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Glioblastoma multiforme (GBM)

Glioblastoma multiforme is virtually synonymous with Astrocytoma Grade IV. These are WHO Grade 4 lesions with a very poor prognosis in most patients. Two of the main distinguishing features are prominent neovascularity and tumor necrosis. These elements often define the GBM as a heterogeneous ("multiforme") unilocular or multiloculated mass. Although the sharp margin of the enhancing rim seen on gross inspection, CT, and MR suggests a discrete lesion, these masses are diffusely infiltrating at the microscopic cellular level. The apparent margin merely reflects the greatest density of tumor cells, with the largest amount of neovascularity.  In effect, the gross inspection and imaging only show the secondary feature of tumor induced vessels - not the tumor itself.  Characteristically, these lesions show microscopic infiltration into the surrounding white matter for several centimeters from the edge of enhancement.  Typically, however, most recurrences begin within 1-2 centimeters of the original rim of enhancement.  It has also been suggested that "multifocal" glioma represents islands of tumor with neovascularity that are microscopically connected by tumor cells without vessels.