Low-Grade Gliomas

Excerpt

Tumors of the central nervous system (CNS) are classified according to the cell lineage of origin. Gliomas are neuroepithelial tumors that originate from glial cells within the CNS. Glial tumors are further classified based on the cell types involved, such as astrocytomas, ependymomas, and oligodendrogliomas. This review focuses on diffuse gliomas of low-grade pathology, specifically World Health Organization (WHO) grade 2 (diffuse infiltrating) gliomas.

In 2016, the WHO introduced a revised classification of CNS tumors that incorporated molecular profiling in addition to histopathology for the first time. In 2021, the WHO updated its classification of these tumors, some of which were diagnosed solely based on their genetic characteristics. Gliomas are now classified into 3 subtypes: isocitrate dehydrogenase (IDH)–mutant astrocytomas, IDH-mutant and 1p/19q-codeleted oligodendrogliomas, and IDH–wildtype glioblastoma. IDH-mutant astrocytomas are assigned WHO grades 2 through 4, with the distinction between WHO grades 2 and 3 based on mitotic activity. IDH-mutant astrocytomas that harbor mutations in CDKN2A/B are automatically designated WHO grade 4 astrocytomas. Oligodendrogliomas, IDH-mutant, and 1p/19q-codeleted tumors can be WHO grade 2 or 3.

The histologic grading is based on cytological atypia, mitotic activity, anaplasia, microvascular proliferation, and necrosis. High-grade gliomas demonstrate all these characteristics, whereas low-grade gliomas (LGGs) typically exhibit only cytological atypia or none of the other findings. LGGs generally are slower-growing tumors compared to high-grade gliomas. However, more than 70% transform into a higher grade or become aggressive within a decade. Pretreatment MRI study results have shown that LGGs typically grow at an average rate of 4.1 mm annually. Survival rates are favorable for low-grade gliomas compared to more aggressive grades. Thus, various factors should be considered, including the toxicity of chemotherapy, radiation therapy, and complications of surgical interventions, to manage LGGs and improve outcomes appropriately.