Gated irradiation with scanned particle beams.

PURPOSE To demonstrate mitigation of the interplay effects of scanned particle beams and residual target motion within a gating window by increased overlap of pencil beams. METHODS AND MATERIALS Lateral overlap was increased by increasing the pencil beam widths or by decreasing the distance between the pencil beams (scan grid). Longitudinal overlap was increased by reducing the distance between iso-range slices. For scanned carbon ion beams, simulation studies were performed and validated experimentally to determine the required parameters for different residual motion characteristics. The dose distributions were characterized by the maximal local deviations representing local over- and underdosage. RESULTS For residual lateral motion, the local deviations were <5% for 2, 4, and 7 mm residual motion within the gating window for a 2-mm scan grid and pencil beams of 10, 14, and 18 mm full width at half maximum, respectively. Decreasing the iso-range slice distance from 3 mm to 1 mm effectively mitigated <or=10 mm water-equivalent range changes. Experimental data reproduced the simulation results. CONCLUSION In charged particle therapy with a scanned beam, interplay effects between gated beam delivery and residual target motion can be decreased effectively by increasing the overlap between pencil beams laterally, as well as longitudinally.