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Introduction: Struma ovarii, a rare ovarian neoplasm, is a monophyletic teratoma composed predominantly of thyroid tissue (>50%).Cystic struma ovarii is a rare variant wherein the thyroid component could be minimal in contrast to struma ovarii. The co-existence of two tumors with different histogenesis in a neoplasm is a rare pathological entity. In the present case, Mucinous cystadenoma which is of surface epithelial origin is seen co-existing with Cytic struma ovarii which is of germ cell origin. Case report : A 25 year old woman presented with complaints of pain right lower abdomen since 1week, on examination a mobile cystic mass of about 10*10 cm size was felt in right adnexa which was deviating the uterus to the left. Her USG revealed an anechoic cystic lesion of 9.5*5.4 cm size (230cc volume) with multiple thin septations arising from right adnexa -possibly mucinous cystadenoma. Her tumor markers and thyroid profile were found to be within normal limits. Laparotomy revealed a 10*8*5cm multi loculated cystic mass with smooth outer surface which was enucleated and cystic fluid drained and sent for cytology. On histopathological examination the cyst was diagnosed as mucinous cystadnoma, however on further examination areas of thyroid tissue were detected at the cyst wall and its presence was confirmed by Immunohisto chemistry where positivity for thyroid transcription factor -1 was noted. Cystic struma ovarii is easily missed unless extensive tissue sampling is done to quantify it. It is recommended that cystic struma ovarii should be considered in the differential diagnosis of all ovarian cystic lesions.