IDH mutation & diffuse astrocytoma/anaplastic astrocytoma. IDH-wildtype is rare both for diffuse astrocytoma (some of these may turn out to be lower grade lesions such as gangliogliomas) and for anaplastic astrocytoma (some of which may be glioblastomas). Some, 1) but not all, 2) studies show little prognostic difference between IDH-mutant Grade II and Grade III astrocytomas. But IDH-mutant cases appear to do better than IDH-wildtype for both diffuse and anaplastic astrocytomas.
Tumors may lose their IDH mutation over time.
The overall age-standardized incidence is 3.5 per million person years with a peak incidence of 8 per million population in the 75 to 79 year age group.
By CT or MRI complex enhancement some may not enhance. Most glioblastomas enhance, but some rare ones do not.
Histology: compared to WHO grade II astrocytomas, there is a high degree of cellularity and increased mitotic activity (more than a few mitoses in a resection specimen), often along with additional signs of anaplasia (distinct nuclear atypia, multinucleated tumor cells & abnormal mitoses). These tumors stain positive for GFAP.
Anaplastic astrocytomas (AA) often exhibit complex enhancement on CT or MRI; however, some may not enhance. 3).
Ring enhancement as is typical in glioblastoma does not occur. Calcifications and cysts are seen in 10–20% of AA 4).
Anaplastic astrocytomas fall under the category of high grade gliomas (WHO grade III-IV), which are pathologically undifferentiated gliomas that carry a poor clinical prognosis. Unlike glioblastomas (WHO grade IV), anaplastic astrocytomas lack vascular proliferation and necrosis on pathologic evaluation.