Assessment of Cartesian Co-Ordinates-based Bladder and Rectal Dose and Variability with Tandem-Ring Angles First Nigerian Experience in High-Dose-rate Brachytherapy

Objective: This study assessed bladder and rectal dose based on Cartesian-coordinates in intracavitary brachytherapy and examined the variations resulting from use of tandem-ring (T-R) of different angles. Methods: Cartesian-co-ordinates of bladder and rectum points were derived on orthogonal-radiographs of 90 patients who had high-dose-rate brachytherapy for cervical cancer at the Department of Radiation Oncology, University College Hospital, Ibadan, Nigeria, The patients were classified in three groups of 30, based on T-R angles 30 degrees, 45 degrees and 60 degrees used, bearing same lengths and diameters across sets. The measured co-ordinates and the related percentage doses to points of interest in the two organs were analysed using EViews and SPSS statistical softwares. Results: The mean rectal point dose (RPD) obtained was 68.97 +/- 13.57 % of the prescribed doses as against 56.57 +/- 11.83% for bladder point dose. While the maximums of the mean RPD were 95.7, 90.5 and 82.5% for T-R angles 30 degrees, 45 degrees and 60 degrees respectively; corresponding values for the bladder point were 72.8, 87.8 and 62.3%. Conclusion: The baseline data obtained in this study served as guidelines for subsequently achieving acceptable values of bladder point dose and RPD at University College Hospital, Nigeria. Special attention should be paid to the application of 2 cm-tandems in relation to the RPD. Advances in knowledge: This is one of the few studies assessing the influence of T-R angles on organs-at-risk. The increase in RPD with the use of intrauterine tandem-2 cm is found to be most pronounced with T-R 30 degrees, and this trend reduced with larger angles.