Deviation of the upper, middle and lower thirds of the rectum during irradiation of rectum cancer.

Background: In patients with rectal cancer, daily fluctuations in rectal volume often lead to large deformations of the target volume that cannot be corrected by adjusting the radiation couch. The main aim of the study was to determine the deviation of all three thirds of the rectum from the reference position and to determine whether these deviations are influenced by the alignments to the bony structures (the sacrum) and the posterior rectal wall.

Patients and methods: The conduct of the study was divided into review of the database, contouring of the anatomic structures on the cone-beam computed tomography (CBCT) images, data collection of the deviations and data output, with the alignment to the bony structures (the sacrum) - and the alignment to the posterior rectal wall performed separately.

Results: Fifty preoperatively irradiated patients were included in the study. The analysis revealed statistically significant differences in terms of alignment to the bony structures for different variations of the rectal thirds in the anterior (+), posterior (+), posterior (-), left (-), right (+) and right (-) directions. With regard to the alignment to the posterior rectal wall, the analysis showed statistically significant differences for different variations of the rectal thirds in directions anterior (+), posterior (+), posterior (-), left (-), right (+), right (-) and bone (+). (The positive value (+) means that the rectum was larger and the negative value (-) means that the rectum is smaller than in the reference position of the CT simulator images.).

Conclusions: The position of the rectal thirds changes daily in alignment with the bony structures (the sacrum), and in alignment with the posterior rectal wall.