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Pathology

The vast majority of NF1-associated OPGs are classified by the World Health Organization (WHO) as grade I astrocytic neoplasms (pilocytic astrocytomas). Similar to pilocytic astrocytomas arising in other brain regions in individuals with NF1, these low-grade gliomas are characterized by a biphasic histologic pattern of more cellular areas alternating with looser cystic regions.[10] Within the less compact areas, there are Rosenthal fibers (tapered corkscrew-shaped hyaline masses) and eosinophilic granular bodies (globular aggregates). These tumors exhibit low mitotic indices with rare mitoses and occasional hyperchromatic nuclei. Despite their benign nature, pilocytic astrocytomas are rather infiltrative tumors with significant microvascular proliferation and the presence of microglia. Immunohistochemical analyses of these tumors reveal robust staining with glial fibrillary acidic protein antibodies, characteristic of astrocytic neoplasms.