Answer: Pilocytic astrocytoma, WHO grade I of IV

Histology: This lesion is predominantly solid and composed of compact fibrillary tissue formed by elongated cells with hair-like (piloid) processes. There are abundant Rosenthal fibers, which are bright red, hyaline, corkscrew shaped fibers. In addition, there are coarse aggregates of hyaline droplets, known as eosinophilic granular bodies. These are the classic features of a pilocytic astrocytoma.

Discussion: Diffuse astrocytoma would lack the piloid cytology, Rosenthal fibers, and eosinophilic granular bodies of a pilocytic astrocytoma. Diffuse astrocytomas lack the circumscription and contrast enhancement which are the radiologic hallmarks of a pilocytic astrocytoma. Ependymoma would be characterized by more epithelioid neoplastic cells forming perivascular pseudorosettes. Piloid gliosis is typically seen at the periphery of a slow-growing lesion, such as an ependymoma, hemangioblastoma or cyst wall. Knowledge that the biopsy comes from the center of a solid mass, not its periphery or the wall of a cyst, helps assure the diagnosis of pilocytic astrocytoma.