Quantifying intrafractional gastric motion using auto‐segmentation on MRI: Deformation and respiratory‐induced displacement compared

BACKGROUND AND PURPOSE: For accurate pre‐operative gastric radiotherapy, intrafractional changes must be taken into account. The aim of this study is to quantify local gastric deformations and compare these deformations with respiratory‐induced displacement. MATERIALS AND METHODS: Coronal 2D MRI scans (15–16 min; 120 repetitions of 25–27 interleaved slices) were obtained for 18 healthy volunteers. A deep‐learning network was used to auto‐segment the stomach. To separate out respiratory‐induced displacements, auto‐segmentations were rigidly shifted in superior‐inferior (SI) direction to align the centre of mass (CoM) within every slice. From these shifted auto‐segmentations, 3D iso‐probability surfaces (isosurfaces) were established: a reference surface for P(Occ) = 0.50 and 50 other isosurfaces (from P(Occ) = 0.01 to 0.99), with P(Occ) indicating the probability of occupation by the stomach. For each point on the reference surface, distances to all isosurfaces were determined and a cumulative Gaussian was fitted to this probability‐distance dataset to obtain a standard deviation (SD(deform)) expressing local deformation. For each volunteer, we determined median and 98(th) percentile of SD(deform) over the reference surface and compared these with the respiratory‐induced displacement SD(resp), that is, the SD of all CoM shifts (paired Wilcoxon signed‐rank, α = 0.05). RESULTS: Larger deformations were mostly seen in the antrum and pyloric region. Median SD(deform) (range, 2.0–2.9 mm) was smaller than SD(resp) (2.7–8.8 mm) for each volunteer (p < 0.00001); 98(th) percentile of SD(deform) (3.2–7.3 mm) did not significantly differ from SD(resp) (p = 0.13). CONCLUSION: Locally, gastric deformations can be large. Overall, however, these deformations are limited compared to respiratory‐induced displacement. Therefore, unless respiratory motion is considerably reduced, the need to separately include these deformation uncertainties in the treatment margins may be limited.