The effectiveness of combined gating and re-scanning for treating mobile targets with proton spot scanning. An experimental and simulation-based investigation

Organ motion is one of the major obstacles in radiotherapy and charged particle therapy. Even more so, the theoretical advantages of dose distributions in scanned ion beam therapy may be lost due to the interplay between organ motion and beam scanning. Several techniques for dealing with this problem have been devised. In re-scanning, the target volume is scanned several times to average out the motion effects. In gating and breath-hold, dose is only delivered if the tumour is in a narrow window of position. Experiments have been performed to verify if gating and re-scanning are effective means of motion mitigation. Dose distributions were acquired in a lateral plane of a homogeneous phantom. For a spherical target volume and regular motion gating was sufficient. However, for realistic, irregular motion or a patient target volume, gating did not reduce the interplay effect to an acceptable level. Combining gating with re-scanning recovered the dose distributions. The simplest re-scanning approach, where a treatment plan is duplicated several times and applied in sequence, was not efficient. Simulations of different combinations of gating window sizes and re-scanning schemes revealed that reducing the gating window is the most efficient approach. However, very small gating windows are not robust for irregular motion.