ESTRO 2021 Abstract Book

S175

ESTRO 2021

registration (DIR) algorithm. Materials and Methods

Ten PCa patients treated with 5x7.25 Gy on a 1.5T MR-Linac were included (50 fractions in total). During each fraction multiple (≥2) T2-weighted MR scans were acquired (110 scans in total, 3 min scan time). A normalized gradient field-based DIR algorithm (EVolution, Denis de Senneville, Phys Med Biol. 2016 Oct 21;61(20):7377- 7396) was used to sequentially register MR scans (MR1-MR2, MR2-MR3, etc.) for each fraction and to propagate the contours (CTV, bladder, rectum). The CTV included the prostate body, GTV with a 4 mm margin, and up to 1/3 rd of the seminal vesicles. The propagated contours were judged by a physician on clinical usability using two criteria. First, for each structure, the need for adaptations within a 2 cm ring (1 cm in craniocaudal direction) around the CTV was assessed on a four-point scale, ranging from none to multiple major adaptations. Second, it was judged if manual adaption of all structures combined could be executed within 3 min, to allow for fast intra-fraction adaptation. Results were stratified by the interval between the MR scans (<10 min versus ≥10 min, ‘short’ and ‘long’), to represent shorter and longer intervals for IF adaptation and re-planning. Results Mean (SD) interval between scans was 5.7 (±1.4) min ('short', n=10 pairs) and 20.0 (±5.2) min ('long', n=50 pairs). Table 1 shows the need for adaptations. In 4 (40%) 'short' and 9 (18%) 'long' cases none of the contours needed manual adaptation. All 'short' interval cases could be adapted within 3 min. For the 'long' interval, 3 min was sufficient in 46 (92%) cases. The remaining four showed large IF rectal deformations, in addition to a need for CTV and/or bladder adjustment.

Conclusion EVolution performed very well for IF contour propagation of bladder and prostate CTV. Overall, rectum contours are acceptable but sometimes need more (extensive) adaptations to perfectly fit the anatomy. Nevertheless, adaptation times were <3 min for the majority of 'long' interval cases and all 'short' interval cases, indicating feasibility for an IF adaptive workflow using repetitive MR imaging and multiple re-planning steps. For cases with extreme IF rectal deformations, 3 min might not suffice. While EVolution provides acceptable results, our future work will include additional algorithms to find the optimal approach. PH-0267 Exploration of feasible motion metrics for bowel motion quantification in pelvic radiotherapy M. Dassen 1 , D. Barten 1 , J. Laan 1 , H. Westerveld 1 , A. Bel 1 , Z. Van Kesteren 1 1 Amsterdam UMC - location AMC, Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective Bowel toxicity is commonly observed in patients treated with pelvic radiotherapy. Especially movements of the bowel into the high-dose area may be an important factor to consider during treatment planning. Therefore, patients would potentially benefit from the incorporation of bowel motion into radiotherapy treatment planning. A deformable image registration (DIR) framework was developed using a 3D cine-MRI acquisition, to enable quantification of bowel motion by analyzing the deformations between consecutive dynamics. The aim of this study was to translate these deformations into a feasible motion metric for application in the field of radiotherapy. Materials and Methods Gynecological patients (n=18) who received external beam radiotherapy (EBRT) were included in this study. In each patient a pre-EBRT 3D cine-MRI was acquired including a total of 160 dynamics per acquisition with an acquisition time of 3.7s per dynamic. Consecutive dynamics were deformably registered (Elastix, ISI) using a B- spline transformation model, resulting in 159 deformation vector fields (DVFs) per MRI set, describing the magnitude of motion. In each scan, the bowel bag volume (excluding bladder, rectum and uterus) was delineated on the first dynamic and co-registered to each set of DVFs. Subsequently, the distribution of the deformation vector lengths situated in the bowel bag volume was used to explore feasible motion metrics. The mean and median vector length were evaluated as motion metrics for the visualization and quantification of all motion in the bowel bag. In addition, the maximum and the 95 percentile (max95%) vector length were tested for the identification of extreme bowel motion by analyzing robustness for potential registration errors. Finally, the most feasible metrics were used to quantify motion by means of a cumulative motion-volume histogram (MVH) and to analyze inter-patient variation of bowel motion using the M50% parameter. Results The deformation vector lengths of a complete set of 159 DVFs showed a non-gaussian distribution of bowel motion (Figure 1). Therefore, the median vector length was more descriptive to the overall bowel motion compared to the mean vector length per voxel. In addition, the max95% metric showed limited sensitivity for registration errors compared to the maximum motion metric. Moreover, this metric was considered to be most