ESTRO 37 Abstract book

S932

ESTRO 37

If proper quality assurance steps are taken, 3D-printed objects can be used accurately and effectively in radiation therapy. It is critically important, however, that the properties of any material being used in patient care be well understood and accounted for. EP-1739 Phantom capable of testing Smart Deviceless4D – four-dimensional CT (4DCT) – system development. T. Kostecki 1 , A. Giłka 1 , A. Kawa-Iwanicka 1 , M. Stefańczyk 1 1 NU-MED CDiTO Katowice, Radiotherapy, Katowice, Poland Purpose or Objective The aim of the study is a development of a phantom capable of testing Smart Deviceless4D(SD4D) 4DCT system, to evaluate the system and to compare it to the Advantage4D(A4D) system. Material and Methods The scanning and registration methods were tested on Dynamic Phantom Model008A (CIRS, Norfolk, USA)(DPM) and the cone-shaped phantom (CSP), specially made for the purpose of presented study, as well as on radiotherapy patients scans. Delineations of structures 'External”, 'Lung” and 'Target” were made using Oncentra MasterPlan (Nucletron, Columbia, USA). The analysis of the obtained values were made using the volume of structures and Dice similarity coefficient (DSC). Results During testing of the systems with DPM, it turned out that the Maximum Intensity Projection (MIP) volume of the Target in the soft tissue window was 29,98ccm for A4D and 18,46ccm for SD4D. SD4D eliminates the need for an external surrogate by using a proprietary algorithm based upon internal anatomical features (i.e. change of the body and lung area and change in the density) (fig1).

Reconstructed Target volume of the CSP using SD4D in each phase remains within the range of 0,39% to 13,04% difference from the known value (4 ccm) while A4D ranged from 0,06% up to 19,63% difference in the "20%" phase reconstruction. At the point where the enlargement of the volume occurred, there was a stitching point of two cine-scan blocks, in which images were merged in slightly different moments of the cycle. The minimum volume of the "Lung" and "External" structures were for A4D respectively 320,78±0,03ccm and 649,08±0,05ccm in the '50%” phase. As for SD4D respectively 321,18±0,20ccm and 649,77±0,29ccm. The maximum of the volume had occurred in the unusual '60%” phase. Knowledge of the maximum exhalation is necessary to determine the peak-exhale location of the tumor. It should be noted that this could be a potential source of errors. MIP images of patients reconstructed using both systems were compared among them. Mean values of DSC in patients were 0,952±0,020. Conclusion The innovative binning of the Deviceless4D requires new testing methods that are suited for exactly those types of sorting systems. Proposed CSP device is capable of this task. The way to minimize the number of cine stitching points should be also discussed, as they affect reconstructed images. Obtained results have shown that, despite the differences in reconstruction techniques, both of the studied systems allow for correct determination of the volumes of interest. EP-1740 3D dose reconstruction in the patient from VARIAN EPID images for IMRT and VMAT treatments F. Younan 1 , J. Mazurier 1 , X. Franceries 2,3 , D. Franck 1 1 Oncorad Garonne, Haute Garonne, Toulouse, France 2 Centre de Recherche en Cancérologie de Toulouse CRCT, Haute Garonne, TOULOUSE, France 3 Laboratoire Plasma et conversion d'Energie LAPLACE, Haute Garonne, Toulouse, France Purpose or Objective The EPID panel (electronic portal imaging device) is increasingly used for dosimetric purposes in external radiotherapy. Two methods are currently available to verify the absorbed dose in the patient: the direct method, which consists on predicting the fluence, acquired with EPID, and the indirect one which redistribute the absorbed dose into the patient. This latter, also referred as a back-projection method, is used in our study to reconstruct the 3D absorbed dose matrix in the patient from EPID images for IMRT and VMAT fields. Material and Methods Images are acquired with the clinac 23iX aS-1000 imager (Varian) and a 6MV beam. To compensate for the lack of uniformity of the detector response and to take into account the influence of the backscattered photons coming from the robotic gantry, a pre-treatment is applied to the portal images. A calibration step is then performed to transform the gray levels of the pixels into absorbed dose in water via a response function. Correction kernels are also used in this step to redistribute the absorbed dose according to the field size and to take into account the penumbra. The next step is the retro-projection of the dose into the patient for all EPID parallel plans with a 1 mm resolution, for each gantry angle. Our algorithm, developed with Matlab version R2015b takes into account the photons scattered into the patient, the attenuation, the beam hardening with depth, the build-up effect, and the beam’s divergence into the patient.

fig1 Diagram of the Smart Deviceless 4D workflow (GE Healthcare) As the DPM has a fixed body, therefore SD4D was unable to properly create MIP reconstruction of the phantom. This led to the construction of the CSP(fig2).

fig2 Axial, sagittal, coronal and 3D reconstruction of the CSP phantom