Net of the colon and rectum D44.9

Epithelial tumour, originating from enterochromaffin cells (EC cells) of the diffuse neuroendocrine system (DENS). Rectal NETs usually do not develop a hormonal syndrome and remain asymptomatic for a long time. They are usually diagnosed at the asymptomatic stage and often in the course of a preventive colonoscopy.

To determine the exact size of the rectal NET, to exclude infiltration of deeper layers of the intestinal wall (muscularis propria) and to exclude lymph node metastases, an endosonography should be performed prior to endoscopic resection (Eick J et al. 2016).

Further diagnostic measures such as MRI of the pelvis, CT, somatostatin receptor scintigraphy are not necessary for well differentiated rectal NET <1cm preinterventionally. For rectal NETs >1cm, for mainly lymph node metastases, for lymphatic and/or angioinvasion, for infiltration of the muscularis propria or for a proliferation rate of ≥2%. In addition, a complete colonoscopy should be performed in all patients with rectal NET to detect and treat multifocal NET as well as synchronous adenomas and adenocarcinomas of the colon.

With mucosal NET with a diameter <1cm the risk of lymph node metastases is only 4%. Neuroendocrine tumors of the rectum of this size with low proliferative activities (Ki-67<2%, G1) that do not grow lymphovascularly or angioinvasively can be called NET without risk factors.

They can be resected endoscopically. Endoscopic resection may be endoscopic mucosal resection without (EMR) or with cap (EMR-C), modified endoscopic mucosal resection techniques (EMR-L; EMR-CMI), or endoscopic submucosal dissection (ESD).

In the case of a risk constellation or non-curative endoscopic resection, surgical-oncological postoperative resection must be performed.

If risk factors (lymphovascular angioinvasion, infiltration of the muscular wall layer (Muscularis propria), lymph node metastases or an increased proliferation rate (Ki-67>10%)) are present, patients without significant comorbidities will be advised to undergo surgery (de Mestier L et al. 2013).

For patients with small (<1cm) rectal NETs and G2 differentiation and patients with rectal NETs between 10 and 20 mm in size and G1 differentiation, there are no standardized studies and guideline recommendations. In these cases, a decision must be made between a local endoscopic or radical surgical procedure, depending on other risk factors and age and comorbidities (Rinke A et al. AWMF Guidelines 2018)

The risk of metastasis in this tumor entity is low (Scherübl H et al. 2011).

Neuroendocrine tumors (NET) of the rectum are 30 to 35 times more common than 40 years ago.

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