Grade is a Dominant Risk Factor for Metastasis in Patients with Rectal Neuroendocrine Tumors

Small (< 2 cm) and diminutive (< 1 cm) rectal neuroendocrine tumors (RNETs) are often described as indolent lesions. A large single-center experience was reviewed to determine the incidence of metastasis and the risk factors for its occurrence.

Cases of RNET between 2010 and 2017 at a single institution were retrospectively reviewed. The rate of metastasis was determined, and outcomes were stratified by tumor size and grade. Uni- and multivariable predictors of metastasis were identified, and a classification and regression tree analysis was used to stratify the risk for distant metastasis.

The study identified 98 patients with RNET. The median follow-up period was 28 months. Of the 98 patients, 79 had primary tumors smaller than 1 cm, 8 had tumors 1 to 2 cm in size, and 11 had tumors 2 cm in size or larger. In terms of grade, 86 patients had grade 1 (G1) tumors, 8 patients had grade 2 (G2) tumors, and 4 patients had grade 3 (G3) tumors. Twelve patients developed metastatic disease. Both size and grade were associated with distant metastasis in the uni- and multivariable analyses, but when stratified by grade, size was predictive of metastasis only for G1 tumors (p < 0.001). Among the 12 patients with metastatic disease, 3 (25%) had diminutive primary tumors, and 9 (75%) had primary tumors 2 cm in size or larger. Diminutive tumors that metastasized were all G2.

Patients with diminutive and small RNETs are at risk for metastatic disease. Tumor grade is a dominant predictor of dissemination. More rigorous staging, closer surveillance, or more aggressive initial management may be warranted for patients with G2 tumors, irrespective of size.