Copy‐Number Alterations in Central Nervous System Tumours: Diagnostic and Prognostic Implications

Copy-number alterations (CNAs) are frequent molecular events in central nervous system (CNS) tumours ranging from whole-chromosome gains or losses to focal alterations involving individual genes or regulatory elements. Beyond their biological roles in tumour initiation and progression, distinct CNA patterns have emerged as highly characteristic features of specific CNS tumour types and have been integrated into the WHO classification of CNS tumours as essential diagnostic or grading criteria. Other CNAs provide independent prognostic or predictive information that may inform clinical management. This review provides a comprehensive, practice-oriented overview of recurrent CNAs across the spectrum of CNS tumours, with emphasis on their diagnostic interpretation in neuropathology. CNAs are classified into a three-tiered framework: (1) diagnostic CNAs, which define tumour type, molecular subgroup, or CNS WHO grade; (2) prognostic CNAs, which provide independent risk stratification or treatment-predictive value without directly determining classification or grade; and (3) supportive CNAs, which represent recurrent alterations that may provide important diagnostic clues and help contextualise CNA profiles, but are not yet sufficiently specific or clinically validated for formal diagnostic, grading, or prognostic application. By integrating this concept from molecular neuropathology, this review aims to provide a clinically relevant reference for the interpretation of CNA profiles in neuropathological diagnostics.