2026 American Academy of Neurology Abstract Website

Objective:

We present an unusual case of high-grade astrocytoma with piloid features (HGAP) which presented 14 years prior to diagnosis as a cystic mass adjacent to the fourth ventricle.

Background:

HGAP is a newly recognized, aggressive tumor entity with a predilection for the posterior fossa with a distinct methylation pattern and often harboring alterations in CDKN2A/B, NF1, BRAF, FGFR1, and ATRX.

Design/Methods:

Natural history of an unusual case of HGAP.

Results:

A 48-year-old man presented with gait difficulty and cognitive dysfunction. Imaging identified obstructive hydrocephalus secondary to a non-enhancing lesion involving the fourth ventricle. Patient underwent endoscopic third ventriculostomy with symptomatic improvement. Surveillance imaging over the years demonstrated a waxing and waning enhancement pattern. Eleven years after initial tumor identification, he developed acute onset right facial numbness due to intralesional hemorrhage. Due to tumor growth, resection was performed fourteen years after initial detection of the tumor. Pathology demonstrated a high-grade astrocytoma which harbored FGFR1, NF1, ATRX, and PIK3R1 mutations, and a complex copy number profile including homozygous deletion of CDKN2A/B, suggestive of HGAP. Patient was subsequently treated with radiation therapy with concurrent and adjuvant temozolomide.

Conclusions:

HGAP is typically an aggressive primary brain tumor; however, as this is a newly designated entity, clinical information is limited. This case demonstrates an unusually long prodrome in a lesion that ultimately was diagnosed as HGAP. The radiographic appearance was also atypical, as fluctuating enhancement has not been reported as a characteristic imaging feature of this tumor type. A subset of HGAPs may be associated with a more indolent course than previously described.