Pilocytic Astrocytoma

KEY FACTS

TERMINOLOGY

• Pilocytic astrocytoma (PA): Well-circumscribed, slowgrowing tumor, often with cyst and mural nodule

IMAGING

• Cystic cerebellar mass with enhancing mural nodule

○ Arises from cerebellar hemisphere and compresses 4th

ventricle

• Enlarged optic nerve/chiasm/tract with variable

enhancement

• Cerebellum (60%) > optic nerve/chiasm (25-30%) > adjacent

to 3rd ventricle > brainstem

• Well circumscribed with little to no edema

• Aggressive appearance (enhancement and MRS) of tumor

is misleading

TOP DIFFERENTIAL DIAGNOSES

• Medulloblastoma (primitive neuroectodermal tumor)

• Ependymoma

• Ganglioglioma

• Hemangioblastoma

• Demyelination

PATHOLOGY

• WHO grade I

• 15% of neurofibromatosis type 1 (NF1) patients develop

PAs, most commonly in optic pathway

• Up to 1/3 of patients with optic PAs have NF1

• Most common primary brain tumor in children

CLINICAL ISSUES

• Clinical presentation varies with location

○ Headache, nausea, vomiting most common

○ Visual loss (optic pathway lesions)

○ Ataxia, cerebellar signs (cerebellar lesions)

• Peak incidence: 5-15 years of age

• Slowly growing tumor with very good prognosis

• Median survival rates at 20 years > 70%

(Left) Axial graphic shows the

characteristic cyst with mural

nodule appearance of a

posterior fossa pilocytic

astrocytoma (PA). These WHO

grade I tumors most

commonly arise in the

cerebellar hemispheres and

compress the 4th ventricle.

(Right) Axial FLAIR MR shows

a classic cyst with mural

nodule appearance ﬈ of a

cerebellar PA in a child. Note

the typical lack of surrounding

edema in the adjacent

cerebellum. Mass effect on the

4th ventricle ſt with

associated hydrocephalus is

common.