Estimation of rectal fluctuations during radiotherapy of rectal cancer using CBCT measured on the bone structures and the posterior rectal wall

This study aims to determine the deviation of the rectum from the reference position in the various parts of the rectum. Fifty neoadjuvant patients were included in the retrospective study. All patients had long-course radiotherapy. At the time of treatment, we acquired six cone-beam computed tomography (CBCT) images. The anatomical structures of the rectum and sacrum were visualized on transverse CBCT images using Eclipse software. In the upper rectum, a statistically significant difference was found between the variable's posterior ± when aligned to bone structures (p = 0.017), while when aligned to the posterior rectal wall, the variables posterior ± represent a borderline statistical significance (p = 0.051). In the lower rectum, a statistically significant difference was found between the variables left ± (p = 0.0001) and between right ± (p = 0.008) when aligned to bone structures. When aligned to the posterior rectal wall, a statistically significant difference was found between left ± (p = 0.006) and right ± (p = 0.005). Daily CBCT imaging and adaptive radiotherapy should be considered, especially in patients with tumors in the highly mobile upper third of the rectum, to minimize these deviations and ensure optimal treatment accuracy.