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Modélisation Monte-Carlo d'un accélérateur linéaire pour la prise en compte des densités pulmonaires dans le calcul de la dose absorbée en radiothérapie stéréotaxique

Abstract : For clinical routine in external Radiotherapy, dose computation is achieved using commercial Treatment Planning Systems (TPS). Since ten years, TPS algorithms have been improved. However they include approximations that are acceptable in most of the clinical cases but they show their limits in some particular conditions for example in presence of small fields and/or low mass y media. And these two conditions are found in the context of stereotactic body radiation therapy (SBRT) of lung tumor. Some studies were published for standard lung densities but none for very low y like in lung during Deep Inspiration Breath Hold (DIBH). This work is a study of dose computation based on a Monte Carlo (MC) model, for different field sizes and mass densities. The first step was to model a TrueBeam(r) linac (Varian, Palo Alto, CA) using data furnished by the manufacturer. This model is built using the Geant4-based GATE platform. The main compounds of the linac head are modeled. Space phase files (i.e. particles files) are furnished by Varian in "IAEAphsp" format and are integrated to the model above the main jaws. To validate this model, a set of simple fields (from 3x3 to 20x20 cm2) in a water phantom is implemented for different photon energies: 6FF, 6FFF, 10FF and 10FFF (FFF = "Flattening Filter Free"). Percentage depth dose (PDD) and lateral profiles are compared to reference measurement in a water tank: respectively 99% and 97% of all the points of these curves passed the Gamma Index test (2% 2mm). Once this validation was completed, a set of simulation was achieved with small field sizes (3x3 to 8x8 cm2) for simple heterogeneous phantoms for which the measurement was achievable. For this purpose, radiochromic films were inserted in phantoms made of PMMA slabs and two types of cork. Cork densities were 0.12 and 0.24 that correspond respectively to lungs during DIBH and free breathing. Results of the MC model for four energies are compared to experimental measurements and to AAA and Acuros Varian's algorithms. AAA algorithm overestimates the dose inside the lung heterogeneity for small field sizes and low density. As an example in the case a 3x3 cm2 field, inside the heterogeneity of density 0.12 an over estimation of 16% in the lung is observed for AAA. The model is finally used for three non-measurable cases: a cylindrical digital reference object and computerized tomography data of a patient during DIBH with a static and stereotactic arc field. Results showed respectively for CT studies an overestimation of dose in the tumor of 7% and 5.4% and in the lungs of 14% and 9.6% by AAA. From a clinical point of view, this means under-dosing the patient and thus a risk of recurrence.
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Sara Beilla. Modélisation Monte-Carlo d'un accélérateur linéaire pour la prise en compte des densités pulmonaires dans le calcul de la dose absorbée en radiothérapie stéréotaxique. Imagerie. Université Paul Sabatier - Toulouse III, 2016. Français. ⟨NNT : 2016TOU30105⟩. ⟨tel-01473898⟩

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