Laboratory Studies
Routine laboratory workup is not helpful as there are no serum biomarkers for oligodendroglioma. The tumor also does not cause any alteration in electrolytes or blood counts. Routine laboratory studies are appropriate if a patient is receiving chemotherapy.
Imaging Studies
Diagnostic imaging studies are the most important part of the workup.
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MRI (with and without gadolinium) is the preferred modality.
T1 images generally demonstrate a hypointense or mixed hypointense and hyperintense mass.
T2 images reveal a hyperintense mass with or without surrounding edema.
- With contrast administration, grade 2 oligodendroglioma generally does not enhance, while a grade 3 oligodendroglioma might. Calcification is also a common finding, potentially reflecting the slow growing nature of these tumors. See the image below.
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Sagittal gadolinium-enhanced T1-weighted magnetic resonance image of a low-grade oligodendroglioma. This image demonstrates no contrast enhancement.
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A study by Megyesi et al compared the MRI characteristics of oligodendroglioma with 1p/19q loss (now called astrocytoma) with those without 1p/19q loss. Tumors with 1p/19q loss (now called oligodendroglioma) were significantly more likely to have indistinct borders, a mixed signal intensity on T1- and T2-weighted images, paramagnetic susceptibility effects, and intratumoral calcification compared with astrocytoma without 1p/19q loss, which more often had a distinct border and a uniform signal on T1- and T2-weighted images. [5]
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A study by Brown et al, using noninvasive quantitative MRI with and without contrast, reliably predicted the co-deletion of chromosomes 1p and 19q with high sensitivity and specificity in suspected low-grade glioma. [6]
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CT scans reveal a hypodense, reasonably well-demarcated mass with moderate surrounding edema.
Intratumoral calcification is common, and hemorrhage is noted occasionally.
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Contrast-enhanced computed tomography scan in a 44-year-old man with a 3-year history of epileptic seizures. This image reveals a calcified hypoattenuating lesion that is invading the corpus callosum.
Computed tomography scan of a low-grade oligodendroglioma. This image reveals a well-demarcated, left frontal hypoattenuating lesion with a small calcification.
Other Tests
Definitive diagnosis is made by surgical resection for histologic and molecular analysis. Maximal surgical resection is associated with improved outcomes.
Histologic Findings
Macroscopic
Grossly, oligodendrogliomas appear as well defined, solid, and pinkish grey, frequently with areas of calcification and sometimes with areas of necrosis and cystic degeneration. Intratumoral hemorrhage may be present.
Microscopic
Oligodendrogliomas are distinctive, consisting of homogeneous, compact, rounded cells with distinct borders and clear cytoplasm surrounding a dense central nucleus, giving them a "fried egg" appearance. See the image below.
Oligodendrogliomas. The classic appearance of the oligodendroglioma is that of a round to oval, water-clear cytoplasm ringing about round to lobulated nuclei. The chromatin appearance is finely threadlike to smudgy, often associated with pointlike basophilic chromocenters, rather than nucleoli.
Classic histologic image of oligodendroglioma. This image shows monomorphous tumoral proliferation that consists of round, regular cells with a small, central, hyperchromatic nucleus surrounded by clear cytoplasm. Few calcifications are present.
Oligodendrogliomas. The cellular density is moderate to high, and the fried-egg appearance dominates the histologic features.
Oligodendrogliomas usually arise in the subcortical location but infiltrate diffusely into cortex around normal neuronal elements and, in superficially located lesions, may extend to the leptomeninges. Within the tumor, branching blood vessels are highly characteristic and divide the cells into discrete clusters.
Oligodendrogliomas. This tumor exhibits oligodendroglial-type nuclei and scanty eosinophilic fibrillar cytoplasm amidst a mucinous background. Such tumors may be considered oligoastrocytomas.
The WHO 2016 Classification of Tumors of the CNS abolished the previously used term “oligoastrocytoma.” To definitively diagnose a glial tumor as oligodendroglioma, the tumor must have both IDH-mutation and 1p/19q co-deletion. Notably, the WHO 2021 Classification of Tumors of the CNS removed the term “anaplastic” from the naming of glial tumors, instead simply referring to them as grade 3.
Most oligodendrogliomas are slow-growing indolent tumors; however, they occasionally behave in a more malignant manner when initially diagnosed, or an indolent tumor may evolve into an aggressive one. Malignant tumors demonstrate increased cellularity, nuclear pleomorphism, endothelial proliferation, mitotic activity, and necrosis. Different grading systems are available for malignant tumors, but most pathologists use a simple two-tier grading system, diagnosing tumors without anaplastic features as oligodendroglioma, grade 2 tumors and as anaplastic oligodendroglioma, grade 3 if several of the malignant features are present. See the images below.
Oligodendrogliomas. Oligodendrogliomas with vascular proliferation and significant mitotic activity are best considered to be anaplastic oligodendrogliomas (World Health Organization [WHO] grade III).
Smear preparation of anaplastic oligodendroglioma. This image reveals increased nuclear pleomorphism and vascular proliferation.
Oligodendrogliomas. Anaplastic oligodendrogliomas frequently take on eosinophilic cytoplasm and hyperchromasia of the nuclei. Such tumors may demonstrate necrosis among its diagnostic features.
Staging
Conventional “TMN” cancer staging does not apply to gliomas as they rarely spread outside the CNS. Instead, gliomas are graded via the World Health Organization (WHO) grading scale from 1 to 4, with 1 being the least aggressive and 4 being the most aggressive. Oligodendrogliomas by definition are either a grade 2 or grade 3 glioma. See the Histologic Findings section for more information on delineation between grades 2 and 3 oligodendrogliomas.
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Classic histologic image of oligodendroglioma. This image shows monomorphous tumoral proliferation that consists of round, regular cells with a small, central, hyperchromatic nucleus surrounded by clear cytoplasm. Few calcifications are present.
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Smear preparation of anaplastic oligodendroglioma. This image reveals increased nuclear pleomorphism and vascular proliferation.
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Contrast-enhanced computed tomography scan in a 44-year-old man with a 3-year history of epileptic seizures. This image reveals a calcified hypoattenuating lesion that is invading the corpus callosum.
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Computed tomography scan of a low-grade oligodendroglioma. This image reveals a well-demarcated, left frontal hypoattenuating lesion with a small calcification.
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Sagittal gadolinium-enhanced T1-weighted magnetic resonance image of a low-grade oligodendroglioma. This image demonstrates no contrast enhancement.
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Oligodendrogliomas. This tumor exhibits oligodendroglial-type nuclei and scanty eosinophilic fibrillar cytoplasm amidst a mucinous background. Such tumors may be considered oligoastrocytomas.
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Oligodendrogliomas. The classic appearance of the oligodendroglioma is that of a round to oval, water-clear cytoplasm ringing about round to lobulated nuclei. The chromatin appearance is finely threadlike to smudgy, often associated with pointlike basophilic chromocenters, rather than nucleoli.
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Oligodendrogliomas. The cellular density is moderate to high, and the fried-egg appearance dominates the histologic features.
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Oligodendrogliomas. Oligodendrogliomas with vascular proliferation and significant mitotic activity are best considered to be anaplastic oligodendrogliomas (World Health Organization [WHO] grade III).
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Oligodendrogliomas. Anaplastic oligodendrogliomas frequently take on eosinophilic cytoplasm and hyperchromasia of the nuclei. Such tumors may demonstrate necrosis among its diagnostic features.
