Astrocytomas are the most common type of childhood glioma, a type of brain tumor. Like all gliomas, astrocytomas consist of glial cells, which form the connective tissue of the brain. Astrocytomas can occur in both the brain and spine; but most often occur in the cerebellum (the part of the brain that coordinates movement and balance), optic nerve and in the brainstem. Anaplastic astrocytomas are a more aggressive type of astrocytoma.