The mediastinal gastroenteric cyst is a rare developmental cyst. In general, the majority of mediastinal gastroenteric cysts are recognized in infancy and are commonly located in the posterior mediastinum. In addition, gastroenteric cysts occasionally contain pancreatic tissue; however, no studies have reported that these cysts influenced the diabetes mellitus status of the patient. We report here an extremely rare case of an adult gastroenteric cyst with pancreatic tissue originating from the anterior mediastinum, influencing the severity of diabetes mellitus in a patient. This case report presents two remarkable findings. First, the location of the gastroenteric cyst (anterior mediastinum) and the patient's age at the time of detection (adulthood) were extremely rare. Secondly, the present case is also unusual in terms of glycometabolism. As this tumour decreased in size, the glycosylated haemoglobin value increased, which suggested a worsening of the patient's diabetes mellitus. The diabetes mellitus further worsened after removal of the tumour.
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