## 4 Discussion  
The results of the present study showed the repeatability of the US-based rBV assessment, confirming the reliability of the method. In this study, due to the lack of volumetric imaging capability of the CyberKnife system, US-based BV assessment was not correlated with radiologically based rBV (e.g., CBCT). Therefore, the consistency of the results of two independent assessment methods cannot be confirmed. However, the data available in the literature indicate the consistency of US-based results with those based on radiologically. [PERSON] et al. compared bladder filling before RTH assessed with the PINIT ultrasound (AUTO mode) and with CBCT and reported comparable results. However, the study did not specify the difference between pBV and rBV [13]. In a phase III randomized study, [PERSON] et al. compared BV using a BladderScan BVI 6100 device and computed tomography (CT) in 190 patients treated for prostate cancer. The radiation oncologist performed three US assessments (considering the average and maximum readings) of the BV immediately before CT, followed by a CT-based BV assessment. There was shown to be a strong positive correlation between both results. A slight underestimate of the US-based rBV was observed [14]. Other authors reported similar results. [PERSON] et al. conclude that the bladder scanner provides an acceptable indicator for monitoring bladder volume in pelvic radiotherapy patients [15]. [PERSON] states that the use of the portable ultrasound bladder scanner in mode A significantly improved the reproducibility of BV [16]. [PERSON] concluded that US was accurate in measuring bladder volume and can be used for daily monitoring in prostate cancer radiotherapy [17]. [PERSON] observed that the measurement of BV caused a relative reduction in the failure rate with an odds ratio of 0.44 and an absolute reduction of 9.1% [18]. The results indicate that the planning of radiotherapy for patients should not involve bladder fillings greater than 350 mL, as it is difficult to reproduce it in rBV. Our data showed that during RTH, patients found it difficult to achieve rBV close to pBV. This finding is consistent with data from the literature. Issues related to bladder anatomy and physiology suggest that the need to urinate generally occurs at a BV between 250 and 300 mL, while at 400 mL, the urge is strong, making refraining from urination difficult [19]. This would explain the large differences in rBV in the group of patients with pBV greater than 349 mL. [PERSON] et al. analyzed BV changes during high-dose IMRT in relation to acute genitourinary \begin{table}
\begin{tabular}{c c c c c} \hline
**pBV** & **N** & **R** & **T(N-2)** & _p-Value_ \\ \hline
100–349 & 386 & 0.376 & 7.942 & \textless{}0.001 \\
350–499 & 168 & 0.138 & 1.799 & 0.074 \\
500–800 & 108 & 0.419 & 4.748 & \textless{}0.001 \\ overall & 662 & 0.577 & 18.147 & \textless{}0.001 \\ \hline \end{tabular}
\end{table} 
Table 6: Correlation of BV for US and CT measurements (pBV and rBV) in each group (Spearman test). \begin{table}
\begin{tabular}{c c c c c c c} \hline
**pBV** & **Type** & **N** & **Mean** & **Median** & **Min** & **Max** & **SD** \\  & **of Measurement** & **(mL)** & **(mL)** & **(mL)** & **(mL)** & **SD** \\ \hline
100–349 & \begin{tabular}{c} CT-US * \\ CT (pBV) \\ US \\ \end{tabular}  & \begin{tabular}{c} 386 \\ 386 \\ 386 \\ 168 \\ 168 \\ 168 \\ 168 \\ 108 \\ \end{tabular}  & \ \begin{tabular}{c} 41.56 \\ 386 \\ 226.37 \\ 184.80 \\ 171.32 \\ 233.98 \\ 333.33 \\ 333.79 \\ 300.45 \\ 295.25 \\ \end{tabular}  &
\begin{tabular}{c} 41.56 \\ 410.0 \\ 100.0 \\ 100.0 \\ 120.0 \\ 120.0 \\ 108.0 \\ 120.0 \\ 108.0 \\ 120.0 \\ 10  
toxicity in 193 patients treated for prostate cancer. rBV was maintained between 200 and 300 mL. The results showed that rBV greater than 180 mL allowed for the reduction in the acute toxicity ratio [20].