Preliminary investigation of X-ray imaging for dose extraction of BANG® polymer gel in intensity modulated radiation therapy Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/v405sc60h

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  • This study investigated gel dosimetry with X-ray CT imaging as a possible means for extracting dose information from a 3D gel dosimeter. Currently Optical CT and MRI are the popular means of dose extraction, but X-ray CT imaging has the advantage of being more convenient and cost effective. The dosimetric system was based on the BANG® polymer gel (MGS Research) and an ordinary clinical X-ray CT unit. The gel system was analyzed for its effectiveness in detecting absorbed dose from a 10 MV Linac unit. This study investigated calibration doses up to 8 Gy and two fractions of an IMRT treatment plan for a total dose of 4.22 Gy. The irradiation plans were generated by the Varian Eclipse® treatment planning system and delivered at OHSU. One week post irradiations the BANG® polymer gels were analyzed using X-ray CT imaging at OHSU. The imaging parameters were unique to this investigation. Post irradiation the BANG® gel dosimeter responded to the absorbed doses through the process known as polymerization. The BANG® polymer gel dosimeter changed chemically and physically. A density change occurring in the location of irradiation allowed for detection via X-ray Imaging. A tube potential of 120 kV was selected for better signal to noise ratio and thin image slices of 1 mm was used for greater spatial resolution. The use of X-ray imaging with these specific imaging parameters proved to be convenient and effective. Images of the gel which received no radiation were studied to build a background evaluation. Post irradiation images were evaluated for dose response. This study showed that X-ray imaging was able to detect the change within the gel due to irradiation with a response of about 0.66 ± 0.03 pixel values per Gy. The BANG® gel dosimeter was characterized for its response to radiation, dose sensitivity, dose resolution, and dose distribution. We found our system to have a high dose sensitivity of 0.96 ± 0.6 H/Gy. The X-ray CT images were able to differentiate between doses with a resolution of 39% within the mean dose. From these finding we were able to build dose distributions and dose maps for our calibration and treatment phantoms. The conclusion of this preliminary investigation found X-ray CT imaging to be successful for dose extraction purposes. We note that there are still areas in gel dosimetry which need additional research such as development in software or code to integrate, analyze the dose response, and compare the results with predicted dose distribution generated by the treatment plans. Using X-ray CT will certainly decrease the cost of the 3D Gel dosimetry systems and with increased clinical use 3D gel dosimetry will soon allow for better quality assurance of radiotherapy treatments.
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