Diffuse glioma is a term used to encompass a variety of poorly marginated infiltrating tumors of the central nervous system, which histologically appear similar to glial cells, specifically astrocytes and oligodendrocytes. These range in biological behavior from very indolent to extremely aggressive and this is reflected in grading that ranges from grade 1 to grade 4.
Importantly, the term diffuse glioma does not include circumscribed astrocytic tumors (e.g. pilocytic astrocytoma, subependymal giant cell astrocytoma, pleomorphic xanthoastrocytoma, etc.) nor tumors with admixed neuronal elements (e.g. ganglioglioma) although there is overlap, particularly with the latter.
Under the current version of the WHO classification of CNS tumors (5th edition, 2021), they are broadly divided into adult and pediatric type tumors, with an increasing emphasis on molecular markers 1.