MESORECT: Influence of bladder depletion on mesorectal movements during radiation therapy of locally advanced rectal cancers: a prospective comparative study

Abstract Neoadjuvant radiotherapy (RT) is a cornerstone in the management of locally advanced rectal cancer (LARC). With the increasing use of intensity-modulated radiation therapy (IMRT), accurate target delineation and treatment reproducibility are critical. Bladder filling may influence anterior mesorectal displacement. This study prospectively evaluated anterior mesorectal motion under "full" versus "empty" bladder conditions. Forty-eight patients were included. Anterior mesorectal distances relative to bony landmarks were measured at six pelvic levels on planning CT (pCT) and seven cone-beam CTs (CBCTs) acquired during treatment under both bladder protocols. Inter-individual means of intra-individual differences and their standard deviations (SD) were calculated to assess variability. Secondary endpoints included bladder volume reproducibility, an in silico dosimetric comparison, and calculation of anisotropic anterior planning target volume (PTV) margins. Anterior displacements were similar between protocols, with consistent forward motion. Variability decreased from upper to lower rectum. In the upper rectum, bladder filling had little effect (SD: empty 6.48 mm vs. full 5.19 mm). In mid and lower rectum, variability was lower with an empty bladder (mid: 4.60 vs. 5.36 mm; lower: 3.03 vs. 3.45 mm). Bladder volume was poorly reproducible under full-bladder conditions (mean 243 cm³, SD 164 cm³) but consistent with empty bladder instructions (mean 73 cm³, SD 58 cm³). Dosimetric analysis showed slightly higher organ-at-risk doses with an empty bladder, remaining within national constraints. An empty bladder protocol with anisotropic margins improves treatment reproducibility while maintaining dosimetric safety in LARC patients.