ESTRO 38 Abstract book

S145 ESTRO 38

planning procedure in the presence of a target deformation. [1]Baldock et al., doi:10.1088/0031-9155/55/5/R01 [2]Mann et al., doi:10.1088/1361-6560/aa51b1 Results The VeroClear material used in the Objet30 Pro printer (Stratasys) showed no significant interaction with the gel and provided a homogeneous dose response with differences of <3 % in the homogeneous irradiation. In addition, a high geometric accuracy when measuring the small field size was found (FWHM: 8.82 mm in VeroClear vs 8.96 mm in BAREX TM ). Varying MR and CT imaging contrast can be realized by combining Agarose, Ni-DTPA and potassium chloride in various concentrations. First measurements showed a good performance of the DIR algorithms in case of linear component shifting but were not fully capable to detect rotations or changes in electron density like bone and air.

By comparing the results for the two cross calibrations the k B,M,Q can be determined for each chamber, and the variation within the chamber-type determined. Results

The factors within both PTW30013 and FC65-G chamber- types were found to be very consistent, with observed standard deviations of k B,M,Q for the PTW30013 of 0.19% (|- ) and 0.13% (//), and for the FC65-G of 0.15% (|-) and 0.17% (//). These variations are comparable with the observed measurement uncertainties (1σ) of 0.14% - 0.20%. Conclusion The consistency of the results for the PTW30013 and FC65- G chambers implies that for these two chamber types it is not essential to measure the k B,M,Q specifically for each individual chamber. Values for each chamber-type, magnetic field strength and relative chamber orientation can therefore be applied from the literature. OC-0291 Development of a deformable phantom for validation of adaptive irradiation methods in MRgRT A. Elter 1,2,3 , S. Dorsch 1,2,3 , P. Mann 1,2 , A. Runz 1,2 , W. Johnen 1,2 , S. Klüter 2,4 , C.P. Karger 1,2 1 German Cancer Research Center DKFZ, Medical Physics in Radiation Oncology, Heidelberg, Germany ; 2 Heidelberg Institute for Radiation Oncology HIRO, National Center for Radiation Research in Oncology NCRO, Heidelberg, Germany; 3 University of Heidelberg, Faculty of Physics and Astronomy, Heidelberg, Germany; 4 University Hospital Heidelberg, Department of Radiation Oncology, Heidelberg, Germany Purpose or Objective One of the key features of MRgRT is the online adaption of treatment plans to account for daily anatomy changes. Due to the complex adaption process, specific end-to-end tests need to be performed that require the development of new phantoms. Such phantoms should include: i. flexible and reproducible geometric insert positioning ii. various anthropomorphic MR and CT image contrasts iii. dose measurement methods (1D-3D) While (i) and (ii) are necessary to test the performance of the deformable imaging registration (DIR) method, (iii) allows verification of beam delivery. For this, a promising method is the use of 3D polymer gels (PG). However, due to the high reactivity with oxygen, its application is yet limited to few container materials and shapes. The aim of this work was to evaluate different 3D printing materials for the use with PG and to develop a phantom that meets (i)-(iii), using arbitrarily-shaped structures filled with PG. Material and Methods Using various 3D printing materials and techniques, PG containers were produced and filled with PAGAT [1] dosimetry gel. The containers were irradiated at a linear accelerator (Linac, Artiste, Siemens) with photons using either a homogenous large field or a small field (1x1cm²) to test the compatibility with the PG. The PG was evaluated by MRI and the results were compared with those obtained in already established BAREX™ containers [2]. Having found that the VeroClear printing material was compatible with the PG, the phantom was designed with variable components meeting requirements (i)-(iii). The phantom was used in a first experiment at a clinical MR- linac (MRidian, ViewRay) to test parts of the adaptive

Conclusion We found a 3D printing material compatible with PG allowing to create arbitrary shaped and PG-filled inserts that can be used for 3D dose measurements. In a first experiment, the developed phantom could be used to test the performance of a DIR algorithm and to trigger a re- optimization process. Future experiments will focus on 3D- dosimetric validation of the adapted treatment plans and the development of a suitable end-to-end test for the validation of complete adaptive irradiation procedures in MRgRT. OC-0292 When we have to apply volume corrections in dosimetry? K. Zink 1 , C. Andersen 2 , L. De Prez 3 , F. Delauny 4 , S. Duane 5 , C. Gomà 6 , M. Pimpinella 7 , P. Teles 8 , J. Tikkanen 9 , M. Pinto 7 1 Universitätsklinikum Marburg- Klinik für