Differences of Ring Applicator Reconstruction Methods in Brachytherapy

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Purpose: Analysis and comparison of existing reconstruction methods of the ring applicator, as well as the development of reconstruction method that is closest to the actual source path in dosimetric planning using brachytherapy.
 Material and methods: Evaluation of reconstruction methods was carried out on the basis of CT images, which were used for dosimetric planning of patient treatment. The obtained data on the positioning of the radiation source using X-ray images were transferred to CT images using matrix transformation methods. A method of reconstruction along the source path is proposed, in which the minimum discrepancies in the location of the source positions in comparison with the coordinates of the actual source positions were obtained. For comparison, the reconstruction of the applicator was performed by three different methods: the method using the manufacturer’s applicator libraries, along the source path, and the method proposed above.
 Results: The results were assessed using a dose-volume histogram. Dose characteristics were selected based on the recommended ICRU Report 89. To assess the presence of the effect of the reconstruction method on the dose characteristics, the Friedman criterion was used. The differences between the doses for different reconstruction methods turned out to be significant (the critical value of the significance level was considered p=0.05). The maximum deviations of the median values of the parameters of the dose-volume histogram were 7.8 % for the “lib” method, 8.6 % for the tracking method, and 7.9 % for the track_new method. We calculated the median values of the deviations of the source positions obtained using the previously described reconstruction methods from the actual stopping positions of the source found experimentally.
 Conclusion: The use of the proposed reconstruction method track_new increases the accuracy of determining the positions of the stop of the radiation source in the applicator and, as a consequence, the accuracy of dose delivery on the brachytherapy apparatus in comparison with the manual tracking method. The methods used in this study can be used when putting the applicators into clinical use as part of a radiotherapy quality assurance program. The obtained results will be useful to medical physicists if it is necessary to choose a technique for carrying out the reconstruction of the applicator in accordance with the available capabilities and imaging methods and, as a result, will increase the accuracy of dosimetric planning and dose delivery on brachytherapy afterloaders.