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Pilocytic Astrocytoma (PCA)

Summary

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  • Pilocytic astrocytoma is a slow-growing, WHO grade 1 glial tumour typically affecting children and young adults
  • Characterised by biphasic histology with compact bipolar cells and loose-textured multipolar cells
  • Imaging features include cystic components with an enhancing mural nodule, most commonly in the cerebellum1

Pathophysiology

  • Arises from astrocytes, a type of glial cell in the central nervous system
  • Typically associated with alterations in the MAPK pathway
    • BRAF-KIAA1549 fusion gene is the most common genetic alteration (found in 60-80% of cases)
    • BRAF V600E mutation occurs in a minority of cases
  • Slow-growing tumour with limited infiltrative potential
  • Often well-circumscribed with a tendency to form cysts

Demographics

  • Most common primary brain tumour in children
  • Peak incidence between 5-15 years of age
  • Slight male predominance (M:F ratio 1.2:1)
  • Accounts for approximately 15% of all brain tumours in children
  • Can occur in adults but less common

Diagnosis

  • Clinical presentation varies depending on tumour location:
    • Cerebellar lesions: ataxia, headache, nausea, vomiting
    • Optic pathway lesions: visual disturbances, proptosis
    • Brainstem lesions: cranial nerve palsies, long tract signs
  • Histopathology:
    • Biphasic pattern with compact bipolar cells (piloid) and loose-textured multipolar cells
    • Rosenthal fibres and eosinophilic granular bodies often present
    • Low mitotic activity and absence of necrosis
  • Immunohistochemistry:
    • GFAP positive
    • Ki-67 proliferation index typically low (<1%)

Imaging

  • CT:
    • Well-circumscribed, hypodense cystic mass with an isodense to hyperdense mural nodule
    • Calcification uncommon (10-20% of cases)
  • MRI:
    • T1: cystic component hypointense, solid component isointense to grey matter
    • T2/FLAIR: cystic component hyperintense, solid component heterogeneous but predominantly hyperintense
    • T1 post-contrast: intense enhancement of solid component and cyst wall
    • DWI: typically no restricted diffusion
    • MR spectroscopy: elevated choline, decreased NAA, absence of lipid/lactate peak
  • Common locations:
    • Cerebellum (60%)
    • Optic pathway/hypothalamus (20%)
    • Brainstem (10%)
    • Cerebral hemispheres (10%)

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  • 50-year-old patient presented with neck stiffness and imbalance.
  • MRI showed a solid-cystic cerebellar lesion with heterogeneous enhancement.

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  • 18-year-old patient presented after a seizure.
  • MRI showed a solid-cystic lesion with no diffusion restriction or haemorrhage in the right mesial temporal lobe.

Treatment

  • Complete resection is usually curative; MAPK-pathway inhibitors (BRAF/MEK) for unresectable or recurrent disease. The cyst-with-enhancing-mural-nodule in the cerebellum is the classic appearance

Differential diagnosis

Differential Diagnosis Distinguishing Feature
Ependymoma Typically intraventricular; PCA is usually extra-ventricular
Medulloblastoma Typically in cerebellar vermis; PCA more often in cerebellar hemispheres
Ganglioglioma Often has calcifications; PCA rarely calcifies
Hemangioblastoma Smaller enhancing mural nodule with angiographic blush; cyst wall does not enhance; associated with von Hippel-Lindau in younger patients
Low-grade glioma Typically infiltrative without cystic component; no discrete mural nodule; T2/FLAIR mismatch sign
Dysembryoplastic neuroepithelial tumour (DNET) Cortical location; PCA is typically deep-seated
Pleomorphic xanthoastrocytoma Superficial cerebral location; PCA is often in cerebellum or optic pathway
Oligodendroglioma Typically in cerebral hemispheres; PCA rare in this location
Abscess Ring-enhancing with restricted diffusion within the cavity; surrounding oedema; no mural nodule

  1. Salles et al. Pilocytic Astrocytoma: A Review of General, Clinical, and Molecular Characteristics. 2020. Journal of child neurology - Open in new tab. ↩