Diagnostic Uses of Molecular Testing
While the case studies in this article demonstrate the use of ancillary testing and provide a suggested strategy for properly subclassifying diffuse gliomas, inherent in this strategy is the assumption that, based upon the initial clinical and pathologic information available, one can accurately categorize the lesion as a diffuse glioma. In reality, such a distinction is not always a straightforward endeavor. It is well recognized that a proportion of low-grade, typically radiologically circumscribed, CNS neoplasms, such as pilocytic astrocytomas and glioneuronal tumors, may infiltrate the surrounding brain parenchyma. In addition, many of these low-grade CNS neoplasms also may have growth patterns that are shared with diffuse gliomas, a diagnostic challenge that often can be further hampered by the inherent limitations involved in obtaining adequate samples for diagnosis from the CNS.
Although there are limitations and caveats, molecular diagnostic testing may be invaluable in properly classifying CNS tumors in such situations. The finding of mutations in the IDH1 or IDH2 genes has been shown to be very valuable in distinguishing low-grade diffuse glioma from both nonneoplastic and low-grade circumscribed neuroepithelial neoplasms that may exhibit growth patterns that can mimic those of diffuse gliomas.15-17 Conversely, finding abnormalities in the BRAF gene in a brain neoplasm that has a low-grade morphology suggests that the lesion may represent one of these low-grade lesions such as a pleomorphic xanthoastrocytoma, pilocytic astrocytoma, or mixed neuronal-glial tumor as opposed to a diffuse glioma.18,19
Depending upon the environment in which one practices, small biopsy specimens may be prevalent, and unfortunately, it is not uncommon to obtain a biopsy that exhibits a histologic growth pattern that is discordant from what one would predict based on the clinical context and imaging findings. Molecular testing may be useful in resolving discordances in such situations. If a biopsy of a ring-enhancing lesion demonstrates a diffuse glioma that doesn’t meet WHO grade IV criteria, applying methodologies that look for genetic features commonly encountered in high-grade astrocytomas may identify genetic abnormalities that suggest a more aggressive lesion than is indicated by the histologic findings. The presence of genetic abnormalities such as homozygous deletion of the CDKN2A gene, TERT promoter mutation, loss of heterozygosity of chromosome 10q and/or phosphatase and tensin homolog (PTEN) mutations, EGFR gene amplification or the presence of the EGFR variant III are a few findings that would suggest the aforementioned sample may represent an undersampling of a higher grade diffuse astrocytoma, which would be important information to convey to the treating clinicians.20-26
Testing In the VA
The goals of the MPWG include promoting increased quality and availability of genetic testing within the VHA as well as encouraging internal referral testing. An informal survey of the chiefs of VA Pathology and Laboratory Medicine Services was conducted in November of 2017 in an attempt to identify internal VA pathology laboratories currently conducting testing that may be of use in the workup of diffuse gliomas (Table 1).
