**3.3. Immunohistochemistry (IHC)**

**3. Histopathology**

342 Neurooncology - Newer Developments

**3.1. Macroscopic appearance**

**3.2. Microscopic appearance**

hypertrophic astrocytes.

grade II OA (OAII) and grade III OA (OAIII).

blurring of the grey white matter junction.

According to the current WHO classification system [1], OA was classified in two subtypes:

On gross pathology, OA was characterized by a soft, well-defined, grey-tan, mucoid or hemorrhagic, calcified mass with or without necrosis that may expand the gyrus, and cause

On histopathologic analysis, OA was characterized by highly cellularlesions composed of both tumor astrocytes and oligodendrocytes that could be separated or intermingled [5], *i.e*. the tumor could be defined as "biphasic" (**Figure 2A**–**C**) or "diffuse" (**Figure 2D**). Astrocytic tumor cells scattered within oligodendroglial cells hadto be recognizedas neoplastic andnotreactive/

**Figure 2.** Histopathologic features of OA. A – OAII with separated astrocytic and oligodendroglial components, x100; B – Id, astrocytic component, x400; C – Id, oligodendroglial component, x400; D – OAII with intermingled astrocytic

*OAII*. The tumor showed a moderate cellularity with no or low mitotic activity. Microcalcifi‐

Reactive astrocytes are present in all gliomas, OA included; in the latter, their distinction from tumor astrocytes was the most important problem since the protean appearance of reactive

and oligodendroglial cells, x200; E – OAIII, x200. All hematoxylin and eosin (H&E).

cations and microcystic degeneration could occur.

IHC was practically based only on GFAP expression. No specific immunohistochemical marker was available for oligodendrocytes [17, 18], although MAP2, OLIG2, Cyclin D1, and alpha-internexin (INA) immunopositivity could be found in the oligodendroglial compo‐ nent. Approximately one third of OAs showed nuclear p53 accumulation, more commonly in the astrocytic cells [19].
