Caraterização Experimental de Feixes de Megavoltagem para Imagiologia por Raios Ortogonais

Dissertação de Mestrado em Engenharia Biomédica apresentada à Faculdade de Ciências e Tecnologia da Universidade de Coimbra. ; Cancer is the leading cause of death in the World and radiotherapy one of the main procedures for treatment. Modern radiotherapy techniques provides increasingly higher conformality to the tumor. However, for successful treatment and to ensure maximum dose exposure to the tumor and avoid the irradiation of organs at risk or healthy tissues it is important detect patient misalignments and anatomical morphological alterations before the treatment, so to carry out the necessary corrections. In this way, a new imaging system to be applied to external X-ray beam radiotherapy was proposed: OrthoCT (orthogonal computed tomography). This technique focusses on acquiring images of the volume to be irradiated immediatly before treatment, when the patient is already positioned and xed for treatment (\on-board imaging") with very low to eventually null dose administered to organs at risk or healthy tissues even when position close to the tumor. OrthoCT relies on orthogonal ray imaging, namely, the detection of scattered photons in the patient that are emitted perpendicularly to the incident beam direction. OrthoCT shows good correlation with the morphological structure of the patient. For that, it is necessary that the beam has an inferior size ( 5 mm x 5 mm) when compared to the typical ones used in patient irradiation. Moreover the scanned pro le requires: (1) high homogeneity so that variations can be associated only to dose/morphological alterations; and (2) high velocity (which favors multi-leaf collimator-based scans in respect to jaw-based scans). The purpose of this dissertation is to de ne the best scan to be applied in OrthoCT, considering the conditions aforementioned. For that some small-beam scans were evaluated experimentally and the obtained pro les were optimize. The more homogeneous pro le achieved experimentally reveals an intensity variability inferior to 1%, thus substantiating ...