High-grade astrocytoma with piloid features in the conus medullaris: a rare presentation of a new World Health Organization diagnosis. Illustrative case.

backgroundNeuroepithelial tumors with histological features of pilocytic astrocytoma (PA), increased mitotic activity with high-grade features of microvascular proliferation, and palisading necrosis have often been designated as anaplastic PAs. High-grade astrocytoma with piloid features (HGAP) was recently classified as its own entity by the World Health Organization (WHO) in 2021, after being grouped under anaplastic PA. This rare tumor typically arises in the cerebellum. The authors present the case of a patient with HGAP in the conus medullaris, a rare occurrence, with only 17 reported HGAP cases in other parts of the spinal cord. OBSERVATIONS: An 83-year-old female patient presented with progressive lower-extremity weakness over 2 weeks. Subsequent imaging revealed an intramedullary spinal cord mass near the conus. Histopathological analysis confirmed a neuroepithelial tumor. Chromosomal microarray revealed the diagnosis of HGAP, an exceedingly unusual tumor to be found near the conus. The tumor characteristics showed uniformly low cellularity and low proliferation of medium-sized ovoid cells with elongated nuclei, embedded in a loose pilocytic matrix. LESSONS: This report presents the case of a patient with HGAP in the conus medullaris, a new WHO diagnosis that needs to be included in the differential diagnosis for conus masses. https://thejns.org/doi/10.3171/CASE24390.