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 receiving fractionated high dose rate gynaecological brachytherapy by using CT-based 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 high-dose-rate brachytherapy using radiographically compatible applicators. The rectal and bladder walls were delineated and DVHs were calculated. Results: Inter fraction variation of the bladder volume (CVmean = 44.1%) was 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 figures 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 and the whole bladder volume (P = 0.018), whereas no correlation was found for the rectum. For both rectum and bladder a linear regression analysis revealed a significant, negative relationship between the whole volume and median dose (P < 0.05). Conclusion: Preferably a CT examination should be provided at every fraction. 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 showed an association between large bladder volumes and dose reductions, the patients will be treated with a standardized bladder volume. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.