The diagnosis of 206 low and high grade adult
gliomas, including 40 oligoastrocytomas, was revised based on the immunohistochemical reactivity for the
ATRX protein, IDH1/2 mutation status and 1p/19q chromosomal status. All
oligodendrogliomas kept the initial diagnosis.
Astrocytomas did not change diagnosis in 30 of 36 cases (83.3 %); four of 36 (11.1 %) cases were reclassified as
oligodendroglioma, one (2.8 %) as
DNT and the other (2.8 %) as reactive
gliosis. Oligoastrocytomas changed diagnosis in 35 of 40 (87.5 %) cases, being reclassified 22 of 40 (55 %) as
astrocytoma, 11 of 40 (27.5 %) as
oligodendroglioma and two of 40 (5 %) as reactive
gliosis. Four (10 %) remained unclassifiable. In one case only (2.5 %), the diagnosis of oligoastrocytoma could not be excluded since
tumor astrocytes and
tumor oligodendrocytes coexisted in mixed
tumor areas. In the GBM
tumor subgroup, GBMO disappeared because they were not substantiated by molecular genetics.
Pilocytic astrocytomas retained ATRX expression. Loss of nuclear
ATRX protein expression was strongly associated to IDH1/2 mutations (p = 0.0001) and mutually exclusive with total 1p/19q co-deletion (p = 0.0001). In astrocytic
tumors, loss of immunoreactivity for the
ATRX protein was significantly associated to the ALT phenotype (p = 0.0003). The constitutive ATRX expression in microglia/macrophages may be misleading, especially in the identification of an oligodendroglial
tumor infiltration. Of paramount importance in the recognition of oligodendroglial and astrocytic
tumor cells were the double immunostainings for ATRX/GFAP, ATRX/IDH1R132H, ATRX/Iba-1 and ATRX/CD68.