Current status of and problems in endoscopic diagnosis and treatment of rectal neuroendocrine tumor

Endoscopists have an increasing chance of seeing patients with a rectal neuroendocrine tumor (NET). However, there are many issues about rectal NET on which no consensus has been reached, including diagnosis, treatment, and management following treatment. With regard to indications for endoscopic treatment, further discussion is required about whether rectal NETs sized 1-1.5cm can be an indication for endoscopic treatment. On the other hand, it is widely accepted that rectal NETs sized <1cm without muscularis invasion are good indications for endoscopic treatment. For rectal NETs sized < 1cm without muscularis invasion, a modified endoscopic mucosal resection (EMR) technique is recommended, such as endoscopic submucosal resection with ligation device (ESMR-L) or endoscopic mucosal resection using a cap-fitted endoscope (EMR-C), due to its effectiveness, safety and low burden for patients. Management following endoscopic treatment is decided based on the results of pathological evaluation of the endoscopically resected specimen. However, depending on the conditions of cell proliferation and lymphovascular invasion, it may be difficult to judge whether radical surgery is required following endoscopic treatment or not. Particularly regarding lymphovascular invasion, it has been reported that lymphovascular invasion is frequently diagnosed even in patients with small rectal NET G1 lesions without muscularis invasion by using immunohistochemical and special staining in pathological evaluation. Further studies on the clinical significance of lymphovascular invasion in patients with rectal NETs are warranted.