Advanced quality assurance methodologies in image-guided high-dose-rate brachytherapy

(two versions: original and short) Original Thesis Abstract: Brachytherapy (BT) was the first form of radiotherapy and it is still effectively used because of its unique physical and biological advantages. Although the principles of BT operation are considered to be relatively simple (since it is based on the correct timing and positioning of radioactive sources), BT has also benefitted from technological advances. The rate of technical inventions and their incorporation into BT treatments has necessitated development of more precise quality assurance (QA) tools. The purpose of this thesis is to introduce a robust QA framework based on radiochromic film (RCF) dosimetry for image-guided high-dose-rate (HDR) BT. These films can be digitized allowing for high spatial resolution visualization of the source dosimetric trace, which can be used to reconstruct the source positions and evaluate the dose distribution simultaneously. To increase the HDR source-tracking reliability, a film-digitization protocol was developed. This protocol evaluates issues related to film scanning and handling, and specifies parameters of film response and mathematical models that relate this response to absorbed dose. The protocol is based on a new linear response function, ‘normalized pixel value’ (nPV), and it was designed to achieve high accuracy while maintaining practicality. It was further improved by using all RGB (Red, Green, Blue) color information available in RCF scanned images, to correct for scanning-related issues. This protocol was tested and validated for six independent RCF dosimetry systems in three different clinics, demonstrating robustness of the method and its ability to mitigate systematic response shifts. The first application of this dosimetry protocol was in the QA of Freiburg Flap (FF) based treatments in HDR surface BT. The current standard of care in the treatment planning of surface BT does not take into account the lack of the backscatter above the FF and the patient skin, since it assumes the HDR source is always surrounded by water. Before comparing the planned and delivered doses, the film response was calibrated and a detailed uncertainty budget was discussed. The RCF dosimetry system was able to report the difference between the calculated and the delivered doses for different setups and to evaluate the use of bolus to reduce these differences. Subsequently, the source-tracking algorithm was developed to precisely localize the HDR source within catheters based on the acquired 2D distribution from the RCF. The algorithm relies on measured-features of the relative isodose lines (blob analysis) such as area, perimeter, weightedcentroid, elliptic orientation, and circularity. A reference library of features was prepared based on the AAPM TG-43 datasets and the correlations were derived between these features and the source coordinates (x, y, z, θy, θz). The measured features are then compared to the referenced ones and the most probable source coordinates are reported. The source-tracking algorithm was verified experimentally with an accuracy of 0.1 mm by having two film sets on opposing ends of the source.