Rare. Most tumors in adults with a histological appearance suggestive of diffuse astrocytoma that do not have an IDH mutation can be reclassified as other tumors with additional genetic testing. Since this group may consist of a variety of tumors with different clinical ramifications, it is considered a provisional category and does not have a WHO grade.
Entities which have been found to be incorrectly diagnosed as diffuse astrocytoma IDH-wildtype include:
Glioblastoma, conventional type or H3F3A mutated
Normal or reactive brain tissue 1).
The histological and molecular features and even the mere existence of diffuse astrocytoma, IDH wildtype, remain unclear. We therefore examined 212 diffuse astrocytomas (grade II WHO) in adults using IDH1(R132H) immunohistochemistry followed by IDH1/IDH2 sequencing and neuroimaging review. DNA methylation status and copy number profiles were assessed by Infinium HumanMethylation450k BeadChip. Only 25/212 patients harbored tumors without IDH1/IDH2 hotspot mutations and without contrast enhancement. By DNA methylation profiling, 10/25 tumors were classified as glioblastoma, IDH-wildtype, and an additional 7 cases could not be classified using methylome analysis, but showed genetic characteristics of glioblastoma. Histologically, all of these 17 tumors were low-grade diffuse astrocytomas. Nevertheless, 10/17 patients experienced early malignant progression. Other methylation classes included diffuse midline glioma, H3 K27M-mutant, diffuse astrocytoma, IDH-mutant, pilocytic astrocytoma, and normal or reactive brain tissue (total n = 8). In conclusion, no convincing diffuse astrocytoma, IDH-wildtype, was identified. Most IDH-wildtype tumors showing histopathological and radiological features of low-grade diffuse astrocytoma exhibit molecular and clinical features of high grade glioma and may represent an early stage of primary glioblastoma. The findings have implications for the biology, classification and neuropathological diagnosis of diffuse astrocytoma, IDH-wildtype in adults 2).