Integrate range shifter in immobilization for proton therapy: 3D printed materials characterization

3D printing is investigated for application in patient immobilization during proton therapy (PT). It potentially enables a merge of immobilization, range shifting and other functionality into one patient-specific structure. Beside minimizing the lateral beam spread due to the removal of air gap it could also reduce the collision risk and the treatment time compared to movable nozzle snouts. In this first study, 9 different 3D printed materials were characterized in detail. The resulting data (Table 1) will serve as input for the design of a printed immobilization structure. The printed test objects showed reduced geometric printing accuracy for 3 materials. Compression testing yielded Young’s moduli from 0.6 MPa to 3445 MPa, without deterioration after exposure to 100 Gy in a MV photon beam. Dual-energy CT methods were used to estimate the effective atomic number Zeff, the relative electron density e and the stopping power ratio SPR. Zeff ranged from 5.91 to 10.43. The SPR and e both ranged from 0.6 to 1.22. The measured photon attenuation coefficients at therapeutic energies scaled linearly with e. In a 62 MeV proton beam, 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 was printing orientation dependent for certain materials. In conclusion printed materials exhibit a wide variation in structural and radiological properties. The quantification of these characteristics enables optimal material selection for the design of a multifunctional 3D printed immobilization structure for PT