[Primary Central Nervous System Lymphoma]

Histopathologic classifications of primary central nervous system lymphoma(PCNSL)are covered by two WHO classifications; WHO classification of Tumors: Central Nervous System Tumours(WHO-CNS)and WHO classification of Tumors: Haematolymphoid Tumours(WHO-HAEM). The International Consensus Classification(ICC)is another recent comprehensive classification of hematolymphoid tumors that covers these disease entities. While these classifications mainly share the same principles for categorizing these tumors, there are several differences that should be noted owing to the nature of WHO-CNS(which covers only the lymphomas manifesting in the CNS, and the anatomic sites are described in the titles of some entities)and the introduction of new entities in the latest version(5^th)of the WHO-HAEM(WHO-HAEM5). In the WHO-HAEM5, two novel entities related to the perception of PCNSLs are introduced: primary large B-cell lymphoma of immune-privileged sites and lymphomas arising in immune deficiency/dysregulation. These entities are largely based on the underlying biology shared by lymphomas that arise from different sites, including the central nervous system, and they combine and re-classify several types that had been defined separately according to their sites and etiologies in previous versions. Classifications of PCNSLs in future revisions of the WHO-CNS might be described as parts of these novel entities, and nomenclatures of some diagnostic entities may be subject to change accordingly. This review will mainly focus on the classification of lymphomas described in the 5^th edition of WHO-CNS, along with its comparison with WHO-HAEM and ICC.