Size matters: Establishing a cut-off for rectal neuroendocrine neoplasm to predict recurrence and standardize surveillance guidelines.

Objectives: This study aimed to identify risk factors for recurrence of rectal neuroendocrine neoplasms, establish a cut-off size for recurrence prediction, and standardize surveillance guidelines.

Methods: This retrospective study analyzed patients diagnosed with rectal neuroendocrine neoplasm at Samsung Medical Center from January 2007 to July 2021. Tumors were classified according to World Health Organization and European Neuroendocrine Tumor Society guidelines. The primary outcome was to determine the ideal cut-off size for predicting recurrence.

Results: A total of 1011 patients (median follow-up: 58 months) were included: 967 with grade (G) I neuroendocrine tumor (NET), 35 with GII NET, and 9 with neuroendocrine carcinoma. Disease-free and overall survival were significantly better in GI NET than in GII and neuroendocrine carcinoma. For NET G1 patients undergoing endoscopic resection, a 0.7 cm cut-off (area under the curve = 0.94) showed 100% sensitivity, 79% specificity, and no recurrence. In contrast, for lymphovascular invasion (LVI)-positive, lymph node-negative NET G1 patients undergoing transanal endoscopic microsurgery/transanal excision or radical resection, an optimal cut-off of 1.5 cm (area under the curve = 0.92) was identified. NET G2 had a 22.9% lymph node metastasis rate, with recurrence risk increasing with size.

Conclusions: For NET G1 tumors ≤0.7 cm without LVI following endoscopic resection, routine surveillance may not be necessary due to the minimal risk of recurrence. Similarly, for LVI-positive, lymph node-negative NET G1 tumors that underwent surgical resection, surveillance may not be required if the tumor is ≤1.5 cm. Additionally, NET G2 tumors require regular follow-up regardless of size to ensure favorable oncologic outcomes. These findings contribute to a risk-based approach for surveillance, optimizing follow-up strategies.