High-Grade Astrocytoma With Piloid Features.

Abstract

Context.—: High-grade astrocytoma with piloid features (HGAP) is a newly recognized glioma defined by its methylation profile. Understanding of its clinical, histologic, and molecular characteristics continues to evolve.

Objective.—: To review the HGAP literature, emphasizing updates in our understanding of the entity since its codification in the 2021 World Health Organization (WHO) Blue Book. Additionally, to present a case series illustrating a single institutional experience with HGAP.

Data sources.—: The English-language HGAP literature from 2018 to 2024 was reviewed. Four cases of HGAP were reviewed, along with relevant medical records.

Conclusions.—: 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.