Author: Eric S. Diffenderfer, Lei Dong, Alejandro Garcia, Wenbo Gu, Michele M. Kim, Alexander Lin, Kai Mei, Peter B. Noël, Boon-Keng Kevin Teo, Lingshu Yin, Jennifer Wei Zou 👨🔬
Affiliation: Department of Radiation Oncology, University of Pennsylvania, University of Pennsylvania 🌍
Purpose: We present a novel 3D-printed range-modulating devices with spatially modulated density for FLASH particle therapy. By varying density distributions, spread-out Bragg peaks(SOBPs) can be generated from mono-energetic proton beams using a variable density range-modulating device(VDRMD). This study aims to demonstrate the flexibility of VDRMDs, compared to conventional uniform-density ridge filters, by stacking smaller modulators to create composite range modulators.
Methods: An integrated inverse optimization framework was developed to simultaneously obtain density maps for two stackable modulators (targeting 2cm and 3cm SOBPs) when coupled together produce an approximately 5cm SOBP. Modulator performance was simulated with MCsquare. The devices were fabricated using PixelPrint technology and Polylactic acid filament. Their range-modulating capabilities were measured using a multi-layer ionization chamber under mono-energetic proton beams, both individually and in a stacked configuration. Additionally, a calibration phantom with infill ratios 20%-100% was 3D-printed and imaged via dual-energy CT to characterize the properties of filament.
Results: The calibration phantom quantified the relationship between infill ratio with electron density, stopping power ratio, and Hounsfield units. Two 6×6cm² VDRMDs were optimized and printed with a pixel resolution of 1 mm and density ranging between 0.2 to 1.0g/cm³. CT scans showed good agreement between designed and printed densities, with mean density differences of 0.0195g/cm³ (3cm device) and 0.0231g/cm³ (2cm device). Measured using a 225MeV proton beam, these devices individually produced SOBP widths of 3.2cm and 2.1cm (proximal-distal depth at 90% isodose). When vertically stacked, the SOBP was 5.6cm. Measurements across multiple proton energies agreed with Monte Carlo predictions.
Conclusion: A novel stackable VDRMD for proton therapy was presented. High manufacturing fidelity was confirmed by CT imaging, and both simulations and measurements demonstrated the feasibility of creating wider SOBPs by stacking. This approach will enable a library of pre-fabricated universal modulators to be assembled for flexible clinical applications.