Characteristics of movement-induced dose reduction in target volume: a comparison between photon and proton beam treatment.

We compared the main characteristics of movement-induced dose reduction during photon and proton beam treatment, based on an analysis of dose-volume histograms. To simulate target movement, a target contour was delineated in a scanned phantom and displaced by 3 to 20 mm. Although the dose reductions to the target in the 2 treatment systems were similar for transverse (perpendicular to beam direction) target motion, they were completely different for longitudinal (parallel to beam direction) target motion. While both modalities showed a relationship between the degree of target shift and the reduction in dose coverage, dose reduction showed a strong directional dependence in proton beam treatment. Clinical simulation of target movement for a prostate cancer patient showed that, although coverage and conformity indices for a 6-mm lateral movement of the prostate were reduced by 9% and 16%, respectively, for proton beam treatment, they were reduced by only 1% and 7%, respectively, for photon treatment. This difference was greater for a 15-mm target movement in the lateral direction, which lowered the coverage and conformity indices by 34% and 54%, respectively, for proton beam treatment, but changed little during photon treatment. In addition, we found that the equivalent uniform dose (EUD) and homogeneity index show similar characteristics during target movement. These results suggest that movement-induced dose reduction differs significantly between photon and proton beam treatment. Attention should be paid to the target margin in proton beam treatment due to the distinct characteristics of heavy ion beams.