Full bladder, empty rectum? Revisiting a paradigm in the era of adaptive radiotherapy.

BACKGROUND AND PURPOSE: Many patients find it challenging to comply with instructions regarding rectum and bladder filling during pelvic radiotherapy. With the implementation of online adaptive radiotherapy, the reproducibility of organ volumes is no longer a prerequisite. This study aims to analyze the sparing of the bladder and the posterior rectum wall (PRW) in conditions of full versus empty bladder and rectum. METHODS: 280 fractions from 14 patients with prostate cancer who underwent adaptive radiotherapy using the Varian Ethos system were analyzed post-hoc. Various metrics for the bladder and PRW were correlated with respect to organ volume. RESULTS: Our analysis quantitatively confirms the advantage of a full bladder during radiotherapy, as metrics V48Gy and V40Gy significantly inversely correlate with bladder filling for each patient individually. While bladder volume did not show a gradual decrease over the course of radiotherapy, it was observed to be higher during planning CT scans compared to treatment sessions. A full rectum condition either significantly improved (in 2 out of 7 patients) or at least did not impair (in 5 out of 7 patients) PRW sparing, as represented by the V30Gy metric, when patients were compared individually. The average V30Gy across all patients demonstrated a significant improvement in PRW sparing for the full rectum condition, with a [Formula: see text]-value of 0.039. CONCLUSION: Despite the implementation of adaptive therapy, maintaining a high bladder filling remains important. However, the recommendation for rectum filling can be abandoned, as reproducibility is not critical for adaptive radiotherapy and no dosimetric advantage per se is associated with an empty rectum. Patients may even be encouraged not to void their bowels shortly before treatment, as long as this is tolerated over the treatment session.