The effect of the amount of bladder filling on normal tissue doses in three-dimensional hdr vaginal vault brachytherapy

Purpose: In this study, it was aimed to compare the dose volume parameters of organs at risk for the bladder filling of 50 cc versus 150 cc in high dose rate (HDR) three-dimensional (3-D) vaginal cuff (VC) brachytherapy (BRT). Materials and Methods: The treatment data of 8 hysterectomized patients with gynecological malignancy who received postoperative pelvic external radiotherapy (RT) + 3-D HDR VC-BRT between March 2015 and August 2015 were analyzed. Retrospectively, computerized tomography (CT) sectional images were used for the study; recontouring was performed on these images and the treatment plans were formed again. Proximal 1/3 vagina had been treated in all patients using cylinder. The dose had been prescribed to the 0.5 cm depth from the vaginal mucosa (cylinder surface). A total dose of 18 Gy (3X6Gy) BRT had been performed. Bladder, rectum, sigmoid, bowel and CTV clinical target volume (CTV) were recontoured 3-D retrospectively. Afterwards, treatment planning was performed by the medical physicisit using BRT treatment planning system. Bladder V50%, D50% and D2cc, rectum D2cc, sigmoid D2cc, bowel D50% and D2cc were recorded from the dose-volume histograms obtained in the treatment planning system. Paired comparisons were made for the parameters above for the bladder filling of 50 cc versus 150 cc. Different bladder fillings were compared using Wilcoxon Signed-Rank Test in the SPSS 15.0 statistics program. Results: Bladder D50% decreases (p=0.012) while bladder D2cc increases (p=0.025) in case of 150 cc bladder filling instead of 50 cc. Rectum D2cc shows a statistical trend for increase (p=0.05), however bowel D50% decreases (p=0.012) in 150 cc bladder filling compared to 50cc. Conclusion: The statistically significant decrease in bladder and especially bowel D50% parameters supports filling bladder with 150 cc instead of 50 cc in 3-D VC-BRT. Key Words: three-dimensional vaginal cuff brachytherapy, bladder filling, normal tissue dose