Introduction/Background Struma ovarii is a germ cell tumor of the ovary, which consists of >50% thyroid tissue. Malignant struma ovarii is a very rare type of gynecologic cancer, accounting 5–10 % of these tumors. The most common type is papillary carcinoma, followed by typical follicular carcinoma.

Methodology A 54-year-old woman, was referred to our department for a 10 cm cystic tumor with hemorrhagic components, originated from the left ovary on lower abdominal Magnetic Resonance Imaging (MRI), finding of endometriosis. A year ago, the size of this tumor was 5 cm. A surgical treatment was suggested then, but the patient refused. Cancer biomarkers were negative. In our department the patient underwent a robotic left salpingo-oophorectomy, which was placed into a sac and synchronous hysterectomy with right salpingo-oophorectomy. Intraoperatively, the mass was characterized macroscopical as endometrioma.

Results The pathology report of the ovarian tumor revealed malignant struma ovarii with thyroid papillary cancer type. The immunohistochemical analysis showed that EMA, CK7, TTF1, PAX8, thyroglobulin, p16, M1B1 (>20% positive) markers were positive. An otorhinolaryngological assessment was requested to define whether the ovarian tumor was originated from a primary thyroid tumor. A thyroidectomy was conducted, in which the pathology report was negative for thyroid cancer. Multidisciplinary team (MDT) decided that no adjuvant therapy was needed, because of the complete resection and the absence of distant metastasis. The patient is scheduled for follow-up with postoperative serum CA125 and lower abdominal MRI every 2–3 months.

Conclusion Thyroid carcinoma arising in struma ovarii or mature cystic teratoma is a rare type of ovarian with no definite guidelines for the management of this condition. The diagnosis of this neoplasm usually is a post-operative finding after histopathological examination of the resected sample, as in our case. Because of the rarity of this tumor, a close follow-up is required.

Disclosures Disclosures All authors declare no potential conflicts of interest with respect to the research, authorship, and/or publication of this case report.