High-Grade Astrocytoma With Piloid Features: Case Series and Review of a Recently Described Brain Tumor Entity

HGAP is an important consideration in the differential diagnosis of isocitrate dehydrogenase-wild-type gliomas and is more frequently encountered in adults. A handful of studies published following the entity's codification in the 2021 WHO Blue Book have refined our understanding of its clinical, histologic, and hallmark molecular characteristics. The most substantial updates include the description of 3 provisional subtypes, further characterization of an association with neurofibromatosis 1 syndrome, identification of new rare molecular alterations, and documentation of a unique case of possible transformation of pilocytic astrocytoma into HGAP. Clues to the diagnosis of HGAP include histologic infiltrating glioma with moderate pleomorphism, posterior fossa location, CDKN2A/B (cyclin dependent kinase inhibitor 2A/B) deletion, MAPK (mitogen-activated protein kinase) pathway alterations, ATRX (alpha thalassemia/mental retardation syndrome X-linked) loss, and association with neurofibromatosis 1 syndrome in some cases; these findings should prompt further molecular testing, including genome-wide DNA methylation analysis, which is currently essential for diagnosis.