Insulinoma-associated protein 1 expression in pancreatic neuroendocrine tumours in endoscopic ultrasound-guided fine-needle aspiration cytology: An analysis of 14 patients

Background Insulinoma-associated protein 1 (INSM1) has been reported to be a useful marker for diagnosing pancreatic neuroendocrine tumours (PNETs). However, INSM1 expression in endoscopic ultrasound-guided fine needle aspiration cytology has not been examined. We evaluated INSM1 expression in the cytology of cases diagnosed with PNETs. Methods We immunocytochemically stained INSM1 and Ki-67 in 14 PNET cases, and according to the 2017 World Health Organisation criteria, seven PNET Grade 1 cases, four Grade 2 cases and three Grade 3/neuroendocrine carcinoma cases were identified. As a control for INSM1 and Ki-67 expression, we used cytological specimens from 15 cases of pancreatic ductal adenocarcinoma. Results In PNET cases, INSM1-expressing tumour cells were identified in all cytological specimens of PNET. The median INSM1 expression rate in Grade 1 cases was 49.8% (mean +/- standard deviation: 55.1 +/- 12.5%, min: 39.3%, max: 74.1%), and in Grade 2 and Grade 3/neuroendocrine carcinoma cases was 81.1% (mean +/- standard deviation: 77.6 +/- 18.6%, min: 50.3%, max: 100%). However, there was no correlation between INSM1 and Ki-67 expression (r = -0.15). The median expression rate in PNET cases was 64.3%, which was significantly higher than that in pancreatic ductal adenocarcinoma cases (P < 0.0001). Conclusion INSM1 immunocytochemistry of cytological specimens obtained from endoscopic ultrasound-guided fine needle aspiration cytology can accurately diagnose PNETs; therefore, INSM1 could be an important diagnostic tool in assessing therapeutic strategies, including molecular-targeted therapy.