Answer: Pilocytic astrocytoma

Histology: A proliferation of bipolar and stellate cells in a mucopolysacarrhide matrix. Scattered microcysts are present. While cells orient to vessels, well-formed perivascular pseudorosettes are not present. No zone of infiltration is present.

Discussion: The differential diagnosis of intramedullary lesions includes 1) ependymoma (by far the most common such tumor in adults), 2) astrocytomas of diffuse and pilocytic type, 3) hemangioblastoma, 4) schwannoma and melanotic schwannoma (rare in the substance of the cord), and 5) rare melanocytic lesions (melanocytoma and melanoma).

The absence of perivascular pseudorosettes and the presence of the myxoid background help exclude ependymoma, a lesion that out of the filum terminale is not myxoid. Unlike diffuse, or infiltrating, astrocytoma, there is no preexisting CNS parenchyma.

The lesion is thus pilocytic astrocytoma, albeit one without Rosenthal fibers. The lesion has some features of a variant of pilocytic astrocytoma known as “pilomyxoid astrocytoma”. This lesion, that usually occurs in the first few years of life, is more monomorphous and myxoid than the classic pilocytic neoplasm. Rosenthal fibers are sparse if present at all.