title: Towards 3D printed multifunctional immobilization for proton therapy: Initial materials characterization.
abstract: OBJECTIVE
  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.
  
  
  METHODS
  Using four different 3D printing techniques, in total eight materials were subjected to testing. Samples with a nominal dimension of 20 × 20 × 80 mm3were 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 Zeff, 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.
  
  
  RESULTS
  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 Zeffranging from 5.91 to 10.43. The SPR and ρeboth 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 Zeff. As opposed to the photon attenuation, the proton range shifting appeared to be printing orientation dependent for certain materials.
  
  
  CONCLUSIONS
  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.
authors: Department of Oncology, Laboratory of Experimental Radiotherapy, KU Leuven - University of Leuven, Herestraat 49, Leuven 3000, Belgium.
authors: Steven
authors: S
authors: Michiels
authors: Department of Medical Engineering, Materialise NV, Technologielaan 15, Haasrode 3001, Belgium.
authors: Antoine
authors: A
authors: D'Hollander
authors: Department of Materials Science and Engineering, Ghent University, Technologiepark 903, Zwijnaarde 9052, Belgium.
authors: Nicolas
authors: N
authors: Lammens
authors: Department of Materials Science and Engineering, Ghent University, Technologiepark 903, Zwijnaarde 9052, Belgium.
authors: Mathias
authors: M
authors: Kersemans
authors: Department of Radiology, KU Leuven - University of Leuven, Herestraat 49, Leuven 3000, Belgium.
authors: Guozhi
authors: G
authors: Zhang
authors: Department of Radiotherapy and Oncology, Saint Luc University Clinics, Avenue Hippocrate 10, Woluwe-Saint-Lambert 1200, Belgium.
authors: Jean-Marc
authors: JM
authors: Denis
authors: Department of Radiation Oncology, University Hospitals Leuven, Herestraat 49, Leuven 3000, Belgium.
authors: Kenneth
authors: K
authors: Poels
authors: Department of Oncology, Laboratory of Experimental Radiotherapy, KU Leuven - University of Leuven, Herestraat 49, Leuven 3000, Belgium and Université catholique de Louvain, Center of Molecular Imaging, Radiotherapy and Oncology, Institut de Recherche Expérimentale et Clinique, Avenue Hippocrate 54, Woluwe-Saint-Lambert 1200, Belgium.
authors: Edmond
authors: E
authors: Sterpin
authors: Department of Oncology, Laboratory of Experimental Radiotherapy, KU Leuven - University of Leuven, Herestraat 49, Leuven 3000, Belgium and Department of Radiation Oncology, University Hospitals Leuven, Herestraat 49, Leuven 3000, Belgium.
authors: Sandra
authors: S
authors: Nuyts
authors: Department of Oncology, Laboratory of Experimental Radiotherapy, KU Leuven - University of Leuven, Herestraat 49, Leuven 3000, Belgium and Department of Radiation Oncology, University Hospitals Leuven, Herestraat 49, Leuven 3000, Belgium.
authors: Karin
authors: K
authors: Haustermans
authors: Department of Oncology, Laboratory of Experimental Radiotherapy, KU Leuven - University of Leuven, Herestraat 49, Leuven 3000, Belgium and Department of Radiation Oncology, University Hospitals Leuven, Herestraat 49, Leuven 3000, Belgium.
authors: Tom
authors: T
authors: Depuydt
mesh_terms: D006801:Humans; D007103:Immobilization; D055595:Mechanical Phenomena; D056888:Patient Positioning; D017785:Photons; D066330:Printing, Three-Dimensional; D061766:Proton Therapy / Q000379:methods*; D014057:Tomography, X-Ray Computed