Current Concepts in the Diagnosis of Gliomas
Oligodendrogliomas and Mixed Gliomas
Richard A. Prayson and Gene H. Barnett
Oligodendrogliomas are tumors composed of cells which resemble oligodendrocytes. This group of
neoplasms has notoriously been the cause of great difficulty and anxiety and poses a diagnostic challenge
for a number of reasons:
Oligodendrogliomas comprise anywhere between 5% and 18% of intracranial gliomas and
approximately 4-5% of all primary brain tumors. They can arise throughout the neuroaxis in the white
matter and their distribution roughly correlates with the amount of white matter in various regions of
the brain. In contrast to astrocytomas and ependymomas, oligodendrogliomas are relatively infrequently
encountered in the spinal cord. Oligodendrogliomas most commonly occur in adults, with a peak incidence
in the 5th and 6th decades of life.
- grading schemas are more nebulous and the reproducibility of such approaches are poor as compared with astrocytomas;
- there is no good immunohistochemical marker for oligodendroglial cells; and
- their diagnosis is complicated by the recognition of tumors with mixed oligodendroglioma/astrocytoma phenotypes (mixed gliomas); and
- similar to astrocytomas, they can be phenotypically heterogeneous.
Clinical presentation is dependent on site and size of the tumor, with seizures and headaches being
the most commonly reported symptoms prior to diagnosis. Often patients present with symptoms of several
years duration. Radiographically, these tumors cannot be reliably distinguished from astrocytomas on CT
scan. The lower grade tumor appears as a hypodense or isodense mass. On MRI studies, low grade
oligodendrogliomas appear as hypointense lesions on T1-weighted images and hyperintense lesions on
T2-weighted images. Focal calcification and intratumoral hemorrhages are a relatively common occurrence.
Higher grade tumors often have a more heterogeneous appearance radiographically due to necrosis and
vascular proliferative changes.
Similar to astrocytomas, there have been several attempts at constructing grading schemas
for oligodendroglial neoplasms. The most widely utilized approaches are two-tier systems in which tumors
are designated as low grade (WHO grade II) or anaplastic (WHO grade III). Grading of oligodendroglial
neoplasms, according to this approach, has been shown to be a significant predictor of survival. The
problem lies in the lack of reproducibility in application of grading parameters. Exactly what the
minimum criteria are for the establishment of a diagnosis of anaplastic oligodendroglioma are far from
agreed upon. In general, the same histologic parameters that are worrisome in the diffuse or fibrillary
astrocytomas are worrisome in oligodendrogliomas: hypercellularity, nuclear pleomorphism, mitoses,
vascular (endothelial proliferation), and necrosis. The relative importance of these various parameters
in the grading of oligodendrogliomas, however, are probably different. In general, tumors with more
worrisome histologic features are more likely to behave in an aggressive fashion. Again, further
complicating grading problems, are issues related to tumor heterogeneity and tumor sampling.
The histologic appearance of the classic low grade oligodendroglioma is familiar to
most. The tumor is marked by a proliferation of cells with rounded nuclei and a scant amount of
cytoplasm. Monotony of cells are a salient feature. Delays in formalin-fixation generate perinuclear
clearing or halos which result in the classic "fried egg" appearance of the tumor cells. It is important
to remember that this finding will not be evident in tissue processed at the time of intraoperative
consultation or in rapidly fixed tissue. In addition, the frozen section procedure generates some
nuclear pleomorphism and hyperchromasia, mimicking astrocytoma. The tumor is often accompanied by a
distinct arcuate, capillary vascular pattern. Perineuronal and perivascular satellitosis and subpial
aggregation of infiltrating tumor cells are relatively common findings. As mentioned before,
calcifications are present in the majority of oligodendrogliomas. Frequently admixed with the more
typical tumor cells are so-called "minigemistocytes", which are small cells with oligodendroglial-type
nuclei, a readily discernible amount of eosinophilic cytoplasm filled with glial filaments, and limited
cytoplasmic processes. Occasionally, cells may contain cytoplasmic, refractile eosinophilic bodies.
Small nodular foci of hypercellularity, which may be quite proliferative if evaluated with MIB-1 or Ki-67
antibodies, can be encountered in an otherwise low grade appearing tumor; the significance of these
hypercellular foci is not certain.
