Polymer gel dosimetry, in conjunction with x-ray computed tomography (x-ray
CT) imaging, is a three-dimensional dosimetric tool that shows promise in the verification
of complex radiation therapy treatments. Previous studies have shown that
x-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 of
the delivered therapy dose distributions, is not well understood. This thesis quantifies
the effects of CT dose on normoxic polyacrylamide gel (nPAG) dosimeters.
The investigation is comprised of four parts. First, quantification of the x-ray
CT dose given during CT imaging of nPAG gels was measured using ion chamber
measurements and filmed dose profiles for a range of typical gel dosimetry imaging
protocols (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 (120
kVp, 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 dose
response 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 within
the gels irradiated with kV photons than MV photons. Thirdly, the measurements
of the given x-ray CT dose as established in the first study and the dose response of
the polymer gel to different photon energies in the second study were correlated to
estimate the induced changes of the nPAG CT number ("NCT ), caused by x-ray CT
imaging of the polymer gel. (CT number is defined to be the measured attenuation
coefficient normalized to water.) For typical gel imaging protocols (as above with
16-32 image averages), it was found that "NCT <0.2 H is induced in active nPAG
gel dosimeters. This "NCT is below the current threshold of detectability of CT
nPAG gel dosimetry. Finally, the traditional method of chemically fixing the dose
response mechanism of nPAG gels by passive oxygenation of the gel, is investigated
to 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 fixation
of nPAG dosimeters, as the diffusion constant was 1.2 ± 0.2 × 10−6cm2/s, or 25% of
the diffusion constant for anoxic PAG gel dosimeters.
In conclusion, it was found that x-ray CT dose in polymer gel dosimeters is not
a concern for standard gel imaging protocols. X-ray CT dose can potentially be a
concern when large numbers of image averages (e.g. >60 image averages) are utilized,
as in gel imaging protocols for high-resolution scans.
| Identifer | oai:union.ndltd.org:uvic.ca/oai:dspace.library.uvic.ca:1828/1162 |
| Date | 11 September 2008 |
| Creators | Baxter, Patricia |
| Contributors | Jirasek, Andrew, Beckham, Wayne |
| Source Sets | University of Victoria |
| Language | English, English |
| Detected Language | English |
| Type | Thesis |
| Rights | Available to the World Wide Web |
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