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Nouvelle méthode de détermination du spectre en énergie initial d'un accélérateur linéaire en vue d'une planification de traitement en radiothérapie par Monte Carlo

Bouchra Habib 1 Bénédicte Poumarède 1 Jean Barthe 1
1 LM2S - Laboratoire Modélisation et Simulation de Systèmes
DM2I - Département Métrologie Instrumentation & Information : DRT/LIST/DM2I
Abstract : Introduction: Since the end of 1990s, treatment planning systems offering Monte Carlo (MC) dose calculation tools have emerged. These methods are considered to be the most accurate in medical physics. However, to calculate the dose by MC, the characteristics of the primary electron beam at the exit of the accelerator after the magnetic deflection must be first determined. Unfortunately, these characteristics are relatively unknown. In MC simulation, the initial energy spectrum is usually approximated to a gaussian distribution by optimizing empirically its mean energy, standard deviation and energy window. This study proposes a novel method faster and more deductive for determining the initial energy spectrum of an accelerator. Materials and Method: The method relies first on several MC simulations (PENELOPE) of depth-dose curves in water by considering a set of monoenergetic initial beams. Afterward, the least squares method with non-negativity constraint is used to determine the energy spectrum. We consider the one who gives the depth-dose curve (reconstructed by linear combination between curves simulated from different energies) that fits the measurements. This method is used to simulate beams from the Saturne-43 (12 MV photon and 18 MeV electron modes) and Siemens-Primus (25 MV photon and 9 MeV electron modes) accelerators. The gamma index test is used to compare the calculated and measured dose distributions. Results: The gamma index results show a good agreement between measured and calculated dose distributions for both treatment modes (photon and electron) and all field sizes. Differences are less than 1%-1mm for depth-dose curves and less than 2%-1mm for dose profiles. Conclusions: These results validate the method developed here for determining the initial energy spectrum of the accelerator. This method determines the best spectrum in the least-squares sense. The results also highlight the speed and simplicity of this method that distinguishes only one degree of freedom including the mean energy, energy distribution (including asymmetry of the spectrum) and the energy window of the primary electrons beam.
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Bouchra Habib, Bénédicte Poumarède, Jean Barthe. Nouvelle méthode de détermination du spectre en énergie initial d'un accélérateur linéaire en vue d'une planification de traitement en radiothérapie par Monte Carlo. 48èmes Journées Scientifiques de la Société Française de Physique Médicale (JS-SFPM - 2009), Société Française de Physique Médicale, Jun 2009, Montauban, France. ⟨cea-02562712⟩

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