2017 Volume 26 Issue 11 Pages 782-791
The World Health Organization (WHO) Classification of Tumors of the Central Nervous System needed to be revised due to the advances in many diagnostic technology since the first edition appeared in 1979, and in 2016 a revised edition of the fourth edition was published. In this revision, the molecular classification was introduced for the first time in diffuse gliomas and embryonal tumors, and the pathological diagnosis concept was greatly changed from one based on classical histological classification to one based more on molecular and genetic classification. This change reflects a policy wherein the tumor definition should be rigid and objectively defined as much as possible, and any tumor cases where genetic analysis cannot be performed or diagnostic evidence are ambiguous are now categorized as NOS (not otherwise specified). Unfortunately, this has led to problems where too many NOS diagnoses are generated.
