Anaplastic oligodendroglioma NOS

Anaplastic oligodendroglioma NOS (not-otherwise-specified) is a diagnosis in the current (2016) WHO classification of CNS tumors denoting a diffuse infiltrating glioma that has histological features of anaplastic oligodendroglioma, but without 1p19q co-deletion status or IDH mutation status being available (either because the test has not been performed, or the markers are equivocal) ¹. 

It is, therefore, equivalent to the diagnosis of anaplastic oligodendroglioma pre-2016, and it is essential to note that the imaging features of anaplastic oligodendrogliomas defined on histological grounds versus those defined using 1p19q co-deletion and IDH mutation are not identical. These are discussed further in the article on oligodendroglioma. 

Importantly, neither ATRX status (usually the opposite of 1p19q co-deletion status) nor additional immunohistochemical markers such as NOGO-A are substitutes for formal 1p19q co-deletion determination, and as such unless both IDH mutation and 1p19q co-deletion are assessed, the formal diagnosis of anaplastic oligodendroglioma cannot be made.