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Modélisation d'un accélérateur linéaire clinique en vue de l'exploitation d'un détecteur à transmission dédié au contrôle qualité en radiothérapie

Abstract : Quality assurance is a key topic in radiation therapy. Indeed, the increasing complexity of the treatment methods led to many additional sources of potential errors, and the verification of treatment is more than ever a task that can be difficult. One of the objectives of the radiophysicists lies in the application of quality assurance procedures to ensure that the dose delivered to the patient is consistent with the oncologist’s prescription.Numerous tools are available to ensure controls that are both effective and reliable, reducing the chances of errors. However, zero risk is not achievable, and the use of new treatment techniques can make these errors difficult to detect. One of the innovative solutions studied in recent years depends on the use of new two-dimensional detectors, embedded on the head of the linear accelerator and placed upstream of the patient. These devices thus allow to collect information on the photon flux delivered by the accelerator.This type of detector has several advantages. Their attenuation is very light, and grant the possibility to use them while treating the patient. Moreover, their location does not give space to ambiguity about the origin of the observed dose deviations. Indeed, a patient positioning error cannot be confused with mechanical misbehavior of the linear accelerator.In this context, the PHYSMED group of the LPSC, in collaboration with the Grenoble Public Hospital, is developing TraDeRa, a detector based on a pixelated planar ionization chamber, embedding dedicated electronics specially designed by the lab. The detector is able to collect a signal map that accurately describes the incoming photon flux, for any clinical field, in real time and without dead zone.The portage of the detector towards clinical routine partially depends on the translation of the signal map supplied by the detector into a reliable dose map that can be used by medical physicists. To begin with, one of the considered solutions is the use of complex Monte Carlo simulations, in order to associate the detector response with a calculated dose deposition in a water tank.The precision to provide on the various Monte-Carlo calculations seemed very important. Therefore, a precise model of the clinical accelerator seemed essential. Thus, we have initiated the modeling of a type Clinac 2100 clinical accelerator for the Monte-Carlo code PENELOPE, in order to reproduce as accurately as possible the accelerator available for our measurements campaign. The knowledge of the primary electron beam characteristics is critical. We have thus developed an original method for adjusting these characteristics based on comparison of measurements with reference simulations. This method has interesting features compared with the usual trial and error process. The objective was to propose a method applicable to any accelerator of the same model, using only a set of reference simulations, thus allowing the faster adjustment (sometimes slightly less accurate than the trial and error method) of the beam characteristics. This method seems to have proved itself worth, and has been used on two other Clinac 2100 accelerators to test its reliability.Next, the full modeling of the detector in PENELOPE coupled and its environment granted us to describe some characteristics intrinsic to conception choices. Thus, we have been able to study the linearity response of the detector in terms of dose and dose rate, characterize the attenuation it opposes to the beam, and evaluate the cross-talk between the different electrodes of the pixelated matrix.In addition, we were able to initiate the work of converting the signal collected by TraDeRa into a dose deposited in the patient. We associated the response of selected electrodes with calculated dose distributions in a water tank model. By decomposing any field into a sum of elementary responses, we have determined a procedure allowing the reconstruction of the dose deposition in the patient.
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Robin Fabbro. Modélisation d'un accélérateur linéaire clinique en vue de l'exploitation d'un détecteur à transmission dédié au contrôle qualité en radiothérapie. Physique Médicale [physics.med-ph]. Université Grenoble Alpes, 2017. Français. ⟨NNT : 2017GREAY078⟩. ⟨tel-01720776v2⟩

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