Answer: Pilocytic astrocytoma

Histology: A biphasic appearance is evident with more cellular areas alternating with myxoid microcysts. In the more cellular areas, the cells appear to have long processes in a densely fibrillar matrix. In areas there are densely eosinophilic hyaline globular bodies as well as densely eosinophilic rod-like structures, the latter termed “Rosenthal fibers”.

Discussion: The histological findings seen in this case are classic for pilocytic astrocytoma, and do not closely resemble any other entity. The lesion is too cellular for reactive gliosis, although one should recognize that Rosenthal fibers may also be seen in longstanding reactive gliosis adjacent to benign lesions. Pilocytic astrocytomas may show cytologic atypia and vascular proliferation, although it is not seen in the current case, and should not be over diagnosed as an anaplastic astrocytoma. Subependymal giant cell astrocytomas are composed of huge cells with abundant eosinophilic glassy or hyaline cytoplasm, resembling ganglion cells.