Purpose: To design, fabricate and test a 3D‐printed filter for proton range spreading in scanned proton beams. The narrow Bragg peak in lower‐energy synchrotron‐based scanned proton beams can result in longer treatment times for shallow targets due to energy switching time and plan quality degradation due to minimum monitor unit limitations. A filter with variable thicknesses patterned on the same scale as the beam's lateral spot size will widen the Bragg peak. Methods: The filter consists of pyramids dimensioned to have a Gaussian distribution in thickness. The pyramids are 2.5mm wide at the base, 0.6 mm wide at the peak, 5mm tall, and are repeated in a 2.5mm pseudo‐hexagonal lattice. Monte Carlo simulations of the filter in a proton beam were run using TOPAS to assess the change in depth profiles and lateral beam profiles. The prototypes were constrained to a 2.5cm diameter disk to allow for micro‐CT imaging of promising prototypes. Three different 3D printers were tested. Depth‐doses with and without the prototype filter were then measured in a ~70MeV proton beam using a multilayer ion chamber. Results: The simulation results were consistent with design expectations. Prototypes printed on one printer were clearly unacceptable on visual inspection. Prototypes on a second printer looked acceptable, but the micro‐CT image showed unacceptable voids within the pyramids. Prototypes from the third printer appeared acceptable visually and on micro‐CT imaging. Depth dose scans using the prototype from the third printer were consistent with simulation results. Bragg peak width increased by about 3x. Conclusions: A prototype 3D printer pyramid filter for range spreading was successfully designed, fabricated and tested. The filter has greater design flexibility and lower prototyping and production costs compared to traditional ridge filters. Printer and material selection played a large role in the successful development of the filter.

Purpose: To compare calculations of the relative biological effectiveness (RBE) for a passively scattered carbon beam to that for a scanned beam with the same physical dose distribution in the spread out Bragg peak (SOBP). It is possible that differences in the spectrum of the fragments between the two beams could significantly affect the RBE, making it difficult to compare clinical doses from scattered and scanned carbon beams. Method and Materials: A simulation program based on the GEANT4 toolkit was developed to mimic measurements and other simulations from a passively scattering carbon beam at the NIRS in Japan. Closely spaced pristine Bragg peaks in water were simulated for both a scanned beam — where depth was adjusted by adjusting energy directly — and for a passively scattered beam — where depth was adjusted by changing the thickness of an aluminum degrader. Pristine peaks were then weighted to fit a pre‐defined SOBP dose distribution. RBEs for both beams were calculated using the linear‐quadratic survival model in use at NIRS. The depth dose, calculated RBE, and effective doses were then compared between the scanned beam simulation and the passive beam simulation. Results: No significant difference was found in the calculated RBE between the scanned carbon beam and the scattered carbon beam. After fitting to the SOBP region of previously published data, no significant difference was found in the dose, RBE, or effective dose between the scanned beams and the scattered beam. Conclusion: This work indicates that it should be possible to equate the biological effective doses between scattered and scanned carbon beams in so much as the LQ RBE model used can predict differences in the beams.