High-grade appendiceal mucinous neoplasm: case report.

High-grade appendiceal mucinous neoplasm is an extremely rare gastrointestinal tumour with nonspecific clinical manifestations that complicate preoperative diagnosis. In women, it may mimic gynecological diseases, particularly ovarian tumours, leading to diagnostic delays and inappropriate surgical approaches. We report the case of a 68-year-old woman from the San Martín region of Peru who presented with abdominal pain, progressive distension, and abnormal vaginal bleeding. Laboratory tests showed elevated carcinoembryonic antigen levels, while transvaginal ultrasound revealed a right adnexal mass suggestive of ovarian neoplasia. During surgery, a mucinous appendiceal mass without ovarian involvement was incidentally identified, prompting modification of the surgical plan to an oncological right hemicolectomy. Histopathological analysis confirmed high-grade mucinous adenocarcinoma of the appendix. The postoperative course was favorable. This case highlights the importance of considering appendiceal neoplasms in the differential diagnosis of pelvic masses in postmenopausal women and the value of intraoperative and histopathological evaluation.