Download or read book Assessment of X-ray Computed Tomography Dose in Normoxic Polyacrylamide Gel Dosimetry written by Patricia Baxter and published by . This book was released on 2008 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Polymer gel dosimetry, in conjunction with x-ray computed tomography (x-rayCT) imaging, is a three-dimensional dosimetric tool that shows promise in the verificationof complex radiation therapy treatments. Previous studies have shown thatx-ray CT imaging of gel dosimeters is robust, easy-to-use, and has wide clinical accessibility. The effects of x-ray CT dose imparted to the gel dosimeter, during imaging ofthe delivered therapy dose distributions, is not well understood. This thesis quantifiesthe effects of CT dose on normoxic polyacrylamide gel (nPAG) dosimeters. The investigation is comprised of four parts. First, quantification of the x-rayCT dose given during CT imaging of nPAG gels was measured using ion chambermeasurements and filmed dose profiles for a range of typical gel dosimetry imagingprotocols (200 mAs (current-time), 120-140 kVp (peak potential energy of photons),2-10 mm slice thickness). It was found that CT doses ranged from 0.007 Gy/slice (120kVp, 2 mm) to 0.021 Gy/slice (140 kVp, 10 mm) for volumetric phantoms. Second, Raman spectroscopy was used to determine the effect of photon energy on the doseresponse of nPAG dosimeters exposed to photon energies from a CT scanner (140 kVp photons) and from a Linac (6 MV photons). A weaker response was exhibited withinthe gels irradiated with kV photons than MV photons. Thirdly, the measurementsof the given x-ray CT dose as established in the first study and the dose response ofthe polymer gel to different photon energies in the second study were correlated toestimate the induced changes of the nPAG CT number ("NCT), caused by x-ray CTimaging of the polymer gel. (CT number is defined to be the measured attenuationcoefficient normalized to water.) For typical gel imaging protocols (as above with16-32 image averages), it was found that "NCT 0.2 H is induced in active nPAGgel dosimeters. This "NCT is below the current threshold of detectability of CTnPAG gel dosimetry. Finally, the traditional method of chemically fixing the doseresponse mechanism of nPAG gels by passive oxygenation of the gel, is investigatedto determine if oxygenation would mitigate the changes caused by x-ray CT imaging of the gels. It was determined that oxygen diffusion was too slow to cause fixationof nPAG dosimeters, as the diffusion constant was 1.2? 0.2? 10?6cm2/s, or 25% ofthe diffusion constant for anoxic PAG gel dosimeters. In conclusion, it was found that x-ray CT dose in polymer gel dosimeters is nota concern for standard gel imaging protocols. X-ray CT dose can potentially be aconcern when large numbers of image averages (e.g.60 image averages) are utilized, as in gel imaging protocols for high-resolution scans.