Vaginal vault brachytherapy: the effect of varying bladder volumes on normal tissue dosimetry

This study was designed to assess the impact of bladder volume on dosimetry to critical normal structures in vaginal vault brachytherapy using a single line source vaginal applicator. 30 consecutive patients undergoing vaginal vault brachytherapy were studied by CT scanning with the applicator in situ and the bladder empty and then with the bladder containing either 35 ml of water (10 patients), 70 ml of water (10 patients) or 100 ml of water (10 patients). The scans were then analysed with isodose distributions overlayed to determine changes in dosimetry. No effect on bladder dose was seen with increasing volume compared with the empty bladder; however, there was a reduction in amount of small bowel within the high dose treatment region as bladder volume increased. With 100 ml bladder volume, the reduction reached 57.5% compared with the empty bladder. We conclude that vaginal vault brachytherapy should be undertaken with a bladder volume of at least 100 ml, which will considerably reduce the amount of small bowel in the irradiation volume with no increase in bladder dose.