Reliability of Ki-67 Determination in FNA Samples for Grading Pancreatic Neuroendocrine Tumors

Neuroendocrine pancreatic tumors (PanNETs) are graded on the basis of their proliferative activity. Cytological samples are commonly the only samples available, but the determination of Ki-67 in cytology and its reliability as a measure of tumor mitotic activity is not well settled. We have retrospectively reviewed all the cases of FNA under EUS control of PanNETs in a 10-year period (2006-2016) in the Hospital Clínico San Carlos (Madrid). We identified 10 PanNET cases with histological correlation. Median age was 49.4 years and the patients were mainly women. PanNETs were located more frequently in the tail of the pancreas, with a median size of 33.8 mm. None of our cases was a grade 3 tumor. The seven grade 1 tumors confirmed in histology had consistent Ki-67 in cytology. In three cases (30 %), there were discrepancies between the Ki-67 index measured in cytology and histology, and the differences ranged from 2 to 15 %; all these cases were grade 2 tumors in histology and were graded as grade 1 tumors in FNA material. Our results are consistent with previous studies which showed understaging when tumor grade was assessed in cytological samples, mainly in G2 tumors. Previous literature has shown that Ki-67 assessment in EUS-FNA samples is a useful tool to rule out G3 tumors, but can be problematic for distinguishing G1 and G2 tumors.