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Pilocytic Astrocytoma (PA)

Last Updated: March 27, 2020

Open Table of Contents: Pilocytic Astrocytoma (PA)

Figure 1: This posterior fossa pilocytic astrocytoma (PA) has a large cyst with an adjacent mass (nodule). The axial image (left) shows that this patient's cytic tumor is compressing and deviating the fourth ventricle with resulting hydrocephalus. Sagittal postcontrast T1WI image (right) shows the sometimes very heterogeneous enhancement apparent in the solid portion of the lesion that belies its underlying low-grade histology.

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

Basic Description

  • 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 PAs associated with NF-1

Clinical Features

  • Patients ages 5-15 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, rarely adjuvant chemoradiation is 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 may 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 may be completely solid, particularly in older patients
  • CT

    • Mixed solid-cystic mass with minimal surrounding edema
      • Solid component iso- to hypodense to gray matter
    • May have calcification; hemorrhage uncommon
    • Strong mural nodular enhancement on contrast-enhanced CT
    • Nonenhancing cystic component, but may show cyst wall enhancement

      • Contrast accumulation in cystic component on delayed images
  • MRI

    • T1WI
      • Cyst: iso- to hyperintense to CSF, but hypointense to parenchyma
      • Solid: iso- 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; may show optic pathway enhancement
    • MR spectroscopy (MRS): aggressive features; elevated choline and lactate, decreased NAA

Imaging Recommendations

  • 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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Hwang JH, et al. Proton MR spectroscopic characteristics of pediatric pilocytic astrocytomas. AJNR Am J Neuroradiol. 1998;19:535-540.

Koeller KK, Rushing EJ. From the archives of the AFIP: pilocytic astrocytoma: radiologic-pathologic correlation. Radiographics. 2004;24: 1693-1708.

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