Three-dimensional Dosimetry of Hadron Beams.

A novel 3-D dosimeter system comprising a leuco-dye doped plastic 3-D dosimeter (PRESAGE(TM) ) and an in-house CCD-based optical computed tomography (CT) scanner has recently been introduced. PRESAGE(TM) is based on a clear polyurethane matrix doped with radiochromic components, leuco dyes, which generate a colour change, and hence optical density change, on exposure to ionizing radiation. A prototype CCD-based scanning system that has been constructed (with a similar configuration of a "first generation" X-ray CT scanner) provides optical attenuation profiles of the irradiated dosimeter following the Beer-Lambert law. A complete plane of data is acquired at each step, obtaining a 2D projection planar image of an object in a relatively short time depending on the hardware and averaging options selected. This source-detector combination measures parallel beam projection geometry, then the sample is rotated, and the acquisition is repeated until the desired number of 2-D projections has been obtained, and thus, data for a complete 3-D region are acquired. This thesis focuses on dosimetry measurements of a 60 MeV proton beam and investigates possible LET dependence and proton energy in PRESAGE(TM). Experimentally, we observed that in the plateau region the dose-response is independent of linear energy transfer (LET) until we get the expected lower response of the dosimeter to the dose deposited at the Bragg maxima caused by the high-LET at this point. We used the track structure theory (TST) to give possible explanations for the LET dependence of PRESAGE(TM). From these calculations, we found that PRESAGE(TM), in general, is not LET independent which makes it unsuitable for proton dosimetry. Then we investigated the behaviour of both an optically absorbing and an optically scattering dosimeter using our optical-CT apparatus with the aim of eliminating optical artefacts due to the optical-CT scanner from that radiation induced effects in our measurements. A calibration protocol/ jig system in which a standard distribution is irradiated in the same dosimeter thus establishing a dose conversion for each batch with specific dose sensitivity was carried out, yielding three-dimensional dose maps that are of sufficient accuracy, resolution and precision to allow verification of complex treatment deliveries