= uncommon form of slowly growing glioma presenting with large size at time of diagnosis

Incidence: 0.3÷100,000 annually; 3^rd most common glioma; 2–5% of all primary brain tumors; 5–25% of all glial neoplasms; <1% of pediatric CNS neoplasms

Path:

1. well-differentiated oligodendroglioma
2. anaplastic oligodendroglioma (20–54%)
3. “mixed glioma” containing neoplastic astrocytes (9%)

Histo: monotonous “fried-egg” appearance = uniformly rounded hyperchromatic nuclei surrounded by perinuclear halo of clear cytoplasm; “chicken-wire” pattern = delicate branching network of capillaries; little mitotic activity

Genetics: deletion of alleles at chromosome loci 19q (50–80%) and 1p (40–92%) in well-differentiated tumors

Median age: 35–45 years; M÷F = 2÷1; adult÷child = 8÷1, 6% in childhood (2^nd peak at 6–12 years)

• Older age is associated with more aggressive tumor behavior!

• long clinical presentation >5 years
• seizures (35–85%), headache, mental status change
• paralysis (50%), visual loss (49%), papilledema (47%)
  • Worse prognosis with neurologic deficit!
• ataxia (39%), abnormal reflexes (37%), meningismus (10%)

Location:

1. supratentorial: frontal lobe (50–65%); temporal lobe (47%); parietal lobe (7–20%); occipital lobe (1–4%)
2. infratentorial: cerebellum (3%); brainstem & spinal cord (1%)
3. others: leptomeninges (“oligodendrogliomatosis”); cerebellopontine angle; cerebral ventricles (3–8%) = “subependymal oligodendroglioma”; retina, optic n.

Site: mostly involving cortical gray matter and subcortical white matter; occasionally through corpus callosum as “butterfly glioma”

• round / oval usually sharply marginated mass
• large nodular clumps of calcifications (in 45% on plain film; in 20–91% on CT)

CT:

• hypodense (60%) / isodense (23%) / hyperdense (6%) mass
  • May NOT AT ALL be detectable by CT!
• cystic degeneration (frequent)
• hemorrhage (uncommon)
• ± erosion of inner table of skull (exophytic growth)

CECT:

• may be adherent to dura (mimicking meningiomas)
• surrounding vasogenic edema (in 50% of low-grade, in 80% of high-grade tumors)
• subtle ill-defined enhancement (15–20%) associated with high-grade tumor

MR:

• well-circumscribed tumor of heterogeneous intensity:
  • hypo- / isointense compared to gray matter on T1WI
  • hyperintense compared to gray matter on T2WI
  • hyperintense signal on DWI + low ADC values in solid tumor portion of high-grade tumors ← restricted water diffusion + lowered extracellular hyaluronic acid
• little edema / mass effect (common)
• solid / mixed moderate peripheral enhancement:
  • moderate to strong for high-grade glioma
  • minimal / none for low-grade glioma
  
  N.B.: lack of enhancement does not equal low-grade
• calcification may not be detected

SPECT (²⁰¹Tl) & PET (¹¹C-L–methylmethionine):

• metabolic rate correlates with histologic grade
• detects hypermetabolic regions within tumor

Prognosticators of malignant behavior:

• increasing age of patient
• mass effect, cyst formation, necrosis
• moderate to strong enhancement ← disruption of blood-brain barrier
  however: up to 25% of high-grade gliomas show faint / no enhancement
• ↓ADC values in non-enhancing solid tumor portion of high-grade tumor
• ↑rCBV on perfusion MRI in solid portions + peritumoral region ← tumor neoangiogenesis

Cx: leptomeningeal seeding via CSF (1–15%)

Rx: gross total resection; PCV chemotherapy (procarbazine, lomustine, vincristine); irradiation reserved for chemotherapy failure

Prognosis: 46% 10-year survival rate with low-grade; 20% 10-year survival rate with high-grade; 3–17 years median postop survival

DDx:

1. Astrocytoma (no large calcifications)
2. Ganglioglioma (in temporal lobes + deep cerebral tissues
3. Ependymoma (enhancing tumor, often with internal bleeding producing fluid levels)
4. Glioblastoma (infiltrating, enhancing, edema, NO calcifications)
5. Central neurocytoma