General
- First tumours to be defined by a specific methylation profile
- Aka: methylation-class anaplastic astrocytoma with piloid features (MC-AAP)
Definition
- Essential:
- An astrocytic glioma AND
- A DNA methylation profile of high-grade astrocytoma with piloid features
- Desirable:
- МАРК pathway gene alteration
- Homozygous deletion or mutation of CDKN2A and/or CDKN2B, or amplification of CDK4
- Mutation of ATRX or loss of nuclear ATRX expression
- Anaplastic histological features
Numbers
- Adult population with a median patient age of 40 years (4–88 years)
Pathology
- A mixture of histological features including those of a high-grade astrocytoma/glioblastoma and pilocytic-like features.
Genetic
- Alterations of the MAPK pathway are frequently combined with homozygous deletion in CDKN2A, CDKN2B, and/or ATRX mutations.
Grading
- More data are required to confirm a CNS WHO grade but current evidence suggests a clinical behaviour similar to grade 3.
Location
- Posterior fossa with the cerebellum being the most common site (74%).
- Recently, a reanalysis of cerebellar GBMs by methylation array revealed a moderately high proportion of HGAPs (25 of 86 patients)
Radiology
- Given that this is a newly recognised type, its radiological features and clinical characteristics are still emerging.
Imaging
- Rim enhancement with lack of central enhancement
- Hyperintense appearance on T2w
- Diffusion restriction and cyst formation did not appear to be features
- O-(2-[18F] fluoroethyl)-l-tyrosine positron emission tomography might be a useful tool for this tumour type.
- Limited paediatric population descriptions
- Nevertheless, they described a single case of HGAP in an 8-year-old girl with a parietal tumour who underwent resection followed by chemoradiotherapy and had an overall survival of 37 months [95]. Like PA, there does appear to be an association between HGAP and NF1
Prognosis
- Poor prognosis
- Marginally better than IDH-wildtype GBMs
- Comparable to that of IDH-mutant astrocytoma
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