Alternative lengthening of telomeres in small well-differentiated pancreatic neuroendocrine tumors: independent validation and cross-platform concordance.

Well-differentiated pancreatic neuroendocrine tumors measuring ≤ 2 cm (small PanNETs) are usually non-functional and most are discovered incidentally. While most are indolent, some behave aggressively and metastasize. Alternative lengthening of telomeres (ALT) has shown promise as a predictive biomarker of recurrence in recent studies. We designed a multi-institutional study to validate the predictive ability of ALT in small PanNET to inform patient selection for operative versus non-operative management. We identified 172 cases of small PanNET diagnosed from 2001 to 2021 in four different institutions. ALT fluorescence in situ hybridization (FISH) could be performed in 133 cases and was compared with clinical outcomes. Among those with FISH, 44.4% were females with a mean age of 65.4 years (range 31-88). Twenty-one cases (15.8%) were ALT-positive. Over a median follow-up of 48 months, a total of 19 progressions (metastasis, recurrence, increase in size, or death) were recorded. ALT positivity was significantly associated with worse progression-free survival [PFS]. In the ALT-positive group, a total of 9 cases showed progression compared to 10 in the ALT-negative group (5-year PFS: 70.2% vs. 88.8%, hazard ratio: 4.69, p = 0.002). In a separate analysis using matched cases, ALT chromogenic in situ hybridization (CISH), demonstrated high concordance with ALT FISH (95.5% for positive and 91.7% for negative cases) enabling cross-platform validation. Unlike FISH, CISH can be interpreted using standard bright field microscopy, facilitating broader implementation. These findings support further evaluation of ALT testing, particularly via CISH, as a potentially scalable and accessible biomarker, while recognizing that formal analytic validation and confirmation in unselected cohorts are required before routine clinical implementation.