Foundations of the Diagnosis and Management of Low-Grade Gliomas

In the past, low-grade gliomas—World Health Organization (WHO) grade I and II tumors—were generally expected to have a much better prognosis than higher-grade (WHO grade III and IV) gliomas. However, diffuse gliomas (WHO grade II), unlike WHO grade I gliomas, are by definition infiltrative, limiting resection and potentially contributing to poor outcomes like those seen with malignant gliomas. Rapid progress in the understanding of the pathogenesis of these tumors indicates that specific molecular factors, especially isocitrate dehydrogenase mutation status and the presence or absence of the 1p/19q codeletion (deletion of the short arm of chromosome 1 and long arm of chromosome 19), are much more important than grade in determining prognosis and response to treatment. These molecular characteristics outweigh the histologic distinctions and have been quickly incorporated into the WHO classification of gliomas. Management of these tumors with surgery, radiation, and chemotherapy has similarly been transformed by these developments, highlighting the need for a customized approach for patients with low-grade gliomas.