WHO 2016: Open questions and practical implications

Abstract

The WHO Classification of Tumors of the Central Nervous System has been a story of success over the past decades. There is general agreement that the WHO Classification represents the basis not only for neuropathological diagnosis but also for prognosis and therapy. The world-wide consensus of using this classification system is a remarkable achievement and may serve as a role model for communities working on other neurological diseases where heterogeneous and conflicting classification systems impair scientific and clinical progress.The most recent WHO Classification has been issued in May 2016 and includes several changes, as detailed in the BBlue Book^ [3] and summarized in an accompanying review article [4] as well as in the paper by Banan and Hartmann published in this issue of Acta Neurochirurgica. As in previous updates of the WHO Classification, a few newly recognized clinico-pathological tumor types have been added, such as diffuse leptomeningeal glioneuronal tumor, epithelioid glioblastoma, and anaplastic pleomorphic xanthoastrocytoma. A few others were deleted, such as protoplasmic astrocytoma, cellular ependymoma, and gliomatosis cerebri. Other tumors were renamed, including fibrillary astrocytoma as diffuse astrocytoma, hemangiopericytoma as solitary fibrous tumor/ hemangiopericytoma, and primitive neuroectodermal tumor (PNET) as CNS embryonal tumor. Tumors that have been known (and diagnosed) for a long time before but somehow had been Bforgotten^in previous WHO Classifications have been added in chapters on lymphomas, histiocytic tumors, tumors of the cranial and paraspinal nerves, and mesenchymal tumors. Finally, brain invasion has been clearly defined as being sufficient for making the diagnosis of atypical meningioma.However, the most dramatic change, which is unparalleled by previous editions, is the inclusion of molecular data in the diagnosis of several tumor types (actually still being a minority), resulting in integrated histological/molecular diagnoses. Examples include Boligodendroglioma, IDH-mutant and 1p/ 19q-codeleted, grade II^and Bclassical medulloblastoma, SHH-activated and TP53 mutant, grade IV^. Compared to the conventional, relatively broad tumor types solely defined by histology, the narrowly defined, combined histological/ molecular newcomers will lead to more homogeneous tumor groups, which are expected to be advantageous for clinical studies and eventually for the management of individual patients. While the updated classification is straightforward, clinically relevant, and based on recent molecular insight, it provokes questions concerning practical application. A few of these questions are listed below, not necessarily complemented with an unequivocal answer.Is 1p/19q analysis required for all cases of diffuse glioma?Codeletion of the short arm of chromosome 1 and the long arm of chromosome 19 is currently considered a defining molecular feature of oligodendroglioma. While in cases of histologically classical oligodendroglioma 1p/19q analysis is essential for making the f...

A simple algorithmic approach using histology and immunohistochemistry for the current classification of adult diffuse glioma in a resource-limited set-up

“…Recently, Paulus also raised the economic issues relating to the practical implications of the WHO 2016 classification 16. One of the questions raised by the author is whether 1p/19q analysis is required for all cases of diffuse glioma, which is left open for more clarification and consensus.…”