Comparison of proton and x-ray conformal dose distributions for radiosurgery applications.

Highly focused dose distributions for radiosurgery applications are successfully achieved using either multiple static high-energy particle beams or multiple-arc circular x-ray beams from a linac. It has been suggested that conformal x-ray techniques using dynamically shaped beams with a moving radiation source would offer advantages compared to the use of only circular beams. It is also thought that, generally, charged particle beams such as protons offer dose deposition advantages compared to x-ray beams. A comparison of dose distributions was made between a small number of discrete proton beams, multiple-arc circular x-ray beams, and conformal x-ray techniques. Treatment planning of a selection of radiosurgery cases was done for these three techniques. Target volumes ranged from 1.0-25.0 cm3. Dose distributions and dose volume histograms of the target and surrounding normal brain were calculated. The advantages and limitations of each technique were primarily dependent upon the shape and size of the target volume. In general, proton dose distributions were superior to x-ray distributions; both shaped proton and shaped x-ray beams delivered dose distributions which were more conformal than x-ray techniques using circular beams; and the differences between all proton and x-ray distributions were negligible for the smallest target volumes, and greatest for the larger target volumes.