Is Intrafraction Motion an Important Consideration in MR Guided External Beam Radiotherapy for Cervical Cancer?

Purpose/Objective(s): During external beam radiotherapy (EBRT) for cervical cancer, large planning target volumes are required to account for potential pelvic organ motion during treatment. These large margins drive treatment toxicity. The introduction of online adaptive MR guided EBRT will enable plan adaption to daily anatomical position, minimizing the impact of inter-fraction changes. Therefore, intra-fraction motion becomes the larger source of residual uncertainty. This study assesses the magnitude and variation of intra-fraction motion at multiple points over an estimated treatment delivery time. Materials/Methods: A prospective MR imaging study acquired scans on a 1.5T diagnostic scanner at 4 time-points over the course EBRT (45Gy/25#.) 6 patients were included in this analysis, At each time-point, motion was captured with 5 alternating axial and 5 sagittal fast T2 Turbo Spin Echo sequences (acquisition time 1min), over approximately 10 minutes. Patients were instructed to empty their bladder and drink 300ml water prior to the scan. The bladder, rectum and a target volume; uterus, cervix and gross tumor were contoured on the initial axial sequence by a single observer. Contours were propagated on the subsequent axial sequence using deformable registration and manually corrected. This was repeated in a stepwise manner. The mean distance to agreement (mDTA), which is the mean of measuring the shortest distance for all points on one surface of one structure to another, was calculated for the target volume with reference to the initial axial sequence for each time-point. The standard deviation (SD) and maximum DTA was also calculated. A linear-mixed effects model was used to analyze within and between patient variability.
Result(s): Based on initial analysis of 6 patients, the mDTA on each sequence was less than 1mm in all cases (range 0.01 to 0.95mm.) The maximum DTA was 9.03mm, in patient 1 at time-point 2, due to rectal passage of gas. The mDTA and SD at each time-point are summarized in the table. There is lower variability between time-points than between patients, with a between patient variance for mean DTA of 0.028 and a time-point to time-point variance of 0.019.
Conclusion(s): This is the most detailed assessment of intra-fraction motion during EBRT for cervical cancer. Although intra-fraction motion is small, the maximum DTA recorded was 9mm and was related to rectal gas, suggesting exception gating would be required if margins are reduced. Finally, this study will inform appropriate treatment margins for online adaptive radiotherapy. [Figure presented]
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