In rare cases, without conclusive data on IDH mutation and 1p/19q codeletion status, tumours are classified as oligodendroglioma, not otherwise specified. The diagnosis of oligoastrocytoma is discouraged in the new WHO classification, which states that only exceptional cases that cannot be conclusively tested for IDH mutation and 1p/19q codeletion that show a mixed oligoastrocytic histology can still be classified as oligoastrocytoma, not otherwise specified 1).
1)
Louis DN, Perry A, Reifenberger G, et al. The 2016 World Health
Organization classification of tumors of the central nervous system:
a summary. Acta Neuropathol 2016; 131: 803–20