Background: Management of <2-cm pancreatic neuroendocrine tumors is controversial. Although often indolent, the oncologic heterogeneity of these tumors particularly related to lymph node metastases poses challenges when deciding between resection versus surveillance.

Methods: We analyzed all patients who underwent resection of primary nonfunctional <2-cm with curative-intent at 8 institutions of the US Neuroendocrine Tumor Study Group from 2000 to 2016. Pancreatic neuroendocrine tumors with poor differentiation and Ki-67 > 20% were excluded. Our primary aim was to create a lymph node risk score that predicted lymph node metastases accurately for <2-cm pancreatic neuroendocrine tumors, utilizing readily available preoperative data.

Results: Of 695 patients with resected pancreatic neuroendocrine tumors, 309 were <2 cm. Of these small pancreatic neuroendocrine tumors, 25% were proximal (head/uncinate), 23% had a Ki-67 > 3%, and only 8% were moderately differentiated. Also, only 9% of all <2-cm pancreatic neuroendocrine tumors were lymph node (+). Indeed lymph node positivity was associated with worse 5-year recurrence-free survival compared with lymph node (-) disease (80% vs 96%; P = .007). Factors known preoperatively to be associated with lymph node metastases were proximal location (odds ratio 4.0; P = .002) and Ki-67 ≥3% (odds ratio 2.7; P = .05). Moderate differentiation was not associated with lymph node (+) disease. Location and Ki-67 were assigned a value weighted by their odds ratio: (distal= 1, proximal= 4, and Ki-67 < 3% = 1 and Ki-67 ≥ 3% = 3), which formed a lymph node risk score ranging 1-7. Scores were categorized into low (1-2), intermediate (3-4), and high (5-7) risk groups. Incidence of lymph node metastases increased progressively based on risk group, with low = 3.2%, intermediate = 13.8%, and high = 20.5%. Only 3.4% of pancreatic neuroendocrine tumors with a Ki-67 < 3% in the distal pancreas were lymph node (+) compared with 21.4% of pancreatic neuroendocrine tumors with a Ki-67 ≥ 3% in the head/uncinate.

Conclusion: This simple and novel lymph node risk score utilizes readily available preoperative factors (tumor location and Ki-67) to stratify risk of lymph node metastases accurately s for < 2-cm pancreatic neuroendocrine tumors and may help guide management strategy.