Resposta radiológica e dosimetria em Phantom físico de cabeça e pescoço para radioterapia conformacional 3D

Abstract

Phantoms are tools for simulation of organs and tissues of the human body in radiology and radiotherapy. This thesis describes the development, validation and, most importantly, the use of a physical head and neck phantom in radiology and radiotherapy, with the purpose of evaluating dose distribution using Gafchromic EBT2 film in 15 MV 3D conformal radiotherapy. The work was divided in two stages, (1) development of new equivalent tissues and improvement of the physical phantom, and (2) use of the physical phantom in experimental dosimetry studies. In phase (1) parameters such as mass density, chemical composition of tissues, anatomical and biometric measurements were considered, as well as aspects of imaging by computed tomography (CT) and radiological response representation in Hounsfield Units (HU), which were compared with human data. Radiological experiments of in-phantom simulated brain pathologies were also conducted. All those results matched human-sourced data, therefore the physical phantom is a suitable simulator that may be used to enhance radiological protocols and education in medical imaging. The main objective in phase (2) was to evaluate the spatial dose distribution in a brain tumor simulator inserted inside the head and neck phantom developed by the Ionizing Radiation Reserch Group (NRI), exposed to 15 MV 3D conformal radiotherapy, for internal dose assessment. Radiation planning was based on CT images of the physical phantom with a brain tumor simulator made with equivalent material. The treatment planning system (TPS), CAT3D software, used CT images and prescribed a dose of 200 cGy, distributed in three fields of radiation, in a T-shaped pattern. The TPS covered the planning treatment volume (PTV) with 97% of the prescribed dose. A solid water phantom and radiochromic Gafchromic EBT2 film were used for calibration procedures, generating a dose response curve as a function of optical density (OD). After calibration and irradiation, the film samples were scanned and digitised, yielding OD data, standard deviation and dose response versus OD. The spatial dose profile reached 70% to 120% of the prescribed dose. Such data was plotted and analysed, showing layers dotted with hot spots. Despite the layered profile, as opposed to the smooth dose profile from the TPS, internal doses in the tumor were within a 5% deviation of the 214.4 cGy evaluated by the TPS. In the gamma index analysis, 83.2% of the points with gamma values smaller than 1 (3%/3 mm) between experimental values and TPS data. The toolset composed by the physical phantom and by the film dosimeters was, therefore, suitable for validation of internal doses in radiotherapy protocols.