the left thyroid nodule was consistent with Atypia of Undetermined Significance (Bethesda III), followed by benign Afirma Genomic Sequencing. The patient is now scheduled for right oophorectomy. Discussion: MCT is the most common type of ovarian germ cell tumor. The incidence of malignant transformation in MCT is rarely reported (<3%). SO is a variant of ovarian teratoma which may present any form of thyroid pathology, and in most cases is a benign condition, but approximately 5-10% of these tumors become MSO, most commonly PTC followed by follicular carcinoma. Most patients are asymptomatic, but some may present with pelvic discomfort or abdominal pain due to mass effect. Treatment is individualized, based on the patient's age and the presence or absence of metastases. A large literature review on patients with MSO found that primary thyroid carcinoma was seen in 6.2% of cases. Although still a small percentage, it is a significantly increased rate compared to the general population and it is important to be vigilant of this increased risk of thyroid carcinoma among these individuals. Patients with MSO confined to the ovary have had an excellent survival outcome. Presentation: Saturday, July 12, 2025