EXPERIMENTAL STUDY OF THE TARGET MOTION EFFECT ON THE DOSE DISTRIBUTION IN SPOT-SCANNING BEAM PROTON THERAPY

This paper presents the experimental results on the intrafractional motion effect on dose distributions in a target in the context of spot-scanning beam proton therapy. The evaluation is performed by a qualitative analysis of the structure and shape of dose distributions and a quantitative analysis of the average dose and dose homogeneity within the region of interest for various motion parameters and acceleration cycles. The results show that intrafractional motion leads to a significant distortion of the dose field shape and structure. Distortions appear as hot and cold spots (overdosing and underdosing areas, respectively) dose field spread along the motion trajectory. These effects are reflected in a 16% reduction in the mean dose in the region of interest and dose homogeneity from 96.7 to 75.5% as the motion amplitude increases from 0 to 10 mm. In this case, the level of distortions weakly depends on time parameters of motion and the accelerator cycle. The observed effects lead to an efficiency degradation of proton therapy in the treatment of moving tumors; therefore, the use of such motion compensation methods as rescanning, gating, tracking, and their combinations is necessary for clinical practice.