Inter fraction variations in rectum and bladder volumes and dose distributions during high dose rate brachytherapy treatment of the uterine cervix investigated by repetitive CT-examinations

Purpose: To evaluate variation of dose to organs at risk for patients recei ving fractionated high dose rate gynaecological brachytherapy by using CT-b ased 3D treatment planning and dose-volume histograms (DVH). Materials and methods: Fourteen patients with cancer of the uterine cervix underwent three to six CT examinations (mean 4.9) during their course of hi gh-dose-rate brachytherapy using radiographically compatible applicators. T he rectal and bladder walls were delineated and DVHs were calculated. Results: Inter fraction variation of the bladder volume (CVmean = 44.1%) wa s significantly larger than the inter fraction variation of the mean dose ( CVmean = 19.9%, P = 0.005) and the maximum dose (CVmean = 17.5%, P = 0.003) of the bladder wall. The same trend was seen for rectum, although the figu res were not significantly different. Performing CT examinations at four of seven brachytherapy fractions reduced the uncertainty to 4 and 7% for the bladder and rectal doses, respectively. A linear regression analysis showed a significant, negative relationship between time after treatment start an d the whole bladder volume (P = 0.018), whereas no correlation was found fo r the rectum. For both rectum and bladder a linear regression analysis reve aled a significant, negative relationship between the whole volume and medi an dose (P < 0.05). Conclusion: Preferably a CT examination should be provided at every fractio n. However, this is logistically unfeasible in most institutions. To obtain reliable DVHs the patients will in the future undergo 3-4 CT examinations during the course of brachytherapy at our institution. Since this study sho wed an association between large bladder volumes and dose reductions, the p atients will be treated with a standardized bladder volume. (C) 2001 Elsevi er Science Ireland Ltd. All rights reserved.