Transparent Photon Detector For The Online Monitoring Of IMRT Beams

Introduction: An innovative Transparent Detector for Radiotherapy (TraDeRa) has been developed for radiotherapy quality assurance (QA). It consists in a pixelated matrix of ionization chambers with a patented electrodes design. Each electrode is connected to in-house designed specific integrated circuits, providing a map of beam intensity and shape, at the linac pulse-scale. The detector aims at real-time monitoring of modulated beam upstream to the patient during delivery sessions, with a field cover up to 40 × 40 cm2. The work described here was realized on a 1:4 scale prototype in order to evaluate the dynamic of measurements, to characterize the raDeRa ability for detecting leaf position errors and to determine the impact of the detector presence on the photon beam. Methods: An acquisition system without charges losses and an electronic calibration procedure were developed to homogenize the response of each electrode and associated readout channel, allowing significant reduction of acquisition biases. An analysis method which takes into account the statistical calibration and measurement errors provides the relevant intensity deviations between two accumulated images of different acquisitions. The measurements under irradiation were performed with a clinical 6 MV X-Ray beam and a low energy high intensity photon beam from the European Synchrotron Radiation Facility (ESRF). Dose calculations are performed with the Monte Carlo code PENELOPE, modeling the full accelerator head and the TraDeRa detector. Results: A 2% attenuation of the 6 MV beam was measured in the presence of TraDeRa and the preliminary simulation study showed no significant modification of the photon beam properties. TraDeRa detects error leaf position as small as 1 mm compared to reference field, for both static(about 10% of local over-response)and modulated fields(about 4% focal over-response, see annex). In addition, measurements are stable over a large dynamic range from low intensity signals, as inter-leaves leaks, to very high intensities as obtained on the medical beamline of ESRF (dose rate thousand times larger than that of a conventional clinical linac). Conclusions: The 1:4 version of TraDeRa shows promising results for IMRT QA, allowing pulse-scale monitoring of the beam and high sensitivity for errors detection. The attenuation seems small enough not to hinder the irradiation while keeping the beam upstream to the patient under constant control. Nevertheless, further simulations will be necessary to convert the signal response of TraDeRa to dose response. In terms of dynamic, the detector is operational for every radiotherapy treatments, including the FFF modes with high dose rates(up to 2400 MU/min).The different versions ofTraDeRa already led to two patents [1] and [2]. A final prototype under development will include 1600 independent electrodes, half of them forming a high resolution area centered on the beam axis. The power supply, acquisition systems and data transmission will be embedded on the device, removing all external dependency and improving the compactness of the whole system. References [1] Brevet FR N° 11/53254. [2] Brevet FR N° 13/54339.