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 the spine. Most often, they occur in the cerebellum (the part of the brain that coordinates movement and balance), optic nerve and brainstem. Anaplastic astrocytomas are a more aggressive type of astrocytoma.