Quantitative Evaluation of the Robustness of Beam Directions Based on Power Spectral Analysis of Water-Equivalent Path Length Image in Charged Particle Therapy

Objective: To investigate the quantitative evaluation of the robustness of particle beam directions against patient setup errors in charged particle therapy. Methods: Power spectral analysis of target water-equivalent path length (WEPL) images in beam's eye views was employed for quantitative evaluation of the robustness of beam directions. The relationship between the beam direction and the 0th moment of the power spectrum was calculated for estimating the robustness of each beam direction. We applied the proposed evaluation method to 7 head-region cancer patients. Results: The mean of the 0th moment at the conventional beam directions, which were empirically selected by a manual method, was smaller than that for the avoided beam directions with a statistically significant difference (p < 0.05), which means that the conventional beam directions were appropriate from the theoretical point of view. Conclusions: The results in this preliminary study may lead to quantitative selection of beam directions based on the relationship between the beam direction and the 0th moment derived from the target WEPL image in charged particle therapy.