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Low-Grade Diffuse Astrocytoma


KEY FACTS

Terminology

  • Well-differentiated but infiltrating neoplasm, slow growth pattern

  • Primary brain tumor of astrocytic origin with intrinsic tendency for malignant progression, degeneration into anaplastic astrocytoma (AA)

Imaging

  • Focal or diffuse nonenhancing white matter mass

  • T2 homogeneously hyperintense mass

  • May expand adjacent cortex

  • Usually no enhancement

    • Enhancement suggests progression to higher grade

  • MRS: High choline, low NAA typical but not specific

  • Perfusion: Relatively lower rCBV compared with AA

  • Cerebral hemispheres most common location

    • Supratentorial 2/3: Frontal and temporal lobes

  • Infratentorial 1/3: Brainstem (50% of brainstem “gliomas” are low-grade astrocytoma)

  • Low-grade astrocytoma may be indistinguishable from other tumors

  • May appear circumscribed on imaging, but tumor cells are often found beyond imaged signal abnormality

Top Differential Diagnoses

  • Other neoplasms: AA, oligodendroglioma

  • Nonneoplastic mimics: Ischemia, cerebritis

Pathology

  • WHO grade II

  • If IDH-1(-) (nonmutated), will behave clinically like AA, glioblastoma

Clinical Issues

  • Seizure is most common presenting feature

  • Majority occur between ages of 20-45 years, mean: 34 years

  • Median survival: 6-10 years

  • Increased survival: Young age, gross total resection

  • More favorable prognosis: IDH1 (+), ATRX (+), MGMT (+)

Coronal graphic shows an infiltrative mass centered in the white matter expanding the left temporal lobe. Axial insert shows mild mass effect upon the midbrain. Low-grade astrocytomas typically affect young adults.

Axial T2 MR in a 34-year-old man with seizures shows a hyperintense infiltrative mass
centered in the right temporal lobe white matter with extension to the overlying cortex. No enhancement was present on contrast-enhanced images. WHO grade II astrocytoma was diagnosed at resection.

Axial FLAIR MR shows a relatively homogeneous hyperintense mass with mild local mass effect, typical of a WHO grade II diffuse astrocytoma. These infiltrative tumors may be focal or diffuse. Fibrillary astrocytoma is the most frequent histologic variant.

Axial MR perfusion in the same patient shows a low relative cerebral blood volume
in the frontotemporal mass, suggesting a low-grade tumor. Perfusion MR has been shown to be helpful in preoperative tumor grading, predicting survival, and guiding biopsy.

TERMINOLOGY

Synonyms

  • Diffuse astrocytoma, grade II astrocytoma, fibrillary astrocytoma, low-grade astrocytoma (LGA)

Definitions

  • Primary brain tumor of astrocytic origin with intrinsic tendency for malignant progression, degeneration into anaplastic astrocytoma (AA)

  • Well-differentiated but infiltrating neoplasm, slow growth pattern

IMAGING

General Features

  • Best diagnostic clue

    • Focal or diffuse nonenhancing white matter (WM) mass

  • Location

    • Cerebral hemispheres, supratentorial 2/3

      • –

        Frontal lobes 1/3, temporal lobes 1/3

      • –

        Relative sparing of occipital lobes

      • –

        Tumors of white matter may extend into cortex

    • Infratentorial 1/3

      • –

        Brainstem (50% of brainstem “gliomas” are low-grade astrocytoma)

      • –

        Occur in pons and medulla of children/adolescents

      • –

        Cerebellum is uncommon location

    • 20% involve deep gray matter structures: Thalamus and basal ganglia

    • Less commonly occur in spinal cord

  • Size

    • Variable

  • Morphology

    • Homogeneous mass with enlargement and distortion of affected structures

    • May appear circumscribed on imaging, but tumor cells are often found beyond imaged signal abnormality

CT Findings

  • NECT

    • Ill-defined homogeneous hypodense/isodense mass

    • 20% Ca++; cysts are rare

    • Calvarial erosion in cortical masses (rare)

  • CECT

    • No enhancement or very minimal

      • –

        Enhancement should raise suspicion of focal malignant degeneration

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