Early and late effects of irradiation on morphometry and residual strain of mouse rectum.

Radiotherapy of malignancies in the pelvis is associated with both early and late intestinal reactions with subsequent alterations in rectal function because this part of the intestine often is included in the radiation field. This report presents data on morphometry of the zero-stress and no-load states as well as residual strains and opening angles of the rectum in mice with early and late radiation injury and in age-matched sham-irradiated control groups. In blood vessels and esophagus, cross-sections at the zero-stress state are not closed rings, but open sectors. The rectal zero-stress state is demonstrated by cutting the rectum into rings and cutting the rings into sectors; each sector is characterized by an opening angle. The opening angle was between 50 degrees and 200 degrees and differed between groups (P < 0.01). Young mice had 25-50% smaller opening angles than the older mice. The irradiated groups had 25-75% smaller opening angles than the normal groups. The residual strain analysis showed mucosal compression and serosal tension in the no-load state. Both at the serosal and mucosal surfaces, variation was found between groups (P < 0.01). The no-load state wall thickness-to-mucosal circumferential length ratio varied along the axis of the organ (P < 0.01) and between groups (P < 0.01). The early irradiated group had higher values than the control group. Furthermore, proximal to the irradiated zone the late irradiated group had significantly higher values compared to the normal group, indicating a higher stress level proximal to the irradiated zone. The zero-stress state must be taken into account when studying pathophysiological problems in which the stresses and strains are important, eg, the storage function of the rectum. Radiation therapy and age remodeled the morphometry and zero-stress state of rectum.