3D printing technology is investigated for the purpose of patient immobilization during proton therapy. It potentially enables a merge of patient immobilization, bolus range shifting, and other functions into one single patient-specific structure. In this first step, a set of 3D printed materials is characterized in detail, in terms of structural and radiological properties, elemental composition, directional dependence, and structural changes induced by radiation damage. These data will serve as inputs for the design of 3D printed immobilization structure prototypes.
Using four different 3D printing techniques, in total eight materials were subjected to testing. Samples with a nominal dimension of 20 × 20 × 80 mm3 were 3D printed. The geometrical printing accuracy of each test sample was measured with a dial gage. To assess the mechanical response of the samples, standardized compression tests were performed to determine the Young’s modulus. To investigate the effect of radiation on the mechanical response, the mechanical tests were performed both prior and after the administration of clinically relevant dose levels (70 Gy), multiplied with a safety factor of 1.4. Dual energy computed tomography (DECT) methods were used to calculate the relative electron density to water ρe, the effective atomic number Z eff, and the proton stopping power ratio (SPR) to water SPR. In order to validate the DECT based calculation of radiological properties, beam measurements were performed on the 3D printed samples as well. Photon irradiations were performed to measure the photon linear attenuation coefficients, while proton irradiations were performed to measure the proton range shift of the samples. The directional dependence of these properties was investigated by performing the irradiations for different orientations of the samples.
The printed test objects showed reduced geometric printing accuracy for 2 materials (deviation > 0.25 mm). Compression tests yielded Young’s moduli ranging from 0.6 to 2940 MPa. No deterioration in the mechanical response was observed after exposure of the samples to 100 Gy in a therapeutic MV photon beam. The DECT-based characterization yielded Z eff ranging from 5.91 to 10.43. The SPR and ρe both ranged from 0.6 to 1.22. The measured photon attenuation coefficients at clinical energies scaled linearly with ρe. Good agreement was seen between the DECT estimated SPR and the measured range shift, except for the higher Z eff. As opposed to the photon attenuation, the proton range shifting appeared to be printing orientation dependent for certain materials.
In this study, the first step toward 3D printed, multifunctional immobilization was performed, by going through a candidate clinical workflow for the first time: from the material printing to DECT characterization with a verification through beam measurements. Besides a proof of concept for beam modification, the mechanical response of printed materials was also investigated to assess their capabilities for positioning functionality. For the studied set of printing techniques and materials, a wide variety of mechanical and radiological properties can be selected from for the intended purpose. Moreover the elaborated hybrid DECT methods aid in performing in-house quality assurance of 3D printed components, as these methods enable the estimation of the radiological properties relevant for use in radiation therapy.
We would like to thank Walter Coudyzer and Hilde Bosmans (University Hospitals Leuven) for providing access to the DECT scanner. Wouter Crijns (University Hospitals Leuven) and Gilles Defraene (KU Leuven) are acknowledged for their support in the execution of the different experiments. The colleagues of UCL and the affiliated University Hospitals Saint-Luc are highly appreciated for various contributions, especially Nancy Postiau (UCL) for providing repeated access to the proton beam line, and Stefaan Vynckier and Antoine Delor for providing access to the Gammex Cheese Phantom. We are indebted to Eefje Verhoelst and Tom Cluckers (Materialise) for providing the 3D printed samples and for various 3D printing related support. Karin Haustermans is partly funded by the Research Foundation Flanders.
1. INTRODUCTION 2. MATERIALS AND METHODS 2.A. 3D printing technologies and materials 2.B. Printing of test object samples 2.C. Mechanical response measurements 2.C.1. Compression tests—Young’s modulus 2.C.2. Photon irradiation—Radiation impact 2.D. Radiological properties calculations based on DECT data 2.D.1. Reference samples and basic formulae 2.D.2. DECT image acquisition 2.D.3. Calculation of ρe 2.D.4. Calculation of Zeff 2.D.5. Calculation of SPR 2.E. Beam measurements 2.E.1. Photon attenuation measurements 2.E.2. Proton range shift measurements 3. RESULTS 3.A. Geometrical printing accuracy 3.B. Mechanical response 3.C. Radiological properties calculations based on DECT data 3.C.1. CT numbers 3.C.2. Relative electron densityρe 3.C.3. Effective atomic number Zeff 3.C.4. Stopping power ratio to water SPR 3.D. Beam measurements 3.D.1. Photon linear attenuation coefficient μtr 3.D.2. Proton range shift 4. DISCUSSION 5. CONCLUSIONS
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