Fine-Needle Aspiration-Based Grading of Pancreatic Neuroendocrine Neoplasms Using Ki-67: Is Accurate WHO Grading Possible on Cytologic Material?

Introduction: The World Health Organization (WHO) has provided grading guidelines for pancreatic neuroendocrine neoplasms (PanNENs) based on mitotic count and Ki-67 proliferation index. Due to the desire to provide earlier tumor grading for clinical management decisions, some groups have proposed grading PanNENs at the time of fine needle aspiration (FNA) using Ki-67 proliferation rates. While a Ki-67 can be performed on FNA cell blocks, there are potential sampling limitations with this technique that may affect the reliability of the Ki-67 result.

Materials and Methods: Forty-nine PanNENs with FNA cell blocks and corresponding resection material were evaluated by immunohistochemistry for expression of Ki-67. Ki-67 proliferation rate was calculated based on cell counts of >500 cells in the highest staining areas. Ki-67 scores from FNA cell blocks were correlated with Ki-67 scores from resection specimens.

Results: The FNA Ki-67 proliferation rates overall did not correlate well with the resection specimen. A linear regression analysis of the correlation between FNA %Ki-67 and resection %Ki-67 showed a slope of 3.2 and an R2 =0.58. The average difference in Ki-67 proliferation rate between FNA and resection was 5.9%. Thirty-nine percent (19/49 cases) of PanNENs showed discordant grading between the FNA cell block and resection specimen. Almost all (18/19) discordant cases demonstrated a lower FNA-based grade as compared to the resection grade.

Conclusions: FNA cell block grading using Ki-67 frequently led to under-grading of the tumor. This finding is consistent with concerns that FNA may not provide accurate grading due to the limited sampling of the tumor.