The classification system for gliomas has undergone significant revisions in the last several years, with the incorporation of molecular criteria and removal of mixed histologic diagnoses. Large-scale molecular studies have elucidated the biological characteristics of low grade gliomas, and enabled grouping based on IDH mutational and 1p19q codeletion status that outperforms histology in predicting patient outcomes. Mutations in ATRX and TP53 are largely mutually exclusive of 1p19q- codeletion in the context of mutant IDH, and are useful in screening patients for further molecular studies.
At our institution, new testing methods for 1p19q codeletion and ATRX were implemented in 2014, and in this presentation we review all cases submitted for 1p19q testing since this time. In comparing histologic to molecular diagnoses, the majority of histologic oligodendrogliomas indeed have 1p19q codeletion, while oligoastrocytomas and GBMOs largely are re-classified as astrocytomas and glioblastomas, respectively. We have also found that loss of ATRX nuclear expression associated with ATRX mutation is highly indicative of 1p19q retention, however the immunohistochemical test can be challenging to interpret and there have been a small number of discordant results.