Optimization of radiotherapy treatment plans based on dose-volume histograms relies on accurate organ delineation. Hollow organs, such as the rectum, are difficult and time-consuming to delineate owing to unclear visualization of the border between wall tissue and filling. Automated hollow organ delineation would be a valuable tool, but its development depends upon improved understanding of the dynamics of the rectum in response to filling. Two reasonable assumptions proposed in the literature are that (1) the rectal wall tissue along a constant length of the rectal cylinder is preserved over time and (2) the rectal wall tissue is distributed homogeneously along the cylinder. Therefore, variations in wall thickness can be explained by variable rectal filling. To investigate these assumptions, transversal cross-sectional areas enclosed by the outer contour (A(out)) and inner contour (A(in)) of the rectum were recorded from digital photographs of cadaver cryo-sections from the U.S. National Library of Medicine's Visible Human Project. In addition, A(out) and A(in) were recorded from 19 CT scans of 5 of our own patients. The transversal cross-sectional area of the wall of the rectum, A(wall)=A(out)-A(in), was calculated. The data derived both from cryo-sections and repetitive CT scans of patients, revealed that there was a significant correlation between A(wall) and A(out), in contradiction to assumption (1) stated above (male cryo-sections: p<0.001, female cryo-sections: p=0.03, repetitive CT scans p<0.001). Moreover, the mean A(wall) calculated from one CT scan differed significantly from the mean A(wall) from other CT scans and was correlated with the mean A(out), i.e. rectal filling (p<0.001). This finding was confirmed by careful analysis of another study (p=0.001) and opposes assumption (2). Hence, the amount of wall tissue within a constant length of rectum is not preserved over time, but increases with increased filling. This implies that the longitudinal length of the rectum decreases in response to distension of the organ.