CNS Atlas
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     Key features:
Clinical: typically occurs in young or middle aged adults (median ~40), rare pediatric cases; 2/3 present with seizures, often long standing, due to cortical involvement.  
Location: vast majority occur in the cerebral hemispheres; cerebellar, brain stem and spinal examples are very rare.
Imaging: Low grade (grade II) cases are typically non-enhancing or heterogenously enhancing, and may be relative well circumscribed; often calcified.
Histopathology: uniform round bland nuclei with perinuclear halos on formalin-fixed sections, perineuronal satellitosis, single cell infiltration of adjacent brain, and 'chicken-wire' capillary vascular network; calcification common; minigemistocytes may be present

Infiltrating glial neoplasm histologically resembling oligodendrocytes. WHO grade II or III.

Differential diagnosis:
Anaplastic astrocytoma
Multiple sclerosis
Central neurocytoma
Dysembryoplastic neuroepithelial tumor

Large deletions, i.e., loss of heterozygosity, on 1p or 19q (LOH 1p/19q) correlates with typical oligodendroglial morphology and prolonged survival.
Approx. 80% of histologically typical oligodendrogliomas are associated with 1p/19q LOH. In this setting, it is unclear whether prolonged survival is related to the natural history of this lesion or to therapeutic sensitivity.
Approximately 70 to 80% of anaplastic oligodendrogliomas are associated with LOH1p/19q. In this setting, LOH1p/19q has been shown to be a marker of response to either radiation or chemotherapy with prolonged survival
Approx. 30% of histologically atypical oligodendrogliomas or mixed oligoastrocytomas are associated with 1p/19q LOH. In this setting, LOH1p/19q has not as of the present time been shown to correlate with either therapeutic responsiveness or prolonged survival
Approx. 5% of astrocytomas, low or high grade, are associted with LOH1p/19q. In this setting, LOH1p/19q has not as of the present time been shown to be associated with either therapeutic responsiveness or prolonged survival
25 to 30% of anaplastic oligodendrogliomas show homozygous deletions of p16. This change correlates inversely with LOH1p/19q and is associated with a worse prognosis
p53 mutations are less common than in low grade astrocytomas, occuring in approx. 10 to 15% of cases. p53 mutations are mutually exclusive with LOH 1p/19q.

Presenting symptoms are similar to those of any mass lesion in the brain, i.e., seizures, symptoms of increased intracranial pressure, or focal neurologic signs related to the location of the tumor.
Seizures are the most common early symptom, related to a tendency for these tumors to involve the cerebral cortex
Anaplastic oligodendrogliomas occur, on average, 7 to 8 years later than grade II tumors

Low grade (grade II) cases are typically non-enhancing or heterogenously enhancing, relative well circumscribed, with minimal surroundinmg edema
Anaplastic (grade III) cases are usually enhancing
80% calcified. Heavy calcification is particularly characteristic
New enhancement may indicated progression

Grade II oligodendrogliomas are characterized by 1) round regular and bland nuclei, 2) high cellularity 3) perineuronal satellitosis and 4) a 'chicken wire' capillary vascular pattern, 4) infiltration
Perinuclear halos are a diagnostically useful artifact associated with delayed fixation. Perinuclear halos are not evident on frozen section, cytology preparations, or in some areas of permanent sections.
Calcification, particularly dense calcification, is more common than in low grade astrocytomas
Minigemistocytes, i.e., gemistocytes with small cell bodies, may be present  
Histologically classic cases are easily diagnosed, but ambiguous cases showing features that overlap with infiltrating astrocytomas are numerous. The rate of interobserver variation among experienced neuropathologists is very high, in the 50% range.

Grading of oligodendrogliomas, in contrast to that of infiltrating astrocytomas, is less standardized. Increased mitotic rates, high Mib-1 proliferative rates, microvascular proliferation and necrosis correlate with shorter survival, and can be used to upgrade tumors to anaplastic (grade III).

Smear preparations at the time of frozen section show a cellular lesion with uniform round bland nuclei and a paucity of glial processes

Clinically useful markers of oligodendroglial differentiation are not currently available
Variable GFAP positivity (minigemistocytes are GFAP positive)
P53 staining less common than in infiltrating astrocytomas
Mib-1 proliferative rates tend to be low (approx. 3 to 8%) relative to the high cellularity of these tumors.
Mib-1 proliferative rate in anaplastic cases may be very high, up to 80 ot 90%

Variable. Average survival for grade II tumors approx. 10 years. 30 to 40 year survivals have been rported in a few cases.
1p/19q deletion correlates with classic oligodendroglioma histology and is often associated with an indolent and protracted clinical course. In the setting of classic oligodendroglioma, it is unclear whether LOH1p/19q is marker of therapeutic sensitivity or a marker of less aggressive natural history.
1p/19qLOH is a marker of response to chemo or radiation therapy and prolonged survival in anaplastic oligodendrogliomas.
Younger age and complete resection also associated with a better prognosis
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Oligodendroglioma: gross specimen showing relatively sharp circumscription
Oligodendroglioma: mild nuclear anaplasia
Oligodendroglioma: note capillary vascular pattern and paucity of perinuclear halos
Oligodendrdoglioma: round bland nuclei, perinuclear halos, capillary vascular pattern
Oligodendrogliom grade II: satellitosis around cortical neurons and blood vessel
Oligodendroglioma grade III: nuclear anaplasia
Oligodendroglioma: minigemistocytes
Oligodendroglioma: palisading of nuclei
Oligodendroglioma: neurons within the tumor indicate infiltration
Oligodendroglioma: calcification and infiltration of cerebral cortex
Oligodendroglioma: chickenwire capillary vascular pattern
Oligodendroglioma grade II: Mib-1 stain characteristically shows a low labeling index contrasted with high cellularity
Oligodendroglioma: smear preparation showing unifrom round bland nuclei with paucity of glial processes
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