The number of duodenal neuroendocrine tumors has recently increased, but their natural history has not been well characterized. Here we report the case of a 59-year-old female undergoing complete resection by endoscopic mucosal resection after monitoring endoscopic morphologic changes and the size increment of a duodenal neuroendocrine tumor over the longest follow-up duration. An elevated lesion was initially detected on the duodenal bulb. Endoscopic biopsy from this lesion was performed three times during the follow-up, but all of the specimens simply demonstrated the presence of gastric metaplasia. Thereafter, in the esophagogastroduodenoscopy performed 14 years after the initial examination, a slight depression was detected in the center of the lesion together with an incremental change of its size. The histopathologic diagnosis of neuroendocrine tumor was made by deeper tissue sampling, and the patient subsequently underwent endoscopic mucosal resection for this tumor and was finally diagnosed with a G1 duodenal neuroendocrine tumor with pT2 cN0, cM0, stage IIa in the TNM classification. This is the first case demonstrating the slowly progressive nature of such a tumor with the longest follow-up of a duodenal neuroendocrine tumor ever reported, which could provide important information regarding the natural history and management of such tumors.
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