Validation of bowel and bladder preparation by rectum and bladder variation in prostate radiotherapy based on cone beam CTs

Purpose: To validate the effect of an individualized, patients-positive, Chinese-style diet bowel and bladder preparation protocol by analyzing bladder and rectum geometric and dosimetric variation in prostate radiotherapy.Methods: We retrospectively evaluated 135 cone-beam computed tomography (CBCT) of 27 prostate patients. Organs at risk on CBCTs were re-contoured by the same radiotherapy doctor for consistency. The dose on CBCTs was recalculated with the forced mean electron density of organs in treatment isocenters to obtain the delivered dose. The volume and delivered dose on CBCTs were compared to planning dose. The relationship between delivered dose and translational shift to volume changes was analyzed.Results: The mean relative volume variations of the rectum and bladder were 112.12 +/- 29.46% and 89.90 +/- 24.77%, respectively. With the registration of CBCTs to planning CT by target matching, the translational shifts <3 mm, <4 mm and <5 mm were approximately 52%, 80% and 92% of 135 CBCTs, respectively. The normalized rectal volume was linearly related to the left-right shift (P = 0.0087, R = 0.0183), and the normalized bladder volume was linearly related to the superior-inferior shift (P = 0.0058, R = 0.0202). Remarkable bladder volume reduction was observed during the radiotherapy course. Dmean, Dmax, V30Gy, V40Gy, V50Gy, V55Gy, V60Gy and V65Gy of the rectum on cumulative or single CBCT were higher than planned (P < 0.05) and mostly met clinical constraints. V30Gy and V40Gy of the bladder on CBCTs were lower than planned (P < 0.05). There was a linear regression relationship between the normalized rectum volume and Dmin, Dmax and V50Gy. Negative regression relationship was found between the normalized bladder volume and Dmin, Dmean, V30Gy, V40Gy, V50Gy, V55Gy, V60Gy and V65Gy.Conclusions: The delivered dose of the rectum met the planning constraints. Additionally, the delivered dose to the bladder was lower than planned dose. The results demonstrate the validity of the bowel and bladder preparation.