Comparison of source dwell positions from different localization methods in the brachytherapy applicator utrecht

Abstract

Objectives: In the routine clinical brachytherapy treatment, the difference of dwell position between using two localization methods, which are x-ray marker and treatment planning system (TPS) applicator modeling library, are observed in the radiographs. The frame of this study was designed to simulate the patients’ treatment position and orientation in the actual clinical setting. The aims of this study were to investigate different localization methods used in Utrecht applicator with the objectives listed as follow: (1) To investigate if there is any significant difference in dwell position by two localization methods: using x-ray marker verse TPS library. (2) To investigate if there is any significant dose difference due to two different mentioned localization methods. Materials and methods: In this study, an Integrated Brachytherapy Unit (IBU) was used to take two general x-ray images. Each set of the images consists of an anterior-posterior (AP) plain x-ray radiograph and a lateral plain x-ray radiograph that were taken to localize the spatial coordinates of the x-ray marker and dummy source. Thirty-one sets of first dwell localization images were acquired at different phantom rotation angle from -15° to +15° with 1° step size. The differences in left-right, inferior-superior and anterior- posterior direction along the patients between x-ray marker and TPS library were compared with the dummy source. The result was presented by the histograms and pair t-test was performed on the coordinates of the dwell positions. The dose differences due to the dwell position differences were studied by simulating the patient's vaginal posterior wall and rectal point. Pair t-test was also performed. Results: There are statistically significant dwell position differences between TPS library and dummy source as the localization methods. For the dose differences, the mean dose difference between using TPS library and x-ray marker as the localization method at patient’s vaginal posterior wall and rectal reference point are 4.40% (SD=2.32) and 2.25% (SD=1.61) respectively. There are statistically significant dose differences between using TPS library and x-ray marker as the localization methods. Conclusion: It has been proved that x-ray marker is a more accurate localization tools because the x-ray marker is able to reflect the actual degree of bending, which is the cause of position shift, within the applicator. To conclude, x-ray marker is recommended to use for localization in Utrecht applicator instead of using TPS library.