Prompt gamma imaging for online monitoring of the dose in hadrontherapy – Gamhadron

The objective of the GamHadron project is to design, optimize, develop and characterize an imaging system for on-line monitoring of the dose deposited during a hadrontherapy treatment. Three-dimensional imaging of the dose deposited in the patient is particularly important in hadrontherapy, because of the high ballistic accuracy of ions (Bragg peak). The only method available so far for such on-line monitoring consists in carrying out PET acquisition, taking advantage of the positron emitters produced by nuclear reactions occurring when ions interact within the tissues. However, this method does not make it possible to obtain images during the treatment (a fortiori in real time), and it does not provide precise quantitative information. An alternative to in-beam PET consists in using the so-called prompt-gamma photons emitted instantaneously in the process of nuclear fragmentation. The energy of these photons is in the MeV range. The points of emission of prompt gammas are correlated to the depth of penetration of carbon ions and to the deposited dose. The solution proposed in this project consists in detecting the prompt gamma photons with a Compton camera, combined to the use of a detection system allowing to tag every incident ion temporally and spatially in the transverse plane. Such a system appears as a credible solution for monitoring the deposited dose in three dimensions, potentially in real time, and with a spatial resolution clinically relevant (2 mm). However, the design and optimization of such a system constitutes a technological challenge for several reasons: (i) the small number of photons emitted requires a high detection efficiency, (ii) the signal has to be extracted from a high level of background noise due to scattered photons, neutrons and charged particles. The use of a collimator coupled to a gamma-camera is possible, but this solution does not allow to achieve the efficiency required to acquire three-dimensional information during the treatment. The use of a Compton camera would make it possible to avoid using a collimator, thus leading to an increase in efficiency by a factor of about 100. However, a Compton camera alone does not present a sufficiently high selectivity to separate the signals of interest from the background (selection of events in coincidence in the scatter and absorption detectors with a time resolution of 5-10 ns). To overcome this difficulty, we propose to use an innovative time-of-flight technique, by placing a tagging system measuring the passage time and transverse position of every ion in the incident beam. Diamond detectors present adequate characteristics for such a tagging system. The time of flight, measured with a time resolution of about 1 ns, between the passage of an ion in the tagging system and the arrival of the prompt gamma in the absorption detector of the Compton camera will make it possible to extract the signals of interest from the background with a considerably increased selectivity. The proposed imaging system will thus present at the same time the needed efficiency and signal-to-noise ratio. The work necessary to develop the proposed imaging system comprises simulations, preliminary in-beam measurements of coincidence rates, developments concerning the different detection modules (diamond detector, silicon scatter detector, absorption scintillating detector), the fast electronics and data acquisition system, as well as the characterization of every component and of the global system operation in ion beams.