Pathology of low-grade gliomas: an update of emerging concepts

Although the term low-grade glioma (LGG) is useful for its connotation of a slow-growing, better prognosis CNS primary neoplasm typically occurring in a young patient, it also serves as a potential diagnostic wastebasket, occasionally leading to conceptual errors, therapeutic uncertainty, or misinterpretation of clinical data. For example, the LGG designation is occasionally invoked as a justification for lumping together biologically unrelated entities such as pilocytic astrocytoma and diffuse astrocytoma. Whereas the former represents a benign and potentially surgically curable neoplasm that virtually never undergoes malignant transformation, the latter is a surgically incurable low-grade malignancy, prone to further malignant progression and eventual fatality. Therefore, although rare cases lacking a clear distinction may be encountered, the term LGG should be abandoned for a more specific diagnosis whenever possible. The primary goals of this paper are to review practical surgical pathology issues related to the diagnosis of diffuse LGGs and to update the reader on emerging clinicopathologic and molecular genetic concepts. Also discussed are current controversies of classification/grading and the role of ancillary testing via immunohistochemical and genetic techniques.