First experimental results of motion mitigation by continuous line scanning of protons

Mitigation of organ motion in active, scanning proton therapy is a challenge. One of the easiest methods to implement is re-scanning, where a treatment plan is applied several times with accordingly smaller weights. As a consequence, motion effects are averaged out. For discrete spot scanning, a major drawback of this method is the treatment time, which increases linearly with the number of re-scans. Continuous line scanning, on the other hand, eliminates the dead time between the positioning of each beam, and in this work, continuous line scanning has been investigated experimentally from the point of view of dose, penumbral width and its effectiveness for re-scanning. As shown by measurements in a homogeneous phantom, dose distributions delivered by continuous line scanning were comparable with those of discrete spot scanning for both geometric and realistic targets, with only a modest degradation of lateral penumbra in the direction of scanning. In addition, delivered dose levels have also been found to agree well between discrete and line scanning. With continuous line scanning, however, more re-scans could be applied without the artefacts seen in discrete spot scanning, with motions of up to 1 cm peak-to-peak amplitude being mitigated by 10 re-scans. For larger motion, in the interest of reducing the volume of irradiated normal tissue re-scanning should be combined with other motion mitigation techniques such as gating or breath-hold.