Critical structure movement in cervix brachytherapy

Background and purpose: To investigate critical structure movement and subsequent dose received during conformal MR-guided cervix brachytherapy. Materials and methods: 21 patients (36 HDR fractions) undergoing brachytherapy for cervical cancer underwent a second MR immediately prior to treatment (pre-treatment MR). Bowel (including sigmoid), bladder and rectum were outlined on both planning and pre-treatment MR scans and dosimetry compared. Results: No statistically significant differences were found between the volumes of the OAR doses across the two scans but there were large variations between patients with differences of up to 3.3 Gy observed. The percentage of fractions for which D-2cc was within 10% of that planned was 61.1%, 41.7% and 47.2% for bladder, rectum and bowel, respectively. The average time between MR scans was found to be 4.75 h (SD +/- 1.2: range 3.2-9.9 h), with no correlation found with critical structure movement within this range. Conclusions: OAR movement is difficult to predict though significant changes occur in individual patients. In 61% of cases in our sample the D-2cc dose changed by at least 10% for at least one OAR from that planned. Pre-treatment imaging with subsequent adjustment of dosimetry will minimise the impact of organ movement on delivered dose. (C) 2013 Elsevier Ireland Ltd. All rights reserved.