Doctors identify warning signs for rare cancers hiding in ovarian cysts

ABSTRACT Objective To describe the clinical and ultrasound characteristics of ovarian mature cystic teratoma containing malignancy. Methods This was an international, retrospective, observational study. We invited 19 (oncological or non‐oncological) gynecological centers to provide clinical and ultrasound information on cases of histologically confirmed ovarian mature cystic teratoma containing malignancy. We reported the description of the tumors by the original ultrasound examiner by extracting information from original ultrasound reports and patient records. In addition, two gynecologists, who were European Federation of Societies for Ultrasound in Medicine and Biology Level II or III in gynecological ultrasound, reviewed all available electronic ultrasound images and used pattern recognition with the aim of identifying the typical sonographic features of ovarian mature cystic teratomas containing malignancy. International Ovarian Tumor Analysis terminology was used to describe the tumors throughout. Results In 10/19 invited centers, no eligible patients were identified. From the databases of nine gynecological centers (three of which were oncological centers), we identified 26 patients with a histologically confirmed malignancy in an ovarian mature cystic teratoma who had undergone preoperative ultrasound examination between September 2012 and August 2024. The median age of the 26 included patients was 48.5 (range, 14–79) years and most (17/26 (65.4%)) were postmenopausal. The most frequent malignant histology was squamous cell carcinoma (13/26 (50.0%)), followed by carcinoid tumor and mucinous malignancy (each 5/26 (19.2%)), endometrioid carcinoma (2/26 (7.7%)) and ganglioglioma (1/26 (3.8%)). Based on the information in the original ultrasound reports, all lesions were unilateral. The median maximum diameter of the ovarian tumors was 110 (range, 21–310) mm. Teratomas containing a mucinous malignancy were the largest (median maximum diameter, 210 (range, 120–310) mm) and teratomas containing a carcinoid tumor were the smallest (median maximum diameter, 29 (range, 21–64) mm). Most tumors containing squamous cell carcinoma, carcinoid tumor, endometrioid cancer or ganglioglioma were described as a unilocular cyst (9/21 (42.9%)) or unilocular‐solid cyst (6/21 (28.6%)) by the original ultrasound examiner and most (16/21 (76.2%)) were described as containing cystic content of mixed echogenicity (i.e. the echogenicity typical of a mature cystic teratoma). All five teratomas containing mucinous malignancies were described as a multilocular or multilocular‐solid cyst and most (4/5 (80.0%)) were described as containing cystic fluid of low‐level or ground‐glass echogenicity. Acoustic shadows were described in 14/26 (53.8%) tumors. Eleven of the 26 (42.3%) tumors had moderate or rich vascularization on color Doppler (color score of 3 or 4). The original ultrasound examiner misclassified 12/26 (46.2%) tumors as a benign teratoma. Good‐quality ultrasound images were available for 25 tumors. On retrospective review of the ultrasound images using pattern recognition, 10/25 (40.0%) tumors appeared as a typical mature cystic teratoma, 8/25 (32.0%) showed features typical of a mature cystic teratoma but contained a solid component (7/8) or multiple cyst locules (1/8), making them suspicious for malignancy, and 7/25 (28.0%) lacked characteristics typical of a mature cystic teratoma and appeared malignant. Conclusions 40.0% of the mature cystic teratomas containing malignancy in our series appeared sonographically indistinguishable from benign teratomas. The presence of solid components within the tumor, especially if vascularized on color or power Doppler, large tumor size, advanced age and postmenopausal status raised the ultrasound examiner's suspicion of malignancy. Serum CA 125 level appears to have a limited role as a risk factor for malignancy, as it is not elevated in most reported cases of squamous cell carcinoma arising in mature cystic teratomas. In contrast, squamous cell carcinoma antigen levels may be more useful. © 2026 International Society of Ultrasound in Obstetrics and Gynecology.