TraDeRa : développement d'un détecteur pour le contrôle en ligne et en temps réel des traitements de radiothérapie

International audienc

Development of a transparent photon detector for the online monitoring of IMRT beams

National audienc

Development of a transparent photon detector for the online monitoring of IMRT beams

National audienc

Simulation of the head of an accelerator: Calculation optimization and statistical comparison methods

International audienceIntroduction: The group Developments and Application for Medicine of the LPSC, in collaboration with the Public Hospital of Grenoble, is developing the TraDeRa detector (Transparent Detector for Radiotherapy). This device includes an ionization chambers matrix and provides 2D signal maps for any irradiation field, upstream to the patient.Converting the detector's signal into dose in the patient is a challenge. To do so, Monte Carlo simulation is a powerful tool, and will give accurate results if correctly parameterized. The work described here focuses on the optimization of the treatment head simulation (especially on the target, the source of X-rays) and the determination of the correct nominal energy and radial distribution of electrons on the target. In order to calibrate the detector, one must reach a perfect agreement between the observables from the simulation and from the beam of our reference accelerator.Methods: The interactions of the electrons in the target are numerous and time consuming to simulate. A study was conducted on both parameters of different particles transport methods and variance reduction in PENELOPE Monte Carlo code, which remains a reference for electron transport for the considered energies [1]. We have optimized a set of parameters to keep a reasonable computation time without biasing the physical observables.The determination of the initial characteristics of the electron beam (nominal energy/radial distribution of electrons on the target) is done by trial and error process. Several simulations are performed at various energies and radial electron distributions. The dose deposited in a simulated water phantom is compared with the depth-dose curves and dose profiles acquired under irradiation. We propose two efficient methods of comparison, the calculation of the Kolmogorov-Smirnov test and an original extension with more sensitivity.Results: The set of optimized parameters provides an overall increase on the simulation efficiency of nearly 300%. The useful secondary particles generation rate (stored in the PSF) was also increased by about 250%.Determining the characteristics of the electron accelerator of our reference beam is ongoing, we currently have simulated three energies around 6 MeV. In the appendix are presented the results of two comparative tests for these three energies. These tests including measurement uncertainties are robust and will accurately lead to the beam nominal energy with only five simulated energy sets. To date, the simulation has been running for six months on the IN2P3 computing center.Conclusions: The developed statistical tests are robust and will allow us to accurately determine the characteristics of the beam by comparing the simulated and the measured depth-dose curves and dose profiles. We will then be able to validate the reference accelerator simulation and calibrate the detector in terms of dose. This will allow us to reconstruct the 3D dose matrix in water from upstream information collected on a TraDeRa simulated model

Transparent photon detector for the online monitoring of IMRT beams

International audienceIntroduction: 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