Grade 2 pancreatic neuroendocrine tumors: overbroad scope of Ki-67 index according to MRI features

To evaluate the value of MR imaging features in stratifying Grade 2 (G2) pancreatic neuroendocrine tumors (PNETs) using the 5% cut-off value of the Ki-67 index as reference standards.

Between January 2010 and October 2016, 41 G2 PNET patients (One patient had 3 tumors) with preoperative MR imaging were included. Tumor grading was based on the revised 2016 World Health Organization classification of PNETs. MR imaging features included size, shape, consistency, T1-w and T2-w signal intensities, enhancement pattern, apparent diffusion coefficient (ADC) ratios (tumor/normal pancreatic parenchyma).

16 Ki-67 index < 5% tumors (SKIT, 37.2%) and 27 Ki-67 index ≥ 5% tumors (LKIT, 62.8%) of G2 were evaluated. The LKIT showed solid consistency (85% vs. 50%, P < 0.05), incomplete envelope-like reinforcement in a delayed phase (74% vs. 62%, P < 0.05), and liver or lymph node metastases (67% vs. 31%, P < 0.05) more frequently than did SKIT. However, ADC ratios of LKIT were smaller than SKIT (0.85 ± 0.23 vs. 1.29 ± 0.39, P = 0.001). Using binary logistic regression analysis, the ADC ratio was an independent significant differentiator of SKIT from LKIT. The AUROC of ADC ratios was 0.816 ± 0.07. The optimal cut-off value for the identification of LKIT was 1.25 × 10^-3 (sensitivity 96.3%, specificity 62.5%).

MRI features may identify the overbroad scope of G2 PNETs and help predict Ki-67 values, as a surrogate for tumor aggressiveness, in G2 PNETs. An optimal cut-off value for predicting Ki-67 status (≥/< 5%) was 1.25 × 10^-3 of ADC ratio.