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.
- 1. Louis DN, Ohgaki H, Wiestler OD, Cavenee WK "WHO Classification of Tumours of the Central Nervous System. 4th Edition Revised" ISBN: 9789283244929