Incidental Rectal Neuroendocrine Tumor Detected During Screening Colonoscopy Managed With Transanal Minimally Invasive Surgery (TAMIS): A Case Report.

Rectal neuroendocrine tumors (NETs) are uncommon neoplasms that are increasingly detected due to the widespread adoption of colorectal cancer screening programs. They typically present as small submucosal nodules, facilitating early diagnosis and minimally invasive treatment. We present the case of a 58-year-old woman with a history of type 2 diabetes mellitus who underwent screening colonoscopy while asymptomatic. During the procedure, an 18-mm non-polypoid submucosal lesion was identified on the anterior rectal wall, 8 cm from the anal verge. Contrast-enhanced computed tomography of the chest, abdomen, and pelvis demonstrated no evidence of regional or distant metastasis. Complete local excision was performed using transanal minimally invasive surgery (TAMIS) without complications. Histopathological examination revealed a well-differentiated grade 1 rectal NET staged as pT1bN0M0, with a Ki-67 proliferation index of 1%, positive immunohistochemical staining for chromogranin A, negative surgical margins, and no evidence of lymphovascular or perineural invasion. Follow-up imaging demonstrated no evidence of local recurrence or distant metastatic disease, and the patient remains under clinical and radiological surveillance. This case highlights the importance of colorectal cancer screening programs for the incidental detection of rare rectal NETs and supports TAMIS as a safe and effective treatment option for selected patients with localized disease.