OBJECTIVE: To describe the clinical and ultrasound characteristics at the time of diagnosis of primary ovarian immature teratoma with no other germ cell tumor components described on histopathology. METHODS: This was a retrospective study of women with a histological diagnosis of primary ovarian immature teratoma who had undergone a preoperative ultrasound examination between 1998 and 2024. Cases were identified from the databases of 17 contributing ultrasound centers and the International Ovarian Tumor Analysis (IOTA) database. The descriptions of the ultrasound images of the tumors made by the original ultrasound examiners using IOTA terminology were reported. In addition, grayscale and color or power Doppler ultrasound images or videoclips were retrieved for all tumors. Two independent ultrasound examiners reviewed the retrieved material and searched for specific ultrasound characteristics of immature teratomas using pattern recognition. We present their agreed description of the tumors. RESULTS: In total, 64 patients with ovarian immature teratoma were included, of which 38 (59.4%) were obtained from the IOTA database (IOTA studies phase 1, 1b, 2, 3, 5 and 7). The median age of the patients at diagnosis was 24.5 (interquartile range (IQR), 18.8-31.0; range, 12-50) years. The most common presenting symptoms were abdominal or pelvic pain (38/60, 63.3%) and abdominal swelling (30/60, 50.0%). All immature teratomas were unilateral. The median largest diameter of the tumor was 149.5 (IQR, 125.0-183.8; range, 27-400) mm. Using IOTA terminology, most tumors were described as multilocular-solid (32/64, 50.0%) or solid lesions (22/64, 34.4%). When present, the solid component had a median largest diameter of 98.5 (IQR, 59.8-146.8; range 6-400) mm. Most masses showed minimal (19/63, 30.2%) or moderate (35/63, 55.6%) vascularization on color or power Doppler ultrasound examination. Using pattern recognition, the most typical ultrasound feature was heterogeneous, bizarre echogenicity of the solid components, with hyperechogenic areas, cystic spaces and acoustic shadows. This feature, which we consider pathognomonic, was present in 48/57 (84.2%) immature teratomas in which the solid components were adequately assessable. CONCLUSIONS: The typical ultrasound appearance of an ovarian immature teratoma is a large unilateral adnexal mass with large solid components that is poorly or moderately vascularized. The pathognomonic feature is heterogeneous echogenicity of the solid components with hyperechogenic areas, cystic spaces and acoustic shadows. Preoperative suspicion of immature teratoma can guide treatment, such as offering fertility-sparing surgery. © 2025 The Author(s). Ultrasound in Obstetrics & Gynecology published by John Wiley & Sons Ltd on behalf of International Society of Ultrasound in Obstetrics and Gynecology.

Clinic of Obstetrics and Gynecology University of Milan Bicocca San Gerardo Hospital Monza Italy

Department of Clinical Sciences Malmö Lund University Malmö Sweden

Department of Development and Regeneration KU Leuven Leuven Belgium

Department of Gynecologic Oncology Fondazione IRCCS Istituto Nazionale dei Tumori Milan Italy

Department of Gynecological Oncology and Gynecology Medical University of Lublin Lublin Poland

Department of Metabolism Digestion and Reproduction Faculty of Medicine Imperial College London London UK

Department of Obstetrics and Gynecology Forli and Faenza Hospitals and University of Bologna Bologna Italy

Department of Obstetrics and Gynecology Skåne University Hospital Malmö Sweden

Department of Obstetrics and Gynecology University Hospitals Leuven Leuven Belgium

Department of Oncology Laboratory of Gynecologic Oncology ImmunOvar Research Group KU Leuven Leuven Belgium

Dipartimento Scienze della Salute della Donna del Bambino e di Sanita' Pubblica Fondazione Policlinico Universitario Agostino Gemelli IRCCS Rome Italy

Dipartimento Universitario Scienze della Vita e Sanità Pubblica Università Cattolica del Sacro Cuore Rome Italy

General University Hospital Prague Czech Republic

Gynecologic Oncology Centre Department of Gynecology Obstetrics and Neonatology 1st Faculty of Medicine Charles University and General University Hospital Prague Prague Czech Republic

Institute of Histopathology Catholic University of the Sacred Heart Rome Italy

Preventive Gynecology Unit Division of Gynecology European Institute of Oncology Milan Italy

Queen Charlotte's and Chelsea Hospital Imperial College Healthcare NHS Trust London UK

UniCamillus International Medical University Rome Italy

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