6688 Clinical Presentation and Management of Highly Differentiated Follicular Carcinoma Arising from Struma Ovarii with DICER1 Mutation: A Case Report

Abstract Disclosure: S.M. Seav: None. J. Shah: None. C. Dosiou: None. Background: Struma ovarii (SO) is a rare type of ovarian teratoma comprised of > 50% thyroid tissue. Extra-ovarian spread of SO, called peritoneal strumosis, was previously considered benign given the lack of histological malignant features. The 2020 WHO Classification of Female Genital Tumors has reclassified this as highly differentiated follicular carcinoma arising from struma ovarii (HDFCO), highlighting its low-grade malignant potential.[1] We report a case of HDFCO with extensive peritoneal disease and discuss its management. Clinical Case: A 38-year-old female presented to her OBGYN for irregular menses. Pelvic MRI showed an 8 cm right ovarian mass. Right salpingo-oopherectomy and pelvic peritoneal biopsies revealed well-differentiated thyroid tissue. Thyroid ultrasound demonstrated a multinodular goiter with ATA high risk nodules; 4 of 5 biopsied with benign results. Her thyroglobulin (Tg) was 170 ng/mL (normal 2 – 35 ng/ml). No further intervention was recommended. Two years later she was referred to our clinic. CT of abdomen/pelvis showed multifocal residual disease. We recommended more aggressive management, given the diagnosis of HDFCO. All visible disease was resected in a diagnostic laparoscopy with pelvic and omental biopsies. Pathology showed well-differentiated thyroid tissue. Molecular testing for the FGFR4 mutation associated with HDFCO was negative, but there was a DICER1 mutation c.5428 G>T of undetermined significance. DICER1 mutations are associated with thyroid cancers2. A total thyroidectomy and radioactive iodine ablation (RAI) were recommended. Thyroid pathology was benign. Tg was 9 ng/mL one month post-op. MRI six months post-laparoscopy revealed only one 6 mm omental lesion. Withdrawal I-123 whole body scan (WBS) showed multiple peritoneal implants. Tg was 214 ng/mL and TSH was 72 ulU/mL. The patient was treated with 157 mCi of oral I-131. Given the extensive peritoneal disease, she received prophylactic oral dexamethasone for 2 weeks after RAI to decrease the risk of peritoneal inflammation and bowel obstruction potentially mediated by local inflammation from radiation-related tissue injury. Her Tg three months post-RAI was 4 ng/mL. The surveillance plan is to alternate between RAI WBS and MRI abdomen/pelvis every 6 months. Conclusions: Peritoneal strumosis, previously considered benign, has recently been reclassified as HFDCO, an extremely rare low-grade malignancy with no standardized treatment. We report the first case of extensive HFDCO with a DICER1 mutation. Given the significant peritoneal involvement, our case was treated with total thyroidectomy, surgical excision of all implants, RAI, and prophylactic steroids post-RAI. Further research into the prognostic factors and optimal treatment of HFDCO is needed. Reference: [1]Giovanopoulou et al. Curr Oncol. 2022; 29(12): 9105-9116. 2Sauer et al. Head Neck Pathol. 2022; 16(1): 190–199. Presentation: 6/1/2024