ESTRO 37 Abstract book

S1208

ESTRO 37

cylindrical phantom at the field size of 10cm×10 cm for 6MV photon beam, with the gantry angle varying from 180° to 90° in 10° increment. Each experimental setup was further simulated in the TPS. By adjusting the relative-to water electron density (ED) values of the couch model, the measured attenuation was replicated. The model accuracy of the model A and model B were evaluated by comparing the measured and calculated results at the minimum computational grid (2mm) and maximum computing grid (5mm), respectively. Results The maximum measured and calculated percentage deviation was 4.64 % for the phantom at L position with 5 mm grid at gantry angle 120°. The couch model was included in the TPS at three position, for model A and B under 2 and 5 mm voxel grid size, the maximum mean absorbed dose with couch was reduced to 0.61%, 0.84%, 0.71% and 1.03%, respectively, from 3.69% without couch. Conclusion Model A has a good agreement between measured and calculated dose distributions for all different voxel grid sizes and gantry angles. It can accurately describe the dose perturbations due to the presence of the couch and should therefore be used during treatment planning. EP-2182 Use of 3D printing to generate patient- specific electron beam aperture blocks S. Michiels 1 , B. Mangelschots 2 , C. Devroye 2 , T. Depuydt 1,2 1 University of Leuven, Department of Oncology, Leuven, Belgium 2 University Hospitals Leuven, Department of Radiation Oncology, Leuven, Belgium Purpose or Objective Electron beam collimators for non-standard field sizes or for irregular field shapes are typically fabricated in Cerrobend using Styrofoam molds to shape the aperture. Additive manufacturing is finding its way into several applications in medicine and an increasing number of radiation oncology centres have a 3D printer available on- site. This technology allows the creation of patient- specific molds using a multi-functional 3D printer which can also be used to produce a wide range of other custom-made treatment auxiliaries within the department, such as bolus, dosimetry phantoms and immobilization. This proof-of-concept study describes a 3D-printing-solution for the creation of patient-specific aperture blocks including a dosimetric comparison with conventionally produced apertures. Material and Methods After electron beam treatment planning and determining the aperture of a clinical treatment field, the RT PLAN DICOM was exported from the treatment planning system. From this DICOM-file, a patient-specific 3D-printable mold was created and exported as a standard tessellation language file (.stl-file) using open source programming (Python v3.6) and computer-aided design software (FreeCAD v0.16). The patient ID-number was integrated in the mold and a key was foreseen to attach the mould to a 3D-printed positioning device, ensuring correct alignment of the mold in the block tray (Fig. 1A). This reusable positioning device was designed uniquely fitting onto the block tray and 3D printed as well (Fig. 1B). Finally, two aperture blocks were cast, one using the conventional clinical workflow with a Styrofoam mold and one using the 3D printed mold and positioning device (Fig. 1C). Using these aperture blocks, the clinical plan was delivered onto a polystyrene slab phantom, with radiochromic film positioned perpendicular to the beam axis at a clinically relevant depth of 2 cm. The dose distributions and transversal profiles were compared between both delivered plans. Results Figure 1 shows the patient-specific 3D printed mold and positioning device and the resulting aperture block.

Figure 2A shows the acquired dose distribution using an aperture block created with the 3D printed mold and positioning device. Figures 2B and 2C show line profiles along the X- and Y-axis for both aperture blocks. A gamma index agreement score of 98.5 % between both 2D dose distributions was found, using 1.5% dose-difference and 1.5 mm distance-to-agreement acceptance criteria. Conclusion 3D printing allows for a standardized, operator-friendly workflow for the creation of patient-specific electron beam aperture blocks without the need for specific equipment to fabricate molds. It offers possibilities to increase safety and quality of the process including patient identification and methods for accurate mold positioning.