Optimization and fabrication of a novel 3D-printed variable density range modulation device for proton FLASH beams

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.

FIGURE 1

(a) Schematic illustration of the proposed 2D modulated VDRMD with thickness $T$. The device has repeated square density patterns in the lateral plane and constant density along the depth direction. (b) The 2D density map of single segments consists of 16 pixels. $x_n$ represents the density of $n$th pixel. Unit vectors $u$ and $v$ represent the two lateral directions. (c) The depth dose of a SOBP (blue) modulated from one of the BP curves (red). R is the range of pristine BP and [$z_1,z_M]$ is the depth range of SOBP plateau.

FIGURE 2

(a) The calibration phantom and (b) the transverse plane of the CT scan. The central block has a 20% infill ratio and the upper left block has a 100% infill ratio. (c) The correspondence of relative electron density and SPR to infill ratio with HU. (d) The HU and infill ratio conversion curve.

FIGURE 3

(a) Central axis depth dose in water from a conventional ridge filter designed to achieve a 3 cm SOBP. (b) Central axis depth dose in water from three cone‐shaped and one randomly distributed VDRMDs. Solid blue lines represent the analytically optimized dose profiles based on the assumptions in Section II.2, and red dotted lines are the corresponding Monte Carlo simulation results. (c) 3D rendering of the simulated conventional ridge filter. (d) Corresponding density maps for the VDRMDs.

FIGURE 4

(a) 3D‐printed 6 $\times$ 6 $\times$ 3.8 cm³ VDRMD with 3 cm SOBP. (b) Axial view of CT scan and (c) sagittal view of CT scan. (d) Density profile along the selected line across the device. Blue solid line is the designed phantom and red solid line is the printed device. (e) From left to right: the original designed density pattern resampled to 0.2 mm resolution; the average of density patterns sampled from the printed device; the standard deviation of sampled patterns. (f) The density profiles along every five lines in the 50 $\times$ 50 pixels patterns. Line 10 is shown as red dotted arrow in Figure (e). The blue solid line is the designed density profile, the red solid line is the printed density and averaged over multiple samples, and the dotted line is the printed densities of the samples.

FIGURE 5

(a) The depth dose profiles under different proton beam energies for MC simulation (blue), MLIC measurement with the printed device (red), and MLIC measurement without the printed device in beam (yellow). (b) The experimental setup with MLIC. The printed VDRMD is positioned at the front window of the MLIC.

FIGURE 6

(a) Spot shape of a 180 MeV proton pencil beam without the modulator and with the modulator inserted, with an air gap (AP) ranging from 5 to 20 cm between the modulator and the Lynx surface. (b) Lateral dose profiles of a 5 × 5 cm² 180 MeV proton field measured at a 19 cm depth (mid‐SOBP plateau) with and without the device. (c) Extracted X and Y dose profiles from the selected line profile.