Development of methods for the optimization and quality assurance of modern brachytherapy techniques

High Dose Rate (HDR) brachytherapy is one of the most advanced and effective techniques in radiotherapy, achieving a high tumor-to-healthy-tissue dose ratio while minimizing radiation exposure to normal tissues. This is accomplished by placing radioactive sources within or near the tumor target and designing treatment plans based on three-dimensional imaging. However, clinical outcomes, both in terms of disease control probability and complication probability, are inextricably linked to dosimetric accuracy and require specialized quality assurance (QA) procedures to ensure treatment accuracy and safety. Specifically, for HDR brachytherapy applications involving remote afterloading of the source—which constitute the vast majority of clinical applications—quality checks such as the calibration certificate accuracy, source positioning in selected catheter/applicator locations, catheter/applicator length, irradiation time, and transit dose due to source movement between programmed positions (transit time) are performed using different methods. This increases the time required, workload, and necessary equipment.The purpose of this research was to develop a QA protocol based on pixel-segmented ionization chambers (PIXSICs) that would enable all QA tests (source calibration, source positioning, and irradiation time) to be performed in an integrated and unified manner. This would eliminate the need for multiple equipment items, reduce the time required to implement the QA program, and automate the process to ensure accuracy while reducing clinical workload.Initially, the detector used in the research consists of 1020 segmented ionization chambers (MatriXX Evolution, IBA Dosimetry, Schwarzenbruck, Germany), distributed in a 32×32 rectangular array with a surface area of 24.4×24.4 cm². It was calibrated in dose to water units per signal within the active volume of each chamber using a 6 MV photon beam from an Elekta Versa HD linear accelerator. For the calculation of the air-kerma strength (Sk) of the source, the desired calibration coefficient (in units of air-kerma strength per PIXSIC signal) was determined through Monte Carlo (MC) simulations and a PIXSIC simulation model. After calibration, Sk was determined through irradiations using a specialized catheter-holding setup, placing the source in a stationary position at the center of the detector at six different distances from its surface (z = 1, 2, 3, 4, 5, and 6 cm). Experimental and MC dose distributions were fitted using 2D Lorentzian functions, and source position corrections were applied based on the deviation between experimental and MC distributions. To verify source position and irradiation time, six irradiations were performed at multiple source positions with various step sizes, dwell times, frame times, and distances (z). The experimental source position was determined from the center of the 2D Lorentzian distribution applied, while the dwell time was calculated from the number of frames. Experimental step sizes were derived from the difference between successive source position centers. Experimental results for source position and irradiation time deviations were less than 1 mm and 0.2 s, respectively. Sk, limited to the central 14×14 pixels of the detector (10×10 cm²), varied within 1.2% of the nominal value, with an uncertainty of approximately 1.4% (k=1). This uncertainty is comparable to that of clinically employed well-type ionization chambers (1.4%, k=1).