Accumulated bladder wall dose and urinary toxicity 137 Results One-hundred-and-thirty patients were included, of whom 39 patients (30%) experienced acute urinary toxicity (Table 1). Of these 39 patients, 20 patients reported an increase in IPSS of ³ 10 points during the first three months of follow-up, of whom 10 also started with alpha-blocking medication. The remaining 19 patients reported no increase in IPSS of ³ 10 points but did start with alphablocking medication. Patients with and without acute urinary toxicity were comparable at baseline except for PTV volume (p < 0.001) and alpha-blocker usage at baseline (p = 0.048). Median IPSS for the entire cohort was significantly higher at 1 month (p < 0.001) and 3 months (p = 0.001) compared to baseline, with a larger proportion of patients experiencing moderate (59.5%) or severe (11.9%) urinary symptoms at 1 month (Figure 2). Most patients reported an increase in score for the ‘frequency’, ‘urgency’, and ‘weak stream’ sub-items (results not presented). Median (IQR) DSCmean was 0.99 (0.98-0.99) for the bladder and 0.84 (0.80-0.87) for the bladder wall. Median (IQR) HDmean was 0.05 mm (0.03-0.10) for the bladder and 0.57 mm (0.42-0.83) for the bladder wall. For two cases (1.5%), bladder DSCmean was < 0.95 (0.92 and 0.94), due to large bladder volume differences that caused (small) registration errors in the cranial part of the bladder for ³ 1 registration. Because this was outside the medium-high dose area, these cases were included. Collinearity statistics showed high correlations over the entire range of bladder (wall) dose parameters (Supplementary C). In univariable analysis (Table 2), Bladder D5cm3, V10-35Gy (in %), and Dmean and Bladder wall V10-35Gy (cm3 and %) and Dmean were correlated with the outcome (odds ratios 1.041.33, p-values 0.001-0.044). Except for relative V35-37Gy (%), bladder wall dose parameters showed larger AUC values compared to their bladder equivalent (Table 2). Corrected for age, diabetes, cardiovascular disease, baseline IPSS, and alpha-blocker usage at baseline (Table 3), bladder V10-35Gy (in %) and Dmean and bladder wall V10-35Gy (cm3 and %) and Dmean were still correlated with the outcome (odds ratios 1.04-1.30, p-values 0.001-0.028). Preliminary cut-off points for the dose parameters based on the Youden index were 11.2 Gy and 11.7 Gy for the bladder and bladder wall Dmean, respectively, and 9.0 cm3 for bladder wall V25Gy (Figure 3 and Supplementary D). In our cohort, of all patients with bladder wall V25Gy £ 9.0 cm3 (n = 70), 16.6% reported urinary toxicity. For bladder wall Dmean £ 11.7 Gy (n = 47) this was 10.6%. 7
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