The lower and
"benign" end of the spectrum of fibrillary astrocytic neoplasms
begins with the astrocytoma. In adults, most of these lesions
arise in the cerebral hemispheres where the symptoms, especially
seizures, may be of long standing. In children, the brain stem,
particularly the pons, is favoured. The spinal cord is affected
in both age groups. In the cerebral hemispheres, the
well-differentiated astrocytoma is less common than the
glioblastoma, whereas the opposite is true in the spinal cord.
In any location, the neoplasm alters the colour and texture of
the parent tissue, but this is most obvious in the cerebral
hemispheres, where the imparted yellow stringiness of the white
matter is most readily seen and felt. Cysts filled
with clear yellow fluid may be seen, and gritty grains of
calcium may be encountered in a minority of cases. The
ill-defined borders of these lesions are reminders that
complete surgical excision is an elusive goal. The brain stem
lesions are similarly ill defined and may lobulate the external
surface of the pons and partially surround the basilar artery.
Some well-differentiated solid or cystic brain stem
astrocytomas protrude into the fourth ventricle.
Microscopically, the nuclei of
neoplastic well-differentiated astrocytes are round and
generally uniform. They congregate in a density not markedly
different from that of normal white matter. Mitoses are rare,
and the vascular proliferation and necrosis that characterize
the glioblastoma are not observed. A helpful diagnostic feature
is a vacuolated state produced by small pools of eosinophilic
fluid and known as microcystic change. This is not usually found
either in reactive astrocytosis or in markedly anaplastic
neoplasms. The pink cytoplasm with the multiple filament-rich
processes characteristic of astrocytes mayor may not be seen in
standard histologic sections but are visualized by electron
microscopy or staining for GFAP. These
astrocytic characteristics are exaggerated in the process-laden
astrocytes that predominate in the subcategory of astrocytomas
known as gemistocytic astrocytomas. Recent data
support the concept that gemistocytic astrocytomas behave more
aggressively than expected for low-grade astrocytomas, with a
tendency for more rapid evolution to glioblastoma, and many are
therefore graded as anaplastic astrocytomas.
The ease of histologic
diagnosis of the well-differentiated astrocytoma depends on the
extent to which the lesion differs from normal or gliotic brain.
This difference may not be great, and a firm diagnosis may not
always be possible, especially in small specimens such as those
from the brain stem, spinal cord, or edge of a cerebral lesion.
As much tissue as possible should be submitted for histologic
studies, especially from the macroscopically most abnormal area.
Frozen section examination should also be directed at this
latter site to minimize the likelihood of a "consistent with"
or "I think this is" diagnosis that can be the pathologist's
way of hedging when there is uncertainty about the nature of the