CDKN2A/B status can be assessed by p16/MTAP immunohistochemistry as a practical surrogate in pleomorphic xanthoastrocytoma and high-grade astrocytoma with piloid features.

The presence of a CDKN2A homozygous deletion (HD) plays an important role in the diagnostic approach of neuropathologists to several central nervous system (CNS) tumors. CDKN2A is located on chromosome 9p21, next to CDKN2B and MTAP, and encodes for the protein p16. CDKN2A/B HD constitutes a frequent alteration in some circumscribed gliomas such as high-grade astrocytomas with piloid features (HGAP) and pleomorphic xanthoastrocytomas (PXA). The diagnosis of these tumor types remains complex and having a rapid and easy tool to suggest these diagnoses would be useful. Herein, we studied a cohort of 42 HGAP and PXA to analyze the concordance of CDKN2A/B and MTAP status between 3 different molecular techniques (FISH, DNA-methylation profiling providing copy number variation analysis, and SNP array), and compared the results to those of p16 and MTAP immunohistochemistry. HD was present in 11/12 HGAP and 28/30 PXA, however, with a wide variety of deletion size, including of CDKN2A, CDKN2B, and MTAP genes. There was excellent concordance between p16/MTAP immunoexpression and the genetic detection of HD, demonstrating 100% sensitivity and 100% specificity. In conclusion, combined p16/MTAP IHC presented a positive predictive value for detecting HD and may assist neuropathologists in their diagnostic approach to circumscribed glial/glioneuronal tumors.