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Double neuroendocrine ductal carcinomas in situ coexisting with a background of diffuse idiopathic neuroendocrine cell hyperplasia of breast: A case report and hypothesis of neuroendocrine tumor development

This article reports the case of a 72‐year‐old woman with two nodules of neuroendocrine (NE) ductal carcinoma in situ coexistent with a background of NE cell hyperplasia. Both tumors, 15 and 3 mm in size, were incidentally revealed on computed tomography without any apparent clinical symptoms. The tumors showed similar histological features, and more than 50% of the tumor cells patchily expressed NE markers, such as chromogranin A, synaptophysin, CD56, and somatostatin receptor type 2. The surrounding nontumor ductal cells also showed spotty or linear positivity for NE markers in contrast to the cells of normal atrophic breasts, which rarely present with NE cells. Moreover, focal mucin production was also observed in the peripheral ducts. It is hypothesized that idiopathic breast NE cell hyperplasia with multiple small nests of NE cells may extend to form a true mass of NE neoplasms.