Summary: Struma ovarii is an ovarian teratoma that comprises 2-5% of all ovarian teratomas. Malignant transformation of struma ovarii occurs in less than 5% of all cases, and metastatic disease is even rarer. We report two cases initially diagnosed with benign struma ovarii that presented malignant transformation, specifically highly differentiated follicular carcinoma of the ovary (HDFCO), some years after the first diagnosis. Case 1 concerns a 37-year-old female featuring HDFCO of the right ovary with multiple metastatic foci, who was diagnosed with benign struma ovarii 14 years ago. Case 2 concerns a 26-year-old female diagnosed with HDFCO of the left ovary. This patient was initially diagnosed with benign struma ovarii 6 years ago that recurred 4 years after the diagnosis. Both patients were treated with surgery, adjunctive total thyroidectomy, and radioactive iodine (131I) therapy.
Learning points: Malignant transformation of struma ovarii is very rare (<5%). Diagnosis of HDFCO without extra ovarian dissemination is difficult due to the resemblance of its histological appearance with normal thyroid tissue. There is no consensus on the postoperative treatment of malignant struma ovarii (MSO). Clinical and histological features of MSO should be assessed for the postoperative treatment decisions. TSH suppression and thyroglobulin level measurements are necessary for patient follow-up.