Rationale: Tailgut cyst (TGC) is a rare congenital disease that originates from residues of the tail intestine during the embryonic period. Most TGCs are benign lesions and the malignant transition is very rare.
Patient concerns: A 50-year-old woman attended our department complaining of defecation difficulty for more than 2 months. She reported irregular defecation with a small amount of liquid stool, 3 to 4 times per day.
Diagnosis: Biochemical analysis showed high levels of carcinoembryonic antigen (79.89 ng/mL; normal, 0-3 ng/mL) and carbohydrate antigen 199 (57.60 U/mL; normal, 0-35 U/mL). Abdominal computer tomography and magnetic resonance imaging showed a large cystic mass with enhanced signals. Post-surgical histopathology indicated that the mass was a TGC with adenocarcinoma transition.
Interventions: The cyst was completely resected. Symptomatic treatment was further performed, and the patient recovered well.
Lessons: We reported a rare case of a large TGC with adenocarcinoma transition. CT, MRI, and histopathology are important to diagnose TGC. Complete surgical resection is the first choice to treat TGC.