Optimal timing of the cumulative dose and factors related to acute urinary toxicity in prostate cancer treated using magnetic resonance linear accelerator.

We investigated which timing of cumulative dose measurement is most strongly associated with acute urinary toxicity in magnetic resonance (MR)-guided linear accelerator (Linac) treatment and also explored additional contributing factors. A total of 114 patients with prostate cancer treated with five-fraction MR-Linac therapy were included. Cumulative bladder doses were calculated using deformable image registration at three time points: before treatment (pre-MRI), during position verification (PV-MRI), and after irradiation (post-MRI). Acute urinary toxicity was defined as an increase of ≥ 10 points in the International Prostate Symptom Score within 3 months from baseline. The relationships between cumulative bladder dose and acute urinary toxicity were analyzed, along with non-dosimetric contributing factors. Cumulative bladder doses calculated from PV- and post-MRI showed stronger associations with acute urinary toxicity than those from pre-MRI. The area under the curve values for bladder V37Gy (cc) and V37Gy (%) on post-MRI were 0.69 and 0.70. A higher frequency of motion correction applied for target displacement during PV-MRI was associated with a lower incidence of acute urinary toxicity (p = 0.04). Secondary adapt-to-shape-based recontouring and replanning shortened the MRI-to-irradiation interval, stabilized prostate positioning during irradiation, and enabled adaptation to an enlarged bladder, which may have reduced bladder dose. Therefore, cumulative bladder doses assessed on PV- and post-MRI are moderately associated with acute urinary toxicity. PV-MRI-based target motion correction and replanning may mitigate this toxicity; however, the correlation between motion correction and post-MRI bladder V37Gy (%) was weak despite statistical significance and should be interpreted with caution.