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In the 4th edition of the WHO classification of CNS tumors (2007) [2, 3] oligoastrocytoma was designated as diffusely infiltrating glioma composed of a mixture of two cell types, morphologically resembling both OD and astrocytoma. In contrast to “classic” OD, the histological appearance of which is rather characteristic, the morphology of oligoastrocytomas often lacks typical features, and is highly heterogeneous and variable, thus it is frequently difficult to distinguish these tumors clearly. Criteria for their pathological diagnosis have always varied considerably [4, 13, 22]. It has been recommended that in such neoplasms the oligodendroglial and astrocytic parts should constitute ≥25% of the total volume, otherwise the diagnosis of glioma with specific cell component (e.g., OD with astrocytic component) should be made [23]. However, the precise evaluation of lesion composition may be difficult, since oligoastrocytomas with distinct areas exhibiting one or another type of differentiation are rare, while typically there is an intermixture of both types of neoplastic cells [22]. Thus, there has been always considerable interobserver variability regarding the exact pathological diagnosis in such cases.

Recently, Sahm et al. [24] revealed that 72% of oligoastrocytomas carry 1p/19q codeletion typical for OD, not present in the astrocytic component; thus, the latter was considered not neoplastic, but reactive. At the same time, in cases with molecular alterations typical for astrocytoma, namely p53 immunopositivity and loss of the transcriptional regulator ATRX (alpha thalassemia/mental retardation syndrome X-linked), the abnormalities were present in all neoplastic cells both with astrocytic and with oligodendroglial phenotype. Therefore the authors suggested that in oligoastrocytomas different lineages of differentiation represent merely morphological variation without a genetic basis, and that based on molecular signatures these tumors may be defined as either OD or astrocytomas [24]. In concordance, Jiao et al. [7] revealed that 88% of oligoastrocytomas carry genetic alterations typical for either OD or astrocytomas. Finally, based on the molecular markers Wiestler et al. [25] were able to reclassify the majority of WHO grade III oligoastrocytomas from the NOA-04 study either as AOD or as AA, and revealed a similar clinical course between “histologically” and “molecularly” defined subgroups of neoplasms.

Therefore, the updated WHO classification of CNS tumors (2016) [11] basically abandoned the diagnosis of oligoastrocytoma as a distinct pathological entity, unless the lesion is NOS or represents a very rare example of the neoplasm exhibiting truly morphological and molecular dualism [7, 25, 26]. Similarly, the term “GBM with oligodendroglioma component” is not recommended for use anymore. Actually these tumors are anaplastic oligoastrocytomas with necroses and until recently have been considered as a subtype of GBM carrying somewhat better prognosis [3]; however they do not represent a distinct entity and not infrequently carry 1p/19q co-deletion, the presence of which classifies it as AOD [12].