Prognosis of incompletely resected small rectal neuroendocrine tumor using endoscope without additional treatment.

Background: In recent years, the incidence of rectal neuroendocrine tumors (NET)s has markedly increased due to the widespread use of screening colonoscopy. However, many patients are referred from local clinics after undergoing conventional endoscopic mucosal resection (EMR) for polyps without perceived NET, with a pathological report of incomplete resection. We evaluated the prognosis of incompletely resected small rectal NET without additional endoscopic resection for small rectal NET less than 10 mm in diameter present within the submucosal layer showing good prognosis, due to its rare metastatic potential.

Methods: We retrospectively reviewed patients from 2008 to 2018 at a single center who had had small rectal NET (located in the rectum from the anal verge to 20 cm in proximity) and had undergone 'incomplete resection' using endoscopy with a positive deep margin or with a very small safe deep margin (< 100 um). A small rectal NET was defined as a tumor ≤ 10 mm in diameter, without lymph node nor distant metastasis, and with low grade (G1) according to the WHO grading system.

Results: Of 267 patients who were diagnosed with small rectal NET, 77 were diagnosed with incomplete resection or possible remnant NET. Of those, 55 patients (55/77, 71.4%) were referred from local clinics post EMR diagnosed as polyps. The rate of histologically incomplete resection was highest in endoscopic submucosal dissection (11/21, 52.4%) and lowest in surgical resection (0/9, 0%), while endoscopic submucosal resection with band ligation showed an incomplete resection rate of 4.4% (5/113). After exclusion of 36 patients, namely 21 patients had undergone additional surgical (n = 6) or endoscopic (n = 15) resection and 25 patients who were lost during the follow-up period of 2 years, 31 patients had undergone surveillance with endoscopic evaluation or either a biopsy or radiological evaluation for distant metastasis during a median follow-up duration of 2 years. None of the incompletely resected small rectal NET patients showed local or distant metastasis.

Conclusion: Incomplete resection of small rectal NET with G1 grade has a good prognosis without additional treatment.