Abstract Book

S929

ESTRO 37

3 Heidelberg University, Faculty of Physics and Astronomy, Heidelberg, Germany 4 University Hospital, Radiation Oncology and Radiation Therapy, Heidelberg, Germany 5 Heidelberg Ion beam therapy center, HIT, Heidelberg, Germany Purpose or Objective Ion beam radiography provides an increased dose conformation to the tumor in comparison to standard radiotherapy using photons. Consequently, it poses increased demands on dedicated imaging methods. However, many of the novel imaging methods suffer from a non-availability of dedicated and clinically usable radiation detection devices. To avoid an averaging of the signal from particles of different kind, energy and direction, single particle tracking is crucial for both, imaging with ion beams and non-invasive monitoring with secondary ions. Material and Methods Single particle detectors, which are currently becoming available also for applied research, are attractive to be investigated for their potential in the next generation imaging techniques for ion beam radiotherapy. The capabilities of the pixelated semiconductor detector Timepix, which was developed at CERN, were analyzed for ion beam radiography and monitoring with secondary ions. This modular technology enables to build various radiation detection systems including particle trackers. Moreover, a unique developed method for the ion type identification allows to avoid signal degradation due to secondary particle background. These capabilities enabled us to develop new dedicated imaging methods, which were evaluated experimentally at the Heidelberg Ion Beam Therapy facility in Germany. Results The build ion radiographic system, the world-wide first approach based entirely on pixelated detectors, comprises a forward and a backward tracker, and a detector for energy loss measurements and particle identification. It allowed us to compare proton and helium ion based imaging under identical conditions. At the worst case position in an head-sized PMMA phantom, the spatial resolution was found to be 80% higher for helium ions (< 2 mm) than for protons. A 1 mm inhomogeneity was clearly visualized with helium ions at diagnostic dose level. The developed data processing, including the avoidance of the inherent contamination of the outgoing beam with secondary hydrogen fragments of helium, was shown to increase the contrast-to-noise by 350%. The mini-prototype of the treatment monitoring system based on tracking of secondary ions, which emerge from the irradiated head phantom, was evaluated during a therapy-like 12C treatment of an Alderson head phantom. The assembled detection system was found to be capable of an interfractional tracking of the scanned 12C pencil beam position in the lateral direction. Conclusion Pixelated semiconductor detectors provide unique radiation detection capabilities including single particle detection, tracking and identification. These are highly 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.