Otimização dos parâmetros de aquisição de imagem volumétrica por CBCT, para redução da contribuição de dose, sem alterar a qualidade da imagem em tratamentos com IGRT

Abstract

The CBCT images for IGRT treatment have the objective the positioning and alignment of verifying of the patient through the visualization of bone structures, anatomical and PTV, thus not requiring an image quality similar to those required for diagnostic imaging or radiotherapy planning. The present work has as main objective the reduction of dose contribution the volumetric images of CBCT in the IGRT treatment without significant loss of the image quality (IQ) considering the ALARA (As Low As Reasonably Achievable) principle. Clinical protocols (presets) image acquisition defined in the Full Synergy Accelerator XVi system were changed in relation to the image acquisition parameters (optimized presets) and for comparative IQ evaluation, image acquisitions were performed with the CatPhan 504. The tests used for IQ analysis are uniformity and non-uniformity, CNR and linearity of pixel values, and high contrast resolution. Dose reduction was determined by the definition of dose estimation by X-ray projection, and the results expressed in relation to the Elekta clinical preset, theoretically demonstrating the dose reduction percentage in relation to the alteration of the beam generation parameters and aspects of patient exposure. Throughout the period of realization, a quality assurance program was implemented, addressing geometric, image quality and performance aspects of the XVi system. The quality control results of the XVi system were within the limits reported in the literature or remained linear during the 10 months of testing, consequently reducing the frequency of tests according to document recommendations. The four Elekta clinical presets evaluated with the variation of the image acquisition parameters showed possible optimization for dose reduction. The presets were chosen to have different image volumes, covering the main collimators of the X-ray tube used for image projection. The results showed a dose reduction of 70% for the Fast Prostate S10 preset, 25% for the Fast Head & Neck S20 preset, 65% for the Prestate Prostate M15 and 60% for the Chest M20 preset