Impact of the filling status of the bladder and rectum on their integral dose distribution and the movement of the uterus in the treatment planning of gynaecological cancer.

PURPOSE Determination of the impact of the filling status of the organs at risk (bladder and rectum) on the uterus mobility and on their integral dose distribution in radiotherapy of gynaecological cancer. METHODS In 29 women suffering from cervical or endometrial cancer two CT scans were carried out for treatment planning, one with an empty bladder and rectum, the second one with bladder and rectum filled. The volumes of the organs at risk were calculated and in 14 patients, receiving a definitive radiotherapy, the position of the uterus within the pelvis was shown using multiplanar reconstructions. After generation of a 3D treatment plan the dose volume histograms were compared for empty and filled organs at risk. RESULTS The mobility for the corpus uteri with/without bladder and rectum filling was in median 7 mm (95%-confidence interval: 3-15 mm) in cranial/caudal direction and 4 mm (0-9 mm) in posterior/anterior direction. Likewise, cervical mobility was observed to be 4 mm (-1-6 mm) mm in cranial/caudal direction. A full bladder led to a mean reduction in organ dose in median from 94-87% calculated for 50% of the bladder volume (P < 0.05, Wilcoxon's matched-pairs signed-ranks test). For 66% of the bladder volume the dose could be reduced in median from 78 to 61% (P < 0.005) and for the whole bladder from 42 to 39% (P < 0.005), respectively. No significant contribution of the filling status of the rectum to its integral dose burden was noticed. CONCLUSIONS Due to the mobility of the uterus increased margins between CTV and PTV superiorly, inferiorly, anteriorly and posteriorly of 15, 6 and 9 mm each, respectively, should be used. A full bladder is the prerequisite for an integral dose reduction.