Motion management in scanned particle therapy: beam gating & tracking

Abstract

Treatment of intra-fractionally moving targets with scanned ion beams requires motion mitigation techniques due to interplay effects. We implemented Gating (paused irradiation) and Beam Tracking (position adaptive irradiation) at GSI and performed experimental studies to validate both techniques. Gating requires mitigation of interplay effects within the gating window. An increased overlap, e.g. larger beam spots at constant spacing of rasterpoints was successfully tested. At 5 mm gating windows and 1 mm spacing, 10 mm FWHM beam spot sizes are required. Beam Tracking accuracy was studied in comparison to stationary irradiations with an ionization chamber array. Within the target volume deviations of 0.3 ± 1.5 % were measured. Clinical implementation at the Heidelberg Ion Beam therapy (HIT) will start with Gating. The mid-term goal is Beam Tracking because treatment planning studies for lung tumors showed that Tracking results in a reduced dose to the ipsilateral lung in comparison to Gating.