ESTRO 37 Abstract book

S929

ESTRO 37

carried out to study the optimal measurement geometry and choice of transducer. The dose distributions were calculated with Monte Carlo code FLUKA, while the acoustic simulations were performed with analytical wave transport code k-Wave. The temporal profiles of the dose pulses, in the order of µs, were measured with a scintillating crystal coupled with a PMT and used as an input to the acoustic simulation. Measurements were performed in a Cyberknife™ radiosurgery beam and a PBS proton beam. Both small and wide fields were irradiated on a water tank and in some cases, in order to amplify the acoustic signal, a lead block was submerged in water and placed partially or totally in the irradiation field. Electromagnetic shielding was required for all sensitive electronic components due to the presence of strong electromagnetic fields in the treatment rooms.

analysis, can be used to monitor the position of the proton Bragg peak with mm accuracy. EP-1735 Pixelated semiconductor detectors for next generation imaging in ion beam radiotherapy M. Martisikova 1,2 , T. Gehrke 1,2,3 , R. Felix Bautista 1,2,3 , C. Amato 1,2 , G. Arico 1,2,3,4 , B. Hartmann 1,2,3,4 , R. Gallas 1,2,3 , M. Reinhart 1,2,3 , T. Gaa 1,2,3 , O. Jäkel 1,2,5 1 German Cancer Research Center DKFZ, Department of Medical Physics, Heidelberg, Germany 2 National Institute for Radiation Oncology NCRO, Heidelberg Institute for Radiation Oncology HIRO- Heidelberg- Germany, Heidelberg, Germany 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

Figure 1. Scheme of the experimental setup

Results The experimental setup was optimized according to the results of the Monte Carlo and analytical simulations. According to spectral analysis, the optimal frequency range of ultrasonic transducers was found to be around 200 kHz. The bandwidth of transducers had a strong impact on the detectability of the photoacoustic signal. If lead blocks are used to amplify the signal, its size must be chosen so that its resonance frequency matches that of the transducer. Photo and protoacoustic signals were detected with both photon and proton beams and its shape, position and intensity are in the range of the predicted values.

Figure 2. Measured VS simulated acoustic waves

Conclusion Photoacoustic simulation with FLUKA and k-Wave can reproduce the experimental response of acoustic transducers in dose monitoring applications. The proposed setup can detect photoacoustic and protoacoustic signals originating from the penumbral areas of the treated fields and with the relevant image