Definition

SOL that exhibit purely astrocytic differentiation can be separated into two major categories: diffuse (or infiltrating) and circumscribed.
Complete surgical resection is rarely attained with diffuse astrocytoma because of the inherently infiltrative nature of the neoplastic cells, which extend well beyond the apparent gross margin of the tumor.

Diffuse astrocytoma (WHO II)

Mildly hypercellular, infiltrative
well-differentiated, spindle- to stellate-shaped astrocytes w/ minimal nuclear atypia
separated by a loosely fibrillary (± microcystic) background.
Nuclei are characteristically irregular, angulated, and hyperchromatic.
Mitotic figures are either absent or rare,
no vascular proliferation or necrosis.
Glial fibrillary acidic protein (GFAP) may be highly or weakly expressed.

Two morphologic subtypes are recognized: protoplasmic and gemistocytic.

Feature	Protoplasmic Astrocytoma	Gemistocytic Astrocytoma
Cell Size	Small tumor cells	Large rounded neoplastic astrocytes
Cytoplasm	Scant	Copious amount of glassy eosinophilic cytoplasm
GFAP Expression	Weakly expressed	Strongly positive
Nuclear Location	Not specifically mentioned	Eccentrically located nucleus
Background	Frequently associated with a myxoid or microcystic background	Not specifically mentioned
Cell Shape	Not specifically mentioned	Predominantly gemistocytic
Behavior	Not specified	Despite benign appearance, intrinsic tendency to progress
Morphological Appearance	Composed of well-differentiated, spindle- to stellate-shaped astrocytes	Composed of a predominant population of neoplastic astrocytes