**6. Oligoastrocytoma**

Criteria for mixed astrocytomas / oligodendrogliomas are weakly defined. Not surprisingly interobserver variaibility is great ranging from 9-80% as seen in a study on 155 tumors that were initially classified as oligoastrocytomas (Fuller et al., 2003). Macoscopically these tumors are similar to other diffuse grade II or grade III lesions. Histological diagnosis of oligoastrocytoma requires that both astrocytic and oligodendrogial neoplastic tumor cells are present in the same tumor. These may appear biphasic as two distinct tumor areas or more commonly as intermingled tumor. The minimal amount to which one tumor component has to be present is unfortunately not properly defined. Some authors are satisfied when one single high power field has either astrocytic or oligodendroglial tumor cells, other authors request at least a minimum of 50% neoplastic astrocytes. Separation of astrocytes and oligodendrocytes is not always possible. Every pathologist has seen tumor cells that have features of both lineages. It is important however to distinguish minigemistocytes and gliofibrillar oligodendrocytes in oligodendrogliomas from astrocytes, as they do not warrant the diagnosis of oligoastrocytoma. Single mitoses are compatible with a grade II oligoastrocytoma, however in our institution we have an relaxed approach, when mitoses are increased in a distinct oligodendroglial compartment only. Anaplastic oligoastrocytomas show increased nuclear atypia, elevated cellularity and abundant mitoses. Microvascular proliferations are frequent in grade III oligoastrocytomas. Discrimination of anaplastic oligoastrocytoma from glioblastomas with oligodendroglial differentiation is especially difficult, as WHO criteria allows pseudopalisading necroses to be present in oligoastrocytic tumors. In our institution decision is based on whether necroses are present in astrocytic tumor parts indicating a glioblastoma or is limited to oligodendroglial tumor parts indicating anaplastic oligoastrocytoma. Like histology, immunhistochemistry results are very mixed and represent the immunophenotype of neoplastic astrocytes or oligodendrogytes as discussed in their sections. Grade II tumors have a MIB-1 proliferation index usually less than 6% (Deckert et al., 1989).
