Introduction Duplication of the alimentary tract is a relatively uncommon congenital anomaly and most cases occur in childhood. Malignancy arising from a gastric duplication cyst is extremely rare. We herein report a very rare case of malignant transformation of a gastric duplication cyst. Presentation of case A 47-year-old asymptomatic Japanese woman was referred to our hospital with a large abdominal mass adhered to the stomach. Since there was a possibility of malignant transformation, complete resection of the cyst and segmental gastrectomy without regional lymphadenectomy were performed. Discussion To our knowledge, this is the 2nd report of asymptomatic adenocarcinoma arising from a gastric duplication cyst in the English-language literature. Unfortunately, the patient developed peritoneal metastasis and ascites seven months after the surgery and died. Conclusion From our long-term follow-up experience of this gastric duplication cyst, we recommend making accurate diagnosis as soon as possible with biopsy using endoscopic ultrasonography. When the disease is diagnosed as malignant, we recommend gastrectomy with lymphadenectomy. Even if the disease is diagnosed as benign, we recommend close observation with imaging modalities.
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