A warning to young women: a case of giant ovarian mature cystic teratoma with squamous cell carcinoma transformation in a 22-year-old female and analysis of its molecular features.

Malignant transformation of ovarian mature cystic teratoma (MCT) is rare, with squamous cell carcinoma (SCC) as the most frequent subtype. Because it usually occurs after menopause, cases in young adults may be overlooked. A 22-year-old nulligravid woman presented with progressive abdominal distension and acute worsening abdominal pain. Contrast-enhanced abdominopelvic computed tomography showed a huge mixed cystic-solid mass (22.5 × 14.1 × 27.8 cm) containing fat and calcifications, with a strongly enhancing mural nodule. Preoperative workup included broad laboratory testing and tumor markers, showing elevated serum squamous cell carcinoma antigen (SCC-Ag) and CA 19-9. Fertility-sparing comprehensive staging surgery confirmed well-to-moderately differentiated SCC arising in MCT, staged as International Federation of Gynecology and Obstetrics stage IA. Immunohistochemistry showed mutant-type p53 overexpression, diffuse epidermal growth factor receptor expression, and high programmed death-ligand 1 (PD-L1). SCC-Ag declined to 2.396 ng/mL after surgery. Six cycles of adjuvant paclitaxel liposome (240 mg) plus carboplatin (650 mg) were administered, and the patient remains clinically well with no evidence of disease to date. This case supports early recognition and timely management of suspicious MCTs in young adults.