Abstract
BACKGROUND AND PURPOSE: Ultra-hypofractionated radiotherapy using stereotactic body radiotherapy (SBRT) in five fractions is standard care for patients with low- or intermediate-risk prostate cancer. Rectal preparation aims for consistent rectal filling to improve dose accuracy and reduce toxicity, however, current evidence is inconsistent and based on conventional fractionation. This study compares four rectal preparation strategies in terms of rectal volume, dose delivery, and acute gastrointestinal toxicity. MATERIALS AND METHODS: A total of 56 patients were allocated to four groups with different rectal preparation strategies: (1) no preparation (N = 14); (2) laxatives and a high‑fiber diet before planning CT and during treatment (N = 14); (3) laxatives before planning CT, without laxatives during treatment and without diet (N = 14); and (4) laxatives before planning CT and during treatment, without diet (N = 14). In total, 280 pre‑treatment cone‑beam CT scans were analyzed. Rectal volumes and dosimetric parameters were extracted and compared across groups. Both patient‑reported and physician‑reported gastrointestinal toxicity were evaluated. RESULTS: Groups receiving extensive (laxatives and diet) or no preparation showed slightly greater rectal volume stability compared to groups receiving laxatives or laxatives only before planning CT (p = 0.001). Most dose parameters were comparable between groups. The group that received laxatives only before planning CT showed higher absolute rates of patient and physician reported gastrointestinal toxicity, although these differences were not statistically significant. CONCLUSION: None of all rectal preparation strategies consistently improved rectal volume stability, provided dosimetric benefit, or reduced gastrointestinal toxicity compared with no preparation. These findings do not support a meaningful benefit of routine use of rectal preparation in the context of modern daily CBCT-guided and adaptive radiotherapy workflows. Future research should prioritize patient-centred, less invasive approaches as the current evidence does not justify the routine use of rectal preparation.