ESTRO 37 Abstract book

S930

ESTRO 37

attractive for the development of a new generation of dedicated imaging techniques based on ions. EP-1736 Comparison of two commercial 2D-array phantoms for patient-specific QA of stereotactic treatments A. Nevelsky 1 , E. Borzov 1 , S. Daniel 1 , R. Bar-Deroma 1 1 Rambam Health Care Campus - Faculty of Medicine, Oncology, Haifa, Israel Purpose or Objective Stereotactic radiotherapy treatments (SRT) are increasingly used in modern radiotherapy. Such treatments strongly rely on the use of small fields, either intensity modulated or conformal. The dosimetry of small radiation fields is not trivial due to the problems associated with lateral electron disequilibrium and requires reliable quality assurance (QA) tools. Commercial 2D arrays which are commonly used for QA of IMRT plans can be also applied for QA of SRT plans. However the accuracy of measurements with standard QA phantoms may be limited due to the lack of spatial resolution. PTW 1000SRS is detector array specifically designed for QA of small-fields and characterized by high spatial resolution. The objective of this work was to compare QA results of SRT plans measured with Scandidos Delta4+ phantom with High Resolution option applied and PTW Octavius 4D phantom with 1000SRS detector array. Material and Methods SRT plans of ten previously treated patients were selected for this study. The plans were created with Monaco TPS and delivered with Elekta Versa HD linac. TPS doses were calculated using 1 mm spatial grid and 1% calculation uncertainty. Detectors in the Octavius 1000SRS array are separated by 2.5 mm in its central region. Delta4+ phantom is characterized by 5 mm distance between detectors in its central region while the High Resolution option may be applied to improve the spatial resolution by a factor of 2. For this purpose the measurements were carried out twice: with phantom position at the linac isocentre and with 2.5 mm transverse shift; the measured doses were then merged to obtain High Resolution results. Prior the measurements, the same setup and output correction procedure was applied for both phantoms. For each plan, Gamma 3D index was calculated with 3%/2mm, 2%/2mm and 1%/1mm criteria using the global maximum and 20% dose threshold. For correct results comparison Gamma 3D for Octavius phantom was calculated as mean value for sagittal and coronal slices. Results All results of Gamma 3D index are presented in Fig.1. For the Delta4+ phantom, no difference was observed between gamma indexes obtained with and without the High Resolution mode, for all gamma index criteria. Gamma index results obtained with the Delta4+ phantom were similar to those obtained with the Octavius SRS1000 array for 3%/2mm and 2%/2mm criteria. Only for the 1%/1mm criterion there was statistical difference (p<0.05) between gamma indexes obtained with two different phantoms. However, the 1%/1mm criterion is very rarely used clinically and the difference in gamma indexes obtained with this criterion cannot be related to the difference in spatial resoluton.

Conclusion QA results for SRT plans based on gamma index evaluation did not depend on the spatial resolution of the measurement arrays. This may indicate that the standard 2D array phantoms such as Delta4+ designed for patient- specific QA of IMRT plans can be successfully used also for patient-specific QA of SRT plans. EP-1737 The ADAM-pelvis phantom: from DICOM data to a patient-like model W. Johnen 1,2 , N. Niebuhr 1,2,3 , A. Runz 1,2 , G. Echner 1,2 1 German Cancer Research Center dkfz, Medical Physics in Radiation Oncology, Heidelberg, Germany 2 National Center for Radiation Research in Oncology, Heidelberg Institute for Radiooncology, Heidelberg, Germany 3 Heidelberg University, Department of Physics and Astronomy, Heidelberg, Germany Purpose or Objective Anthropomorphic phantoms enable controlled simulation of organ motion for investigations in MR-guided radiotherapy. These phantoms should enable a systematic end-to-end testing including MRI and CT as well as dose measurements. This was realized with the realization of the ADAM-pelvis phantom (Anthropomorphic, Deformable And Multimodal). Material and Methods MRI and CT DICOM data of patients is used as template to build organ- and bone-models by using segmentations and transforming these to virtual models using the software Geomagic® Freeform® and Autodesk Inventor. Modifications e.g. compartments, production of casts for casting moulds, cavities, joining elements, application for localizers and pockets for dosimetric films or optically stimulated luminescence detectors (OSLD) are inserted for production of the models. Different 3D-printing methods are used for production of the casts and bone-models, whereas the organ-models are built with variable cast techniques using different types of silicone. The type of silicone which is used was optimized by means of its magnitude of elasticity, stability and imaging properties in MRI and CT. Peanut oil, differently loaded agarose gels, and Vaseline® were used as surrogates for the different types of tissues e.g. adipose, muscle and bone marrow. Results : The cast was realized using a cylindrical PMMA case to build a complete pelvic phantom that includes pelvic bones, bladder, prostate and rectum. To imitate interfractional volume variations of the bladder, it can be filled with varying volumes of water. Rectum dilatation is mimicked with a balloon which can be inserted and inflated with water or air. To localize bladder and