Quantification of intra-fraction changes during radiotherapy of cervical cancer assessed with pre- and post-fraction Cone Beam CT scans

Background and purpose: With the introduction of Intensity Modulated Radiotherapy (IMRT) and image-guided plan-of-the-day strategies, the treatment of cervical cancer has become more sensitive to intra-fraction uncertainties. In this study we quantified intra-fraction changes in cervix-uterus shape, bladder and rectum filling, and patient setup using pre- and post-fraction CBCT scans.

Materials and methods: A total of 632 CBCT scans were analyzed for 16 patients with large tip-of-uterus displacement (>2.5 cm) measured in an empty and full bladder CT scan. In all scans, the bladder, cervix-uterus, and rectum were delineated. For rectum and bladder, intra-fraction volume changes were assessed. Systematic cervix-uterus intra-fraction displacements were obtained by non-rigidly aligning the pre-fraction cervix-uterus to that in the post-fraction CBCT. Intra-fraction patient setup changes were obtained by rigidly aligning pre- and post-CBCTs using the bony anatomy.

Results: The mean time between pre- and post-fraction CBCT scan was 20.8 min. The group-mean intra-fraction displacements averaged over the cervix-uterus were 0.1±1.4/1.8±1.5/-2.8±1.8 (LR/CC/AP) mm. The group-mean 5th and 95th percentile intra-fraction displacements were -2.3,2.1/-0.8,4.9/-5.8,0.5 (LR/CC/AP) mm. There was a significant correlation between bladder inflow rate and cervix-uterus motion (r=0.6 and p<0.01). Intra-fraction changes in patient setup were 1.3/0.4/0.6 and 1.4/1.0/1.1 mm (LR/CC/AP), for systematic and random changes, respectively.

Conclusion: Intra-fraction cervix-uterus motion can be considerable and should be taken into account using appropriate PTV margins.