Ovarian endometriosis associated with significantly elevated CA19-9: a case report.

Background: Carbohydrate antigen 19-9 (CA19-9) is a well-established tumour marker primarily associated with pancreaticobiliary and gastrointestinal malignancies. Its elevation in benign gynaecological conditions, particularly in isolated ovarian endometriosis (OE) without malignancy, is rare and can pose a significant diagnostic challenge. Case presentation: In this case report, a 44-year-old female was found to have significantly elevated CA19-9 (up to 2453.1 U/mL), accompanied by left lower abdominal dull pain and weight loss. She was first diagnosed with an ovarian cyst more than 20 years ago, which resolved during pregnancy and recurred postpartum with no special intervention due to the absence of clinical symptoms. Extensive workup, including gastroenterological endoscopy and abdominal magnetic resonance imaging (MRI), excluded gastrointestinal malignancies. Laparoscopy followed by pathological examination confirmed the diagnosis of an ovarian endometriotic cyst (chocolate cyst). Postoperatively, the patient received standardised endocrine therapy, and follow-up assessments showed that serum CA19-9 levels returned to the normal range with no evidence of lesion recurrence. Conclusions: For female patients with elevated CA19-9 combined with a long-standing ovarian cyst and pelvic symptoms, ovarian malignancy should be suspected after excluding gastrointestinal lesions. However, benign diseases should also be considered. The final diagnosis should be based on comprehensive clinical evaluation, additional laboratory and imaging findings, and ultimately confirmed by pathological biopsy. Clinicians need to strengthen the cognitive understanding of this atypical manifestation to reduce misdiagnosis.