The WHO grading system is the most widely used system for grading diffuse astrocytomas (c.2022). It is an adaptation of the now-superseded St Anne-Mayo grading system (also known as the Daumas-Duport grading system).
Classification
Grade 1 is reserved for localized astrocytomas (see WHO classification of CNS tumors for a complete list), and as such, diffuse astrocytomas range in grade from grade 2 to 4:
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grade 2: low-grade infiltrative astrocytoma
cellular atypia only
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grade 3: previously known as anaplastic astrocytoma
increased mitotic activity
cellular anaplasia
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grade 4: previously equivalent to glioblastoma, now depends on molecular characteristics
vascular proliferation
necrosis typically with perinecrotic palisading
Gliomatosis cerebri does not have its own grade but rather is a highly infiltrative version of a low-grade or anaplastic tumor that involves at least three lobes 5. It does, however, confer a worse prognosis.
It is worth noting that there are other grading systems (see diffuse astrocytoma grading) and the pros and cons of each have been the subject of extensive debate.