Prognostic Markers in Meningioma: Effectiveness of FISH‐Assessed CDKN2A/B Homozygous Deletion and Limits of Surrogate p16/MTAP Immunohistochemistry in Predicting Disease‐Specific Survival

AIMS: Homozygous deletion (HD) of CDKN2A/B represents an adverse prognostic biomarker in meningiomas and a diagnostic criterion for CNS WHO grade 3 assignment. However, fluorescence in situ hybridisation (FISH) thresholds and the role of immunohistochemistry (IHC) surrogates remain uncertain. This study evaluated multiple CDKN2A/B FISH cutoffs and assessed the diagnostic and prognostic performance of MTAP and p16 IHC. METHODS AND RESULTS: Ninety-two meningiomas (CNS WHO grades 1-3) were analysed by FISH evaluating ≥ 10%, ≥ 20% and ≥ 30% CDKN2A/B HD thresholds. CDKN2A/B HD was identified in 18.5%, 6.5% and 4.3% of cases using the respective cutoffs and was significantly associated with shorter disease-specific survival across all thresholds (HD ≥ 10% p = 0.001; HD ≥ 20% p = 0.0001; HD ≥ 30% p = 0.043). Patients with HD ≥ 20% and ≥ 30% experienced universal disease-specific mortality within a median survival time of approximately 2 years. MTAP and p16 IHC were performed in matched areas. MTAP IHC showed high specificity (90%-92%) but limited sensitivity (30%-50%), while p16 IHC displayed variable sensitivity (41%-75%) and lower specificity (63%). Combined MTAP/p16 negativity improved sensitivity (up to 100%) but reduced specificity (57%). Neither MTAP nor p16 loss correlated significantly with survival. Spatially heterogeneous IHC patterns corresponded to regional variability in CDKN2A/B deletion by FISH. CONCLUSIONS: CDKN2A/B HD detected by FISH, particularly using thresholds equal or greater than 20%, is strongly associated with poor outcome in meningiomas. MTAP and p16 IHC, alone or combined, lack sufficient accuracy as independent surrogates but may guide tissue selection for molecular testing in heterogeneous tumours.