Recognition of higher grade oligodendrogliomas is problematic. The increased cellularity and nuclear
pleomorphism often cause diagnostic confusion with astrocytomas. In general, anaplastic
oligodendrogliomas still maintain a certain degree of monotony with regard to cytologic appearance and
often are accompanied by more typical appearing lower grade areas at the edge of the lesion. Mitotic
activity is usually readily identified and often exceeds 5-6 mitotic figures per 10 high power fields.
Occasionally, tumors with a glioblastomatous phenotype (with necrosis) that appear to be oligodendroglial
in derivation are encountered. Some think it is appropriate to designate such lesions as WHO grade IV.
Indication of possible oligodendroglial lineage is important on the pathology report; these patients are
potentially more likely to respond to chemotherapy than histologically similar tumors of astrocytic
lineage. Unfortunately, immunohistochemistry adds very little to the routine diagnosis of these cases
and there is no antibody recognized as a reliable marker of oligodendroglial differentiation. The
minigemistocytes often demonstrate striking GFAP immunoreactivity in contrast to typical
oligodendroglioma cells that are GFAP negative or show only weak positivity.
Cell proliferation marker studies of oligodendrogliomas have shown a correlation between labeling
indices and histologic grade. In one study of 44 histologically pure oligodendrogliomas, MIB-1 labeling
indices ranged from 0-42.3 (median 1.2). The low grade tumors (N=31) had a median index of 0.5 versus
the anaplastic tumors (N=13) which had a median index of 6.2. As in the case of astrocytomas,
evaluation of cell proliferation markers in the setting of oligodendrogliomas needs to take into account
issues of tumor sampling and heterogeneity, the effects of differences in methodology of immunostaining,
and interobserver variability in evaluating stained sections.
Grade for grade, oligodendrogliomas tend to do better than their astrocytoma counterparts. Low grade
tumors (WHO grade II) survive between
8-20 years versus 2-10 year survival range in the anaplastic tumors (WHO grade III). Correlates with
poor survival beside histologic grade have included increased patient age, poor Karnofsky performance
status, ring-enhancement radiographically and vascular endothelial growth factor expression. Prognostic
features associated with a better outcome in some studies have included young age (<40 years) and high
Karnofsky score. In contrast to the astrocytomas, a majority of oligodendrogliomas are chemoresponsive,
with about two-thirds of them exhibiting a radiographic response following PCV chemotherapy
(procarbazine, lomustine, vincristine). A number of recent studies have shown correlation between those
oligodendroglial tumors that demonstrate allelic losses on chromosomes 1p and 19q and
chemoresponsiveness. Heterozygosity for chromosomes 1p and 19q is present in the majority of both low
and high grade oligodendrogliomas. Recognition of a tumor as having an oligodendroglioma component is
important from a therapeutic standpoint in that they are more likely to be treated with chemotherapeutic
regimens and are more likely to respond to such treatment as compared to astrocytomas. Patients who
demonstrate a chemotherapeutic response may survive an additional 9-10 years. Those anaplastic
oligodendrogliomas that are not chemoresponsive generally result in patient death within 2 years. Other
genetic alterations frequently observed in oligodendroglial neoplasms are summarized in Table 8:
Table 8: Common Molecular Genetic Alterations Associated with Oligodendrogliomas.
(Borrowed from Reifenberger G et al, 2000)
Recent work by Ino et al suggests that anaplastic oligodendrogliomas can be divided genetically into
four therapeutically and prognostically relevant subgroups:
- tumors with combined but isolated losses of 1p and 19q have a marked durable response to chemotherapy and prolonged survival;
- tumors with 1p loss and other genetic alterations or no 19p loss have a shorter duration of response to chemotherapy and shorter survival;
- tumors lacking 1p loss but with TP53 mutations which may respond to chemotherapy but recur quickly and have a poor survival; and
- tumors lacking 1p and TP53 mutations which respond poorly to chemotherapy and have poor survival.
One of the most difficult and at times frustrating differential diagnostic considerations
for oligodendroglioma involves distinguishing it from the so-called mixed glioma (oligoastrocytoma).
