The ability to vary the proton energy (depth of beam penetration) and modulate 
the dose distribution at the end of range permits delivery of an increased dose 
to the designated cancer-containing volume with a reduced dose to overlying 
normal brain tissue. The evolution of childhood CNS malignancy following therapy 
is reviewed to identify radiation response variables indicating where the proton 
dose distribution will improve the therapeutic ratio. The review documents that 
of the 1262 children expected to develop CNS malignancy in 1989, only 43% will 
survive 5 years. About 75% of those with medulloblastoma and over 90% with 
astrocytoma die from persistent (in-field) disease. When the patient has been 
treated with radiation, it is accepted that disease persistence indicates the 
cancer dose was insufficient. Potentially 536 children could show an improved 
incidence of local control and improved survival from an increased cancer dose 
available from proton irradiation. As the total dose and volume of brain 
irradiated is increased about 1800 cGy, brain dysfunction increases, producing a 
spectrum of functional and intellectual deficits which are age and volume 
related. About 900 irradiated patients would have fewer in-field histologic and 
functional changes if the dose to normal brain, or the volume of brain 
irradiated, is reduced by an improved dose distribution. A proton beam treatment 
plan, delivering a cancer dose of 7400 cGy, is simulated for a thalamic 
astrocytoma. The dose distribution of this plan is compared with an x-ray plan 
used to treat a patient, in which a dose of 5400 cGy was delivered to the 
astrocytoma. Comparative isodose distributions and dose-volume histograms 
indicate a decreased integral dose to normal brain and a decreased volume of 
normal brain irradiated, even as the cancer dose is boosted 2000 cGy with 
protons.