Dr. Martinez of Thomas Jefferson University Hospital has no relevant financial relationships to disclose.)
This article includes discussion of oligodendroglioma and oligoastrocytoma, oligodendroglioma, anaplastic oligodendroglioma, oligoastrocytoma, and anaplastic oligoastrocytoma. The foregoing terms may include synonyms, similar disorders, variations in usage, and abbreviations.
Oligodendrogliomas are diffusely infiltrating, slow-growing, and chemoresponsive gliomas defined by the presence of mutated isocitrate dehydrogenase 1 (IDH1) or IDH2 and whole-arm chromosomal losses of 1p and 19q. The integration of genetic parameters IDH, ATRX, and 1p/19q has resulted in fewer diagnoses of oligoastrocytoma and improved correlations with prognosis and treatment response for diffuse gliomas as a group. The authors review the evolving approach to the diagnosis and management of oligodendroglioma and oligoastrocytoma with respect to the 2016 World Health Organization classification update.
• Oligodendrogliomas are diffusely infiltrating, slow-growing, and chemoresponsive gliomas defined by the presence of mutated IDH1 or IDH2 and whole-arm chromosomal losses of 1p and 19q.
• The term oligoastrocytoma is infrequently used as molecular analysis facilitates reclassification of this group into oligodendroglioma or astrocytoma; however, rare cases of dual-genotype oligoastrocytoma have been reported.
• The goal of treatment is to prolong survival and delay further progression, and a multidisciplinary approach with surgery, radiation, and chemotherapy is often warranted.
• The optimal timing of postsurgical treatment is variable and may be guided by assessing the risk of early progression against the risk of treatment-related adverse effects.
Historical note and terminology
In 1926, Bailey and Cushing conceived a classification system for glial tumors and are credited with coining the term oligodendroglioma. Their system relied on histopathological features and emphasized morphological similarities with different neuronal cell types, such as oligodendrocytes and astrocytes, at different developmental stages. Tumors exhibiting histologic characteristics of both astrocytoma and oligodendroglioma were referred to as mixed glioma or oligoastrocytoma. A grading system for gliomas initially proposed by Kernohan in 1938 and modified by others acknowledged a possible correlation between the degree of tumor anaplasia and biological behavior. In 1979, the World Health Organization published its initial consensus classification and grading system for central nervous system neoplasms. It was internationally well-received, and periodic updates maintain its relevancy.
Before the 2016 update of the World Health Organization classification system, oligodendroglial tumors were segregated into 2 groups: grade II (low-grade) and grade III (anaplastic). Although the favorable prognosis of oligodendroglioma over astrocytoma had long been recognized, the clinical significance could not be defined by histology alone. There was little reason to distinguish oligodendroglioma from astrocytoma, let alone oligoastrocytoma, particularly in the context of clinical trials. This changed following initial reports of the chemosensitive nature of oligodendroglioma, resulting in increased attention to such tumors (Cairncross et al 1998).
The discovery of molecular and genetic alterations, such as simultaneous whole-arm deletions of chromosomes 1p and 19q (1p/19q codeletion) and mutations in isocitrate dehydrogenase (IDH), offered further insight into the biology and behavior of diffuse gliomas. Both features are associated with improved prognosis. The prognostic and predictive significance was striking enough to warrant revision of the existing World Health Organization classification for brain tumors to integrate molecular analysis into the overall diagnosis. Oligodendroglioma is now defined by the presence of IDH mutation and whole-arm 1p/19q loss; the presence of an IDH mutation in the absence of 1p/19q codeletion results in a diagnosis of astrocytoma. As a consequence, the term oligoastrocytoma is irrelevant except in the absence of diagnostic molecular testing, in which case the designation “not otherwise specified” (NOS) is used. Reports of gliomas exhibiting genetic evidence of both oligodendroglioma (1p/19q codeletion) and astrocytoma (TP53 and ATRX mutations) are rare and likely represent 2 morphologically and molecularly distinct subclones within a single IDH-mutant diffuse glioma.
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