## 3 Results
### Reliability and Repeatability of US Assessment  
The results of the Passing-Bablok regression indicate that the rUS1 and rUS2 assessments are equivalent in terms of linearity and slope. A slope coefficient close to 1 and the lack of a significant difference from 1 confirm that both measurements yield similar results. This allows the conclusion that the US assessments are repeatable and, therefore, their results are reliable (Table 3). This applies to measurements performed by the same operator as well as by two different operators.  
For repeatability analysis, 177 ultrasound assessments performed by two independent operators were included. The BV assessments performed by two different operators were consistent and equivalent, according to both the Wilcoxon statistical test (\(p=0.932\)) and the Bland-Altman plot analysis (Figure 2). Furthermore, the results of the Passing-Bablok regression indicate that the slope coefficient is close to 1, which means that there is a high agreement between the two assessment methods (Table 3). The mean difference between the assessments was very small (0.76 mL) and most of them were in the range between \(-66.06\) mL and 67.58 mL, being consistent and reliable (Table 4).  
Furthermore, the consistency of the rBV assessment performed by the same operator was verified on the basis of 495 paired ultrasound assessments. The rBV assessments performed by the same operator are consistent and equivalent. The result of the Passing-Bablok regression indicates that the slope coefficient is close to 1, which means that there is high agreement between the two measurement methods. The confidence interval includes the value 1, which indicates that there are no significant differences (Table 3). This is also confirmed by the agreement intervals for the Bland-Altman plot (Figure 3).  
\begin{table}
\begin{tabular}{c c c c c c c} \hline
**BV** & **N** & **Mean** & **Median** & **Min** & **Max** & **SD** & \(p\)**-Value** \\ \hline CTplan & 142 & 340.8 & 320.0 & 89.0 & 930.0 & 173.4 & \textless{}0.001 \\ rUS1 & 672 & 214.7 & 199.0 & 70.0 & 483.0 & 81.9 & \textless{}0.001 \\ rUS2 & 672 & 218.1 & 204.0 & 61.0 & 505.0 & 83.0 & \textless{}0.001 \\ \hline \end{tabular}
\end{table}
Table 1: Bladder volume measured by ultrasound (US) and computed tomography (CT).  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Fraction** & **N** & **Mean** & **Median** & **Min** & **Max** & **SD** \\ \hline
1 & 116 & 227.4 & 219.0 & 74.0 & 462.0 & 83.9 \\
2 & 116 & 224.1 & 204.0 & 93.0 & 472.0 & 84.6 \\
3 & 116 & 211.8 & 198.0 & 74.0 & 453.0 & 75.8 \\
4 & 116 & 210.7 & 198.5 & 84.0 & 428.0 & 75.8 \\
5 & 116 & 208.7 & 189.0 & 77.0 & 483.0 & 87.9 \\ Overall & 580 & 216.5 & 200.5 & 74.0 & 483.0 & 81.8 \\ \hline \end{tabular}
\end{table}
Table 2: Variation in real bladder volume during subsequent treatment fractions.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline
**BV** & **Slope** & **95\% CI (Slope)** & **Intercept** & **95\% CI** & **Linear** & **Linearity** \\
**(Intercept)** & **Dependence** & **_p-Value_** \\ \hline Overall & 1.013 & 0.987–1.045 & 0.408 & \(-5.865\)–5.671 & Yes & 0.4716 \\
2 operators & 0.971 & 0.905–1.055 & 4.661 & \(-8.62\)–16.28 & Yes & 1 \\
1 operator & 1.018 & 0.989–1.052 & 0.109 & \(-6.79\)–5.74 & Yes & 0.481 \\ \hline \end{tabular}
\end{table}
Table 3: Repeatability of ultrasound assessment of bladder volume (results of Passing-Bablok regression).