Feasibility study to achieve ultra-fast treatment delivery to treat moving targets using universal and dynamic ridge-filter and high-intensity beams

To treat moving tumors effectively with PBS proton therapy, it is required to use motion mitigation techniques such as breath-hold, rescanning, and gating. However, these techniques are not efficient in terms of delivery time. Treatment delivery time in PBS proton therapy depends on beam-on time and the dead time (time required to change energy-layers and/or lateral position). To shorten the beam-on time, we increased the beam current reaching the patient by developing new beam optics for PSI’s PROSCAN beamline and Gantry 2. Experimentally, we obtained factor 5 high beam current while achieving clinical beam size at the isocenter. To reduce the dead time, we developed a novel energy modulation unit that comprises two identical ridge filters placed just before the isocenter. Both ridge-filters are movable relative to each other to change the Bragg peak’s characteristics dynamically. By using a dynamic ridge-filter we could reduce the number of energy layers and spots required to generate dose distribution while being independent of energy. For the feasibility study, we evaluated the effect of the ridge filter on three different geometrical shaped homogeneous water phantoms assuming single-beam plans with 1Gy/fraction. We calculated the dose delivery time using high beam currents and ridge-filter and compared it with the clinical setting. For all shapes, we managed to reduce the delivery time at least by 60% compared to clinically used settings. These methods to reduce delivery time are easily adaptable for other proton therapy facilities to treat moving targets