This remains a particularly problematic issue. It is recognized that neuroectodermal stem cells which
have proliferative capacity and are highly migratory have the ability to differentiate along divergent
cell lines. 0-2A progenitor cells, identified in rodent models, are capable of differentiating into
either oligodendrocytes or type 2 astrocytes. Ultrastructural and immunohistochemical evidence of rare
cells which demonstrate evidence of neuronal differentiation exist in oligodendroglioma.
The precise definitions of what a mixed glioma represents histopathologically vary quite from
pathologist to pathologist, i.e. the reproducibility of the diagnosis is quite poor and its diagnosis is
sometimes established by lack of concensus as it is by concensus. Tumors that have geographically
distinct areas of astrocytoma and oligodendroglioma are typically designated as mixed glioma. The
minority cell type of the tumor generally exceeds some arbitrary proportion of the total neoplasm,
usually between 20% and 30%. This obviously implies that in small biopsies or incompletely
examined/sampled tumors, the diagnosis should be made with great caution.
Particularly troublesome are tumors that appear to have intermixed areas consisting of
neoplastic oligodendrocytes and astrocytes. This is a well-described phenomenon in many low grade
oligodendrogliomas. The problem lies in the degree to which it is observed and at what point one decides
to diagnose the lesion as a mixed glioma versus oligodendroglioma versus astrocytoma. Unfortunately due
to the variability of definitions used in the literature in studies examining mixed gliomas (or in some
instances lack of definition), it is difficult to really appreciate the clinical significance of making
this diagnosis. It is generally believed that these tumors have an intermediate survival grade for
grade, between oligodendroglioma and astroctyoma. Molecular biologic studies have indicated that mixed
oligoastrocytomas which demonstrate genetic abnormalities commonly observed in oligodendrogliomas,
particularly LOH on chromosomes 1p and 19q, tend to behave more like oligodendrogliomas. Tumors not
demonstrating those losses, tend to behave more like their astrocytoma counterparts. Until this has been
sufficiently tested, the general philosophy of many is to error on the side of recognizing an
oligodendroglioma component to the tumor in order to ensure that the patient may potentially benefit from
a trial of chemotherapy.
Table 9 summarizes other differential diagnostic considerations of oligodendroglioma
including the dysembryoplastic neuroepithelial tumor, central neurocytoma, and clear cell ependymoma:
Table 9: Differential Diagnosis of Oligodendroglioma
| ||Oligodendroglioma ||Dysembryoplastic Neuroepithelial Tumor ||Central Neurocytoma ||Clear Cell Ependymoma|
|Age ||Adulthood ||Childhood ||Adulthood ||Adult/Childhood|
|Location ||White matter based (frontal lobe most common) ||Cortical based (temporal lobe most common) ||Intraventricular ||Intraventricular|
|Infiltration ||+ ||- ||- ||-|
|Multinodular ||- ||+ ||- ||-|
|Cell heterogeneity ||- ||+ ||- ||-|
|Floating neurons ||- ||+ ||- ||-|
|Minigemistocytes ||+ ||- ||- ||-|
|Perineuronal satellitosis ||+ ||± ||- ||-|
|Rosettes/pseudo-rosettes ||- ||- ||- ||+|
|GFAP ||Minigemistocytes (+) ||Astrocytic component (+) ||Weak coexpression ||+|
|Synaptophysin ||- ||Neural component (+) ||+ ||-|
All of these tumors are marked by the presence of a population of small rounded cells resembling
oligodendroglial cells. The dysembryoplastic neuroepithelial tumor (DNT) arises primarily in children
and in the setting of chronic epilepsy. The lesion is marked by the presence of oligodendroglial-like
cells arranged against a microcystic background and associated with neuronal and astrocytic components.
None of the components of the tumor demonstrate significant cytologic atypia. The DNT lesion is
predominantly cortical based and multifocal and has an excellent prognosis. The central neurocytoma is
classically a ventricular based lesion comprised of neurally differentiated (synaptophysin positive)
cells. Rarely, ependymomas may demonstrate clear cell change resembling oligodendroglioma. The key to
differentiating this lesion lies in the recognition of more recognizable areas of ependymal
differentiation or demonstration of ultrastructural evidence of ependymal differentiation
(blepharoplasts, cilia, or microvilli).
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