PURPOSE: One of the components of radiotherapy (RT) in medulloblastoma/primitive 
neuroectodermal tumors is the prophylactic irradiation of the whole brain (WBI). 
With the aim of reducing late neuropsychologic morbidity a CT-scan-based 
dosimetric study was undertaken in which treatment was confined mainly or 
exclusively to supratentorial sites considered at high risk for disease 
recurrence.
METHODS AND MATERIALS: A comparative dosimetric study is presented in which a 
three field (two laterals and one posterior) proton plan (spot scanning method) 
is compared with a two-field conventional WBI 6 MV x-ray plan, to a 6-field 
"hand-made" 6 MV x-ray plan, and to a computer-optimized 9-field "inverse" 15 MV 
x-ray plan. For favorable patients, 30 Gy were delivered to the ventricles and 
main cisterns, the subfrontal and subtemporal regions, and the posterior fossa. 
For the unfavorable patients, 10 Gy WBI preceeded a boost to 30 Gy to the same 
treatment volume chosen for favorable patients. The dose distribution was 
evaluated with dose-volume histograms to examine the coverage of the targets as 
well as the dose to the nontarget brain and optical structures. In addition, the 
risks of radiation-related late neuropsychologic effects after WBI were 
collected from the literature and used to predict normal tissue complication 
probabilities (NTCPs) for an intelligence quotient deficit after treatment with 
photon or proton beams.
RESULTS: Proton beams succeeded better in reducing the dose to the brain 
hemispheres and eye than any of the photon plans. A 25.1% risk of an IQ score 
<90 was predicted after 30 Gy WBI. Almost a 10% drop in the predicted risk was 
observed when using proton beams in both favorable and unfavorable patients. 
However, predicted NTCPs for both optimized photon plans ("hand made" and 
"inverse") were only slightly higher (0.3-2.5%) than those of proton beams. An 
age-modifying factor was introduced in the predictive NTCP model to assess for 
IQ differences in relation with age at irradiation. Children with ages between 
age 4 to 8 benefitted most from the dose reduction in this exercise (similar 
NTCP predictions for both proton and "inverse" plans).
CONCLUSION: Modulated proton beams may help to significantly reduce the 
irradiation of normal brain while optimally treating the supratentorial subsites 
at higher risk for relapse. A decrease in morbidity can be expected from protons 
and both optimized proton plans compared to WBI.