Lymph node metastasis of grade 1 neuroendocrine tumor from rectal mixed neuroendocrine non-neuroendocrine neoplasm: A case report.

Mixed neuroendocrine non-neuroendocrine neoplasms of the gastrointestinal tract are rare, with only a few cases documented in the literature. In particular, there are no reports of a rectal mixed neuroendocrine non-neuroendocrine neoplasm with regional lymph node metastasis of the neuroendocrine tumor component. Here, we present the case of a man in his early 70s with an incidental finding of multiple colonic polyps during a routine colonoscopy, including a 0.9-cm sessile polyp in the rectum. Repeat colonoscopy was performed for endoscopic mucosal resection. Histopathological examination revealed a biphasic tumor comprising a well-differentiated adenocarcinoma and a grade 1 neuroendocrine tumor, leading to a diagnosis of rectal mixed neuroendocrine non-neuroendocrine neoplasm. Because the adenocarcinoma component had invaded the submucosa and involved the resection margin, an ultralow anterior resection was performed. Notably, postoperative pathological analysis revealed metastasis of the neuroendocrine tumor component to a regional lymph node, an unusual finding not previously reported in the literature. Despite this, the patient remained recurrence-free for 2 years without adjuvant therapy. To the best of our knowledge, this is the first such case to be reported in the literature. Given the limited understanding of mixed neuroendocrine non-neuroendocrine neoplasms, particularly cases with low-grade neuroendocrine components, further case accumulation is necessary to develop standardized treatment guidelines.