Reclassification of pineal tumor as high-grade astrocytoma with piloid features through methylation profiling: illustrative case.

backgroundThe recent WHO 2021 CNS classification system defines new tumor types and subtypes based on molecular features. Among them, high-grade astrocytoma with piloid features (HGAP) is a glioma subtype whose classification depends on its DNA methylation profiling signature. These tumors can have a varied histological appearance and are often misclassified. OBSERVATIONS: A 41-year-old male presented with right-sided weakness and headache. MRI detected a heterogeneously enhancing mass in the pineal region. The patient underwent an endoscopic third ventriculostomy, and a biopsy sample was obtained. The pathology was initially consistent with a high-grade neuroepithelial tumor. Repeat imaging 3 weeks postoperatively demonstrated an interval increase in tumor size. He underwent a suboccipital craniectomy for gross-total resection. DNA methylation profiling was performed and was consistent with the diagnosis of HGAP. The patient was given chemoradiotherapy due to the high-grade pathology. LESSONS: Integration of molecular pathology has led to crucial and clinically relevant changes in CNS tumor classification. Utilizing these advanced diagnostic techniques to classify tumors could lead to changes in management, targeted therapeutics, and enrollment in clinical trials. https://thejns.org/doi/10.3171/CASE24778.