Integrated assessment of malignancy in <i>IDH</i>‐mutant astrocytoma with p16 and methylthioadenosine phosphorylase immunohistochemistry

In the fifth edition of the World Health Organization's (WHO) classification of tumors of the central nervous system (CNS), molecular analysis is required for not only determining each tumor type but assessing its prognosis based on malignancy (CNS WHO grade). A notable example is the loss of tumor suppressor gene cyclin-dependent kinase inhibitor 2A (CDKN2A), and CDKN2A homozygous deletion (HD) is a novel CNS WHO grade 4 marker in isocitrate dehydrogenase gene (IDH)-mutant astrocytoma. However, incorporating molecular workup into the "routine diagnostics" of each brain tumor type remains a major challenge, especially in resource-limited settings, including low- and middle-income countries. We herein validated the usefulness of p16 and methylthioadenosine phosphorylase (MTAP) immunohistochemistry (IHC) as potential surrogates for the assessment of CDKN2A status in 20 IDH-mutant astrocytoma cases. Of note, loss or retention of p16 and MTAP could accurately predict CDKN2A HD (p16: 87.5%, MTAP: 88.9%) or non-HD (p16: 100%, MTAP: 100%) with a single marker alone. Importantly, we revealed contributing factors to gray-zone IHC results (p16: 5-20%, MTAP: mosaic), including (1) hemizygous deletion of CDKN2A, (2) degenerative findings, and (3) intratumoral CDKN2A HD heterogeneity, the detailed histologic and molecular assessment of which would be a key to achieving integrated assessment of malignancy in IDH-mutant astrocytoma. We characterized the pitfalls of each method and provided for the first time a practical flowchart of astrocytoma grading, contributing to a normalization of WHO2021-based molecular diagnostics in resource-limited settings.