Ovarian Collision Tumor in a Pediatric Patient: A Mature Teratoma Associated with a Combined Tumor Containing a Mucinous Cystadenocarcinoma Component

<b>Background</b>: Collision tumors, especially in the ovary, are a rare phenomenon where two distinct types of tumors develop adjacent to each other within the same organ but remain separate histologically. We present a case of the first collision ovarian tumor in a 14-year-old girl consisting of a combined tumor and a mature teratoma. <b>Case Report</b>: A 14-year-old girl presented with abdominal swelling for the past three months, without other symptoms. Ultrasound (US) examination of the abdomen revealed a large cystic mass with multiple septa, filling the entire abdomen from the diaphragm to the pelvis. Magnetic resonance imaging (MRI) showed an intraperitoneal mass, inseparable from the right ovary, measuring 22 cm × 13 cm × 30 cm. Serum tumor markers were within normal limits. The tumor mass was completely extirpated along with the fallopian tube. Histological and immunohistochemical analysis determined that it was a mucinous cystadenocarcinoma, characterized by a transition pattern from benign and borderline components to an adenocarcinoma component with a smaller mature teratoma. Six-month follow-up revealed no recurrence or postoperative complications. <b>Conclusions</b>: As the first documented case, this case provides valuable insights into pediatric ovarian neoplasms, guiding future diagnostic and therapeutic approaches.