Determination of the water equivalent thickness of 3D printed samples for the therapeutic proton beams

Abstract

The objective of this work is to investigate the possibility of using products made by three-dimensional printing methods to solve tasks related to proton radiotherapy. Three-dimensional printing can be widely used for the production of dosimetric phantoms, as well as immobilisation and shaping devices, due to its ability to produce objects with complex shapes. In medical physics, the concept of water equivalent thickness is used as a parameter to determine the properties of a material with respect to its interaction with protons. This parameter is determined from Bragg curves obtained in a water phantom. In this study, experimental and Monte Carlo based calculations were used to determine Bragg curves for proton beams with energies of 100 and 150 MeV. Bragg curves were obtained in a water phantom for an open field and with samples of given thickness made of different materials using different 3D printing parameters. The plastic samples investigated were created by fused filament fabrication with fill factors of 80 %, 90 %, and 100 %. The resulting physical density values ranged from 0.97 to 1.18 for natural PLA plastic (polylactide) and from 1.14 to 1.35 for BFCopper (polylactide base with copper impurities) samples. Several characteristics of the Bragg curves, including Bragg peak position, 80 % dose distal falloff and water equivalent thickness, were determined for all samples. The relative water equivalent thickness values for the 3D printed samples investigated ranged from 1.0 to 1.3. The obtained data were compared with known values for materials commonly used in proton beam therapy. It was shown that the values of the relative water equivalent thicknesses of the materials studied were close to those of known tissue equivalent materials. This indicates that products produced through fused filament fabrication can be used for proton beam therapy applications.