Anaplastic astrocytomas are WHO grade III lesions, with imaging appearances and prognosis between those of diffuse low-grade astrocytomas (WHO grade II) and glioblastomas (WHO IV), and similarly, they are classified on the basis of IDH mutation as IDH-mutant, IDH-wild-type and NOS (when IDH status is unavailable).
On imaging, these tumors share common features with diffuse low-grade astrocytomas, however, they tend to present with contrast enhancement.
Anaplastic astrocytomas occur in adulthood with a peak incidence between 40 and 50 years of age, which is older than low-grade astrocytomas and younger than glioblastomas .
As is the case with most parenchymal brain tumors, patients typically present in one of three ways: seizures, focal neurological deficit or symptoms of increased intracranial pressure.
The key features present in anaplastic astrocytomas that are absent in low-grade tumors are mitotic activity and cellular pleomorphism. Unlike glioblastomas, however, they do not demonstrate necrosis or vascular proliferation.
CT appearances are intermediate, appearing as regions of low attenuation with positive mass effect. Enhancement is variable.
Anaplastic astrocytomas appear similar to low-grade astrocytomas but are more variable in appearance and a single tumor demonstrates more heterogeneity.
The key to distinguishing anaplastic astrocytomas from low-grade tumors is the presence of enhancement which should be absent in the latter (although one should note that variants, especially gemistocytic astrocytomas, can demonstrate enhancement). The pattern of enhancement is very variable .
Unlike glioblastomas, anaplastic astrocytomas lack frank necrosis, and as such central non-enhancing fluid intensity regions should be absent .
- T1: hypointense compared to white matter
- T2: generally hyperintense but can be heterogeneous in cases with blood calcification
- T2-FLAIR: relative hypointensity of most of the tumor except a hyperintense rim (T2-FLAIR mismatch sign)
- T1 C+ (Gd)
- very variable but usually at least some enhancement is present
- presence of ring enhancement suggests central necrosis and thus glioblastoma rather than anaplastic astrocytoma
- MR spectroscopy
- increased choline-to-creatine ratio
- NAA preserved or mildly depressed
- no significant lactate
- intermediate levels of myo-inositol (lower than low grade, but higher than GBM)
- MR perfusion: elevated cerebral blood volume
Treatment and prognosis
Compared to glioblastomas, there are relatively few trials looking at treatment regimens for anaplastic astrocytoma . General principles are the same, however, with surgical resection when possible being the treatment of choice with or without subsequent radiotherapy and/or chemotherapy. This depends on the treating clinician's preference, the degree of resection, patient demographics and whether or not the tumor has recurred.
As is the case with everything about anaplastic astrocytomas, the prognosis is also intermediate between low-grade astrocytomas and glioblastomas. Typically patients succumb to their tumor in 2-3 years, often with transformation into a glioblastoma .
The differential, given the heterogeneous and variable appearance of these tumors, is relatively wide and includes:
- other astrocytomas
- cerebral metastases
- often multiple
- located at grey-white matter junction
- absent NAA, absent myo-inositol
- ring enhancement
- older age group
- subacute cerebral infarct
- gyriform enhancement
- vascular territory
- tumefactive demyelination
- open ring enhancement