Prevalence of CD99 protein expression in pancreatic endocrine tumours (PETs)

Aims : To determine the prevalence of CD99 expression in pancreatic endocrine tumours (PETs). We evaluated CD99 expression and analysed Ki67 labelling by immunohistochemistry in PETs. Methods and results : Thirty‐eight PETs from 33 patients were analysed. CD99 immunoreactivity was consistently observed in normal islets of the pancreas, regardless of the cell type. Tumours comprising more than 30% CD99+ cells were defined as positively immunoreactive for CD99. CD99 expression was observed in 20 of the 38 PETs examined, but not in any of the pancreatic tumours of other histological subtypes (10 ductal adenocarcinomas, five intraductal papillary‐mucinous tumours, and two acinar cell tumours). Loss of CD99 expression was related to markers of worse prognosis for PET, including gross local invasion, metastasis to the lymph nodes or other organs, lymphatic or blood vessel invasion, and neuroendocrine carcinoma (NEC). Thus, CD99 expression may have an efficiency comparable to that of high Ki67 labelling index (5% or more) for prognostication. Conslusions : CD99 expression was observed frequently and exclusively in PETs, and loss of CD99 expression in PETs was found to be associated with ominous prognostic indicators.