Thyroid papillary cancer elements arising from struma ovarii with benign peritoneal strumosis: Utility of iodine‐123 imaging in diagnostics and treatment planning

KEY CLINICAL MESSAGE: In this case of struma ovarii a right‐sided ovarian mass contained features of papillary thyroid cancer. Diagnostic iodine‐123 revealed multiple foci of extraovarian spread, likely as a manifestation of concomitant peritoneal strumosis. Unilateral oophorectomy, partial peritonectomy, and adjuvant iodine‐131 treatment were performed for successful curative treatment. ABSTRACT: Struma ovarii is a rare form of mature teratoma defined by a predominance of thyroid tissue. Approximately 5% of all ovarian strumae exhibit malignant transformation. Due to their extreme rarity, there has been a lack of consensus concerning uniform diagnostic criteria. Appropriate, risk‐stratified treatment strategies also remain widely unelaborated, based only on a small number of cases reported in the literature. We describe the case of a 35‐year‐old female, who presented after undergoing unilateral oophorectomy for a right‐sided ovarian mass. Histological workup revealed a struma ovarii containing papillary thyroid cancer (PTC). Postoperative I‐123 scintigraphy with single photon emission computed tomography (SPECT) detected multifocal extra‐ovarian spread to the peritoneum, containing likely benign strumosis upon pathological examination. The subsequent treatment strategy involved an ablative concept including total thyroidectomy and subsequent I‐131 radioiodine therapy. Throughout a 3‐year follow‐up, the patient has remained without recurrence with thyroglobulin levels ranging below detection limits. Surgical resection with adjuvant radioiodine therapy is a curative therapeutic strategy in cases of struma ovarii with thyroid‐type carcinoma and peritoneal strumosis. Its benefits lay in avoiding more extensive surgery, potentially maintaining fertility, facilitating follow‐up, and minimizing the risk of recurrence. Reliable criteria for risk stratification are needed to identify patients who are most likely to benefit from this treatment approach.