Molecular classification of adult diffuse gliomas: conflicting IDH1/IDH2, ATRX, and 1p/19q results.

Until recently, the diagnosis of brain tumors was primarily based on microscopic examination of hematoxylin and eosin-stained tissue sections. The updated World Health Organization (WHO) Classification of Tumours of the Central Nervous System incorporates genetic alterations into the classification system, with the goal of creating more homogenous disease categories with greater prognostic value. Hence, under the new classification system, the diagnosis of diffuse gliomas incorporates the evaluation of mutations in the IDH1 and IDH2 genes and simultaneous deletion of chromosomes 1p and 19q. For example, although under the 2007 WHO classification system, oligodendrogliomas could be diagnosed based solely on the presence of characteristic histologic features, the newly molecularly defined entity of "oligodendroglioma, IDH-mutant and 1p/19q codeleted" requires the presence of both an IDH1 or IDH2 mutation and 1p/19q codeletion. Given that diagnosis requires evaluation of critical genetic alterations, molecular diagnostics is becoming an increasingly important aspect of clinical oncologic neuropathology practice. As molecular testing is applied more frequently to the diagnosis of brain tumors, inconsistent or conflicting molecular information will create diagnostic challenges. Here we present 6 cases of diffuse glioma that presented a diagnostic challenge due to conflicting molecular testing results. These cases exemplify some of the potential complications that arise when introducing the new 2016 central nervous system WHO classification system diagnostic criteria into routine clinical practice. We aim to alert the general practice pathology community to these potential conflicts to help mitigate the risk of potential misdiagnosis.