Anaplastic (Malignant) Astrocytoma

The anaplastic astrocytoma is intermediate in anaplasia between astrocytoma and glioblastoma multiforme. In its common location in the cerebral hemispheres of adults, it is also intermediate in the age of highest incidence (40 to 50 years), duration of preoperative symptoms, and degree of macroscopic abnormality. The lesion also occurs in the brain stem and spinal cord.

The increment of cellularity over the well-differentiated astrocytoma makes macroscopic abnormality more apparent, so that the anaplastic astrocytoma is better defined from the surrounding brain. Microscopically it is also more abnormal, as the cells are more populous, more pleomorphic, and more often present in mitotic division. The cells may invade the cerebral cortex to surround neurons (perineuronal satellitosis) and mass within a subpial position. Cells with cytoplasmic features of astrocytes are often prominent, but less well-differentiated cells may also be seen. Absent is the necrosis that characterizes glioblastoma multiforme, although vascular proliferation may be seen in limited degrees.

The diagnosis of anaplastic astrocytoma depends on the histological distinction between well-differentiated astrocytoma on one hand and glioblastoma multiforme on the other. The better differentiated varieties of the anaplastic astrocytoma are separated from the well-differentiated astrocytoma by imprecise and subjective differences in cellularity and pleomorphism. The criteria for distinguishing these two lesions may therefore vary among observers, and the dividing line may wander even in the same microscopist from observation to observation. At the other extreme the anaplasia of this lesion overlaps with that of glioblastoma, and there is no well-established criterion to separate these two lesions on the basis of cellularity or anaplasia alone. As discussed below, the glioblastoma is often diagnosed only when necrosis and/or vascular proliferation are present. If one holds rigorously to this rule, many glioblastomas will be underdiagnosed as anaplastic astrocytomas in small specimens, such as needle biopsies, that do not happen to include necrosis or vascular proliferation. It is not surprising, therefore, that some patients with "anaplastic astrocytomas" in the cerebral hemispheres do not survive for the 2 years expected for such patients.