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Pilocytic Astrocytoma

Last Updated: October 8, 2020

Figure 1: This posterior fossa PA has a large cyst with an adjacent mass (nodule). (Left) Axial image showing that this patient's cystic tumor is compressing and deviating the fourth ventricle, resulting in hydrocephalus. (Right) Sagittal postcontrast T1-weighted image showing the sometimes very heterogeneous enhancement apparent in the solid portion of the lesion that belies its underlying low-grade histology.

Figure 2: Axial (left) and sagittal (right) postcontrast T1-weighted images demonstrate a cystic lesion with a brightly enhancing mural nodule/mass. This appearance is very typical for juvenile PAs but must be distinguished from the similar-appearing hemangioblastoma when in the posterior fossa, particularly when in older children and young adults.


  • Slow-growing, well-circumscribed tumor arising from astrocytic precursors
  • Most common pediatric primary intracranial neoplasm


  • WHO grade I
  • Biphasic pattern of astrocytes (Rosenthal fibers and multipolar cells)
  • Syndromic or sporadic
    • Optic nerve/chiasm pilocytic astrocytomas (PAs) associated with neurofibromatosis type 1


  • Patients aged 5 to 15 years most common
  • No gender predilection
  • Overall good prognosis
    • 10-year survival, >90%
  • Common presenting signs/symptoms
    • Headaches, nausea, vomiting, ataxia, seizures, visual loss if optic pathways involved
  • Treatment
    • Cerebellar: tumor resection; adjuvant chemoradiation is rarely recommended, as residual tumors rarely grow and may even regress spontaneously
    • Optic pathway: slow-growing tumors often monitored without treatment; tumor debulking, radiation, and/or chemotherapy for symptomatic or rapidly enlarging tumors


  • General
    • Usually well-circumscribed cerebellar hemispheric lesion but can also be supratentorial
      • Mass effect, effacement of fourth ventricle common
    • May arise from optic nerve, optic chiasm, or hypothalamus
    • Minimal surrounding edema
    • Most often cystic mass with mural nodule but can be completely solid, particularly in older patients
  • CT
    • Mixed solid-cystic mass with minimal surrounding edema
      • Solid component isodense to hypodense to gray matter
    • Can have calcification; hemorrhage uncommon
    • Strong mural nodular enhancement on contrast-enhanced CT images
    • Nonenhancing cystic component, but may show cyst wall enhancement
      • Contrast accumulation in cystic component on delayed images
  • MRI
    • T1WI
      • Cyst: isointense to hyperintense to cerebrospinal fluid but hypointense to parenchyma
      • Solid: isointense to hypointense to gray matter
    • T2WI
      • Cyst: hyperintense to brain parenchyma
      • Solid: usually hyperintense to parenchyma
      • Hyperintense optic pathway lesions
    • FLAIR: hyperintense solid and cystic components
    • DWI: typically no diffusion restriction
    • T1WI+C: enhancing solid component ± cyst wall enhancement; can show optic pathway enhancement
    • MRS: aggressive features; elevated choline and lactate, decreased NAA


  • MRI with contrast

For more information, please see the corresponding chapter in Radiopaedia.

Contributor: Rachel Seltman, MD

DOI: https://doi.org/10.18791/nsatlas.v1.03.01.31


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