Validation of conformal radiotherapy treatments in 3D using polymer gel dosimeters and optical computed tomography

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Holmes, Oliver
Radiotherapy , Gel Dosimetry
Polymer gel dosimeters are a three dimensional (3D) dosimetry system that may be conveniently applied for verifying highly conformal radiation therapies where standard dosimetry techniques are insufficient. Polymer gel dosimetry with optical computed tomography (OptCT) can be used to measure spatial dose distributions with high resolution. While long experience with MRI has yielded many studies reporting on experiments involving validation of clinical deliveries using polymer gel dosimeters, there are very few studies of this type where OptCT is used. OptCT is a relatively new technique and consequently has not yet been adopted into the clinical environment. As a result, methods and software tools for integrating OptCT measurements into clinical systems are not available. Previous studies from the Medical Physics research group at the Cancer Centre of Southeastern Ontario (CCSEO) and Queen’s University have therefore been limited to simple deliveries and two dimensional (2D) comparisons. In this thesis various software tools and calibration techniques have been developed to allow comparative analysis between OptCT measurements with dose distributions calculated by treatment planning software. Further, a modification of the γ-evaluation (Low et al. 1998) is presented whereby the vector components of γ are used to identify the sources of disagreement between compared dose distributions. Test simulations of the new γ-tool revealed that individual vector components of γ, as well as the resulting vector field can be used to identify certain types of disagreements between dose distributions: especially spatial misalignments caused by geometric misses. The polymer gel dosimetry tools and analysis software were applied to a clinical validation mimicking a prostate conformal treatment with patient setup correction using image guidance. In one experiment greater than 90 % agreement was found between dose distributions in 4%T 50%C NIPAM/Bis dosimeters (Senden et al. 2006) measured with the Vista OptCT unit and dose distributions calculated by Eclipse treatment planning software.
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