ESTRO 2023 - Abstract Book

S1661

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ESTRO 2023

L. Wang 1 , D. McQuaid 1 , S. Mason 2 , M. Blackledge 2 , H. McNair 1 , E. Harris 2 , S. Lalondrelle 3

1 Royal Marsden Hospital NHS Trust, Radiotherapy department, Sutton, United Kingdom; 2 Institute of Cancer Research, Department of Radiotherapy and Imaging, Sutton, United Kingdom; 3 Royal Marsden Hospital NHS Trust and Institute of Cancer Research, Department of Radiotherapy and Imaging, Sutton, United Kingdom Purpose or Objective Motion of the low-risk clinical target volume (CTV-LR) during cervix cancer radiotherapy can be extensive and complex. Motion of the uterocervix has been documented to relate to bladder and rectal size, but predictive models based on these alone have been imperfect. We aim to create a model that describes and predicts interfractional motion on an individualised basis, using anatomical, pathological and demographical features. Materials and Methods 160 images from 20 cervix cancer patients who received radical radiotherapy were evaluated. For each patient, the full bladder CT, empty-bladder CT, planning MRI and 5 cone-beam CTs chosen at random were contoured (CTV-LR, bladder, rectum) following the EMBRACE-II protocol. In addition, 23 features of the planning CTs and patient demographics were measured, including various measures of organ shape, and contents, and analysed for their influence on subsequent motion of points U (tip of uterine fundus), C (posterior cervix) and center of mass of CTV-LR (CoM) assessed using linear regression and Wilcoxon signed-rank test. Results Variation was observed in bladder volumes (mean 239 ml, range 25-829), rectal diameters (mean 2.6 cm, range 1.1-5.7) and euclidean distance moved by U (mean 2.2, range 0.2-6.9), C (mean 1.3 cm, range 0.2-3.3), and CoM (mean 1.0 cm, range 0.1-3.1). For the overall cohort, bladder volume and rectal diameter did not correlate with distance moved by U, C or CoM (R2 all < 0.1). However, some individuals had stronger correlations; for U motion against bladder volume, 8 patients had R2 > 0.7 (Figure 1).

Patients with a stronger bladder-motion correlation had larger sigmoid volume on CT (p = 0.006) and more motion of CTVLR between full and empty-bladder CTs (more motion of the uterine fundus tip, p = 0.020; larger maximum surface-to-surface distance, p = 0.025; larger Hausdorff distance, p = 0.044). Details in Table 1.