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2Brain

grade astrocytomas do not induce vascularproliferation and therefore do not enhanceon CT or MRI. Oligodendrogliomas andmixed tumors comprising both astrocytesand oligodendrocytes may be histologicallybenign with modest neovascularity.

Anaplastic astrocytomas and anaplasticoligodendrogliomas are intermediate-gradeneoplasms that arise from low-grade glio-mas as a result of additional mutations intumor-suppressing genes as the preexisting tumor ages. Like high-grade gliomas, theyform blood vessels and will enhance withIV contrast administration. They are not ne-crotic, which is the pathologic hallmark ofglioblastoma. Anaplastic gliomas are typi-cally more heterogeneous than low-gradetumors on both CT and MRI (Fig. 2.31).

◆ Low- and Intermediate-Grade Glioma

Low-grade gliomas are primary CNS tu-mors, the majority of which are either as-trocytomas, oligodendrogliomas, or mixed oligoastrocytomas. They are a common cause of a first seizure in previously healthy adults. Headache and sensory or motor deficits are other presenting symptoms. In contrast to malignant gliomas, which are usually discovered in older patients, low-grade tumors frequently occur in the third and fourth decades.

On CT, a low-grade astrocytoma appearsas a nonenhancing, poorly defined, homog-enous low-density tumor. The characteristicMRI appearance is that of a diuse, non-enhancing mass, hypointense to brain onT1-weighted images and hyperintense onFLAIR and T2-weighted images. Most low-

Fig. 2.31a–fa–d Low-grade mixed oligoastrocytoma. CT shows a homogeneous low-attenuation mass involving the right frontal cortex and white matter. The mass is high in signal on T2-weighted MRI and low in signal on T1-weighted MRI, and it does not enhance after administration of MR or CT contrast material.

e,f Anaplastic oligodendroglioma. Partially calcied, partially cystic right frontal mass (CT) with scat-tered foci of enhancement on postgadolinium T1-weighted MRI.