626 Uniformly Indolent Behavior of Rectal and Appendiceal Neuroendocrine Tumors ≤0.5 cm on Long-Term Follow-Up: Toward New Terminology and Staging Considerations

Document Type

Conference Proceeding

Publication Date

3-23-2026

Publication Title

Lab Invest

Keywords

abdominal pain, adult, African American, aged, appendiceal neuroendocrine tumor, appendix, Caucasian, clinical article, colonoscopy, conference abstract, coronary artery disease, depth of invasion, diagnosis, female, follow up, hemicolectomy, human, human tissue, incidental finding, Kaplan Meier method, lamina propria, male, microadenoma, minimal residual disease, neuroendocrine tumor, neurosecretory cell, perineural invasion, rectal neuroendocrine tumor, rectum, surgery, tumor volume, very elderly

Abstract

Disclosures: Pegah Dejban: None; Qing Chang: None Background: Neuroendocrine tumors (NETs) have been well studied, and most organs have criteria distinguishing preneoplastic proliferations from true tumors. In the lung, proliferations ≤0.5 cm are termed tumorlets, and in the pancreas, lesions <0.5 cm are called microadenomas. By contrast, in the rectum and appendix, any neuroendocrine proliferation is classified as a NET, with stage determined by depth of invasion, even though those ≤0.5 cm typically behave indolently. This study evaluates the clinicopathologic features and outcomes of rectal and appendiceal NETs in this size range. Design: We reviewed patients with rectal and appendiceal NETs ≤0.5 cm diagnosed between 2010 and 2020, with 5–15 years of follow-up. Variables included demographics, clinical presentation, pathologic features, and outcomes assessed by Kaplan–Meier analysis (Figure 1). Results: Thirty patients (15 rectal NETs, 15 appendiceal NETs) were identified. Rectal NETs: Median age 59 years (38–74); 40% male and 73% African American. Most were incidental findings on screening colonoscopy (86% polyps, 14% nodules). All were grade 1 with Ki-67 < 3%, staged as pT1a (5 confined to lamina propria, 10 to submucosa), without lymphovascular (LVI) or perineural invasion (PNI). None required additional resection; all remained disease-free (follow-up 0–111 months). Appendiceal NETs: Median age 58 years (15–85); 33% male and 93% Caucasian. Presenting symptom was abdominal pain with imaging showing a dilated appendix but no mass. Median tumor size 0.3 cm (0.1–0.5). All were grade 1 with Ki-67 < 3%. Nine were pT1 (2 submucosal, 7 muscularis propria) and six pT3 (5 subserosa, 1 mesoappendix). One pT3 tumor with LVI had no residual disease at hemicolectomy. All were recurrence-free (follow-up 2–132 months), with one death due to coronary artery disease (Table 1). [Formula presented] [Formula presented] Conclusions: In line with NCCN guidance, additional treatment or surveillance appears unnecessary for T1 lesions in this size range. We propose exploration of the term ‘neuroendocrine cell cluster/tumorlet’ for well-differentiated rectal NETs ≤0.5 cm invading the lamina propria, and consideration of a Tis category for appendiceal NETs ≤0.5 cm involving submucosa or muscularis propria.

Volume

106

Issue

3

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