Gliomatosis Cerebri

I was diagnosed yesterday following my biopsy post op. I am a 50 year old male who has been a paramedic for 30 years. I have a 7 year old daughter. I Had a case of vertigo 7 months ago while working on the ambulance. CT scan showed a lesion in the right frontal lobe. Neurologist got an MRI, lumbar puncture, and labs before then referring me to a neurosurgeon oncologist in New Orleans who then got a perfusion MRI and did a biopsy. Suppose to start chemo and whole brain radiation in 2 weeks. Since the biopsy I’ve had transient pins and needle like tingling in my left arm and leg. The doctor has put me on Keppra. My biopsy showed grade 2 astrocytoma glioma with only 1 mitotic division seen. All scans and MRI’s are stable with no changes. If gliomatosis is so aggressive and kills so quickly, why?