Assessment of ARX expression, a novel biomarker for metastatic risk in pancreatic neuroendocrine tumors, in endoscopic ultrasound fine‐needle aspiration

BACKGROUND: The transcription factors ARX and PDX1, and alternative lengthening of telomeres (ALT) were recently described as prognostic markers for resected non‐functional pancreatic neuroendocrine tumors (PanNETs). ALT positive tumors with ARX expression relapse most often. Currently, tumor size is the only preoperative marker used to decide whether or not to operate, thus additional preoperative prognostic markers are needed. Therefore, it is critical to assess the performance of these biomarkers on preoperative cytologic specimens. METHODS: Endoscopic fine‐needle aspiration cellblock material and corresponding surgical specimens of 13 patients with PanNETs were assessed for histology, immunohistochemical staining of ARX, PDX1, Synaptophysin, Ki67, and telomere‐specific fluorescence in situ hybridization to detect ALT, and then associated with clinicopathological features. Scoring for ARX and PDX1 was performed blinded by two independent observers. RESULTS: Of the 13 surgical specimens, 8 were ARX+/PDX1−, 2 ARX−/PDX1+, and 3 ARX+/PDX1+. Concordance between cytologic and surgical specimens for ARX protein expression was 100%, whereas concordance for PDX1, ALT, and WHO tumor grade was 85%, 91%, and 73%, respectively. There was a perfect inter‐observer agreement in ARX and PDX1 scoring. CONCLUSION: ARX can reliably be determined in cytologic specimens and has low inter‐observer variability. For cytology, false‐positive PDX1 expression was observed, possibly due to contamination or sampling, while ALT had a false‐negative case due to incomplete sampling. As previously observed, tumor grade is underestimated in cytologic specimens. Thus, ARX and ALT are the most promising markers to predict metastatic behavior in PanNETs, thereby warranting further validation in larger studies.