Fig. 6
From: Does setup on rectal wall improve rectal cancer boost radiotherapy?
Schematic explanation of undetected mobility in absence of imaging contrast. The rectum is simplified as a tube, having only contrast with its surroundings and no internal anatomical landmarks, similar to the contrast of the rectal wall on CBCT. Distensions can be detected in this situation since displacements occur perpendicular to the image contrast (green arrows). However, rotations as well as displacements along the tube, e.g. cranial-caudal movement in case of the rectum, cannot be observed detected since they occur parallel to the image contrast (red arrows)