Abstract
Struma ovarii (SO) is a rare subtype of ovarian teratoma composed of more than 50% thyroid tissue. Extraovarian spread of SO, called peritoneal strumosis, was previously considered benign given the lack of histological malignant features. However, the 2020 World Health Organization Classification of Female Genital Tumors reclassified peritoneal strumosis as highly differentiated follicular carcinoma of ovarian origin (HDFCO), highlighting its low-grade malignant potential. We present a 38-year-old woman with SO treated initially with right salpingo-oophorectomy, with recurrence 2 years later with multifocal metastatic lesions in the abdomen and pelvis that was successfully treated with surgical resection, total thyroidectomy, and 157 mCi of I-131. Tumor molecular testing revealed a pathogenic DICER1 variant (c.5428G>T, exon 25). A second variant (c.319delins13, exon 4) was of uncertain significance. Germline testing confirmed the second DICER1 variant and also identified an increased risk variant in the APC gene (c.3920T>A). This is a rare case of extensive HFDCO with DICER1 variants, which has been associated with thyroid cancer. Given the germline DICER1 variant, this may also represent the first reported instance of DICER1 syndrome manifesting as HDFCO. Further research into the prognostic factors and optimal treatment of HFDCO is needed.