In radiation therapy, gel dosimetry is used to measure radiation doses for treatment verification. Gel dosimeters have the ability to record dose information in three dimensions. The objective of this thesis was to fabricate a transparent cryogel radiochromic dosimeter with poly(vinyl alcohol) (PVA) as the gelling agent. A transparent dosimeter may be analyzed using an optical read out technique, which is desirable. PVA cryogels can be made transparent by adding dimethyl sulfoxide (DMSO).
Measurements of dose response were performed and various parameters were adjusted, including: numbers of freeze-thaw cycles (FTCs); concentrations of PVA; DMSO concentration. The measured absorption coefficient increased linearly with dose up to approximately 10 Gy. The sensitivity was increased for higher PVA concentrations, larger numbers of FTCs, and less DMSO. The resulting dosimeter was stable and showed no significant dose rate or photon energy dependence.
The cryogels were later formed into 5 mm thick films and used as a tool for performing in vivo dosimetry. The dose response of the radiochromic bolus was characterized by irradiating it on a flat surface at different gantry angles. The dose measured in the bolus was approximately 0.80 of the dose measured by Gafchromic film at the skin surface, taking the obliquity into account. IMRT treatments were delivered to a RANDO phantom. The radiochromic bolus was used to measure skin surface dose in two dimensions at various locations. The 0.80 factor was used to calibrate the bolus, which was then compared to an accompanying film measurement. Good agreement was observed between the measurements (>95% gamma pass rate), suggesting the radiochromic bolus may be suitable for in vivo applications.
The radiochromic bolus was then used to evaluate errors associated with the breath hold technique often used with left chest wall tangential irradiation. Treatment plan incorporating the radiochromic bolus was delivered at the planned position and shifted anterior-posteriorly (A/P) up to 5 mm. Large discrepancies from the planned two dimensional skin surface distribution were observed for shifts as small as 3 mm in the A/P direction. The study demonstrated that the cryogel was sensitive to small positioning uncertainties for chest wall irradiations, potentially allowing for the detection of clinically relevant errors.
Other potential formulations of PVA-based radiochromic cryogels are discussed briefly as avenues to future research projects. / Thesis / Doctor of Philosophy (PhD)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/18634 |
| Date | 08 December 2015 |
| Creators | Eyadeh, Molham |
| Contributors | Diamond, Kevin, Medical Physics |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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