Variable versus fixed modulation of proton beams for treatments in the cranium

Dose distributions in the cranium with fixed and with variably modulated proton beams were compared. The variable modulation was designed to tailor the proximal high-dose region of each field to the target volume surface whereas the fixed modulation beams had a constant modulation determined by the greatest extent of the target. Dose-volume histograms of normal tissues were compared, as were the estimated complication probabilities. Twelve patients with chordomas or chondrosarcomas of the base of skull who had been treated to approximately 70 cobalt Gray equivalent (CGE) were evaluated. Dose distributions of the actual treatments were compared to those which would have been delivered had the proton beams been variably modulated; two patients for whom x-ray plans were available were also evaluated. The greatest difference in dose between the variable and fixed modulation proton beams, averaged over all the patients, was 13.8 CGE (8.0-18.0 CGE range). Much of this reduction occurred in the brain, particularly the temporal lobes. In those temporal lobes receiving significant doses, variable modulation reduced the volume receiving more than 54 CGE by 3.0 cc; all temporal lobes had at least a 5 CGE difference to some portion, half had more than 10 CGE and three more than 15 CGE difference to some portion. The optic structures, brainstem and spinal cord received from 1 to 3 CGE less dose with the variability modulated beams. Eight of the parotid glands received more than 20 CGE to more than half their volume with the fixed modulation beams; in these, variable modulation reduced the mean dose by 5.3 CGE. The reduction in integral dose with variable as compared to fixed modulation was in the range 3 to 12%; this gain was considerably less than the gain for uniformly modulated proton beams over x-rays in the two patients for whom x-ray plans were available.