Abstract
BACKGROUND: For proton FLASH therapy, range-modulating devices are inserted in the beam path to create a spread-out-Bragg-peak (SOBP) for ultrafast delivery using a single energy pencil beam scanning technique. Current design typically consists of uniform density spikes with range modulation achieved by changing the area and height of the spikes, which has limited structural stability and modulation flexibility. PURPOSE: We present a new class of 3D-printed range-modulating devices for particle therapy with spatially modulated density. METHODS: PixelPrint technology (Laboratory for Advanced Computed Tomography Imaging, University of Pennsylvania, PA) was used to 3D-print the variable density range-modulator, by continuously varying the ratio of filament to air in each voxel. With specific thickness and spatial density modulation, SOBP of varying widths can be created. A calibration phantom was 3D printed and scanned by a dual-energy computed tomography (CT) scanner to characterize the physical and radiological properties of the PixelPrint technology. We developed an inverse optimization algorithm to generate the density map for producing SOBP from monoenergetic proton beam and verified by MCsquare (http://www.openmcsquare.org/), an open-source Monte Carlo (MC) simulation platform. The range modulation characteristics were measured using a multi-layer ionization chamber (MLIC) under monoenergetic proton field irradiation. RESULTS: The proposed optimization framework generated the density distributions for multiple SOBP widths. MC simulation verified the width and flatness of created SOBPs. The CT scan of a 3-cm SOBP modulator showed good fidelity of the desired density distribution, except for the highest density regions. MLIC measurements confirmed the accuracy of the produced SOBP with multiple proton beam energies. CONCLUSION: A novel variable density range-modulating device for proton therapy was successfully developed. These devices have the potential to be handled easily and significantly speed-up proton therapy treatment delivery.