## 4 Discussion
Thus, it seems reasonable for such patients to maintain the pBV within the range of 200-300 mL, both due to the reduction in potential toxicity and the urgency sensation, which can improve patient comfort and the reproducibility of the treatment plan. To ensure that CT scanning for planning is performed in the optimal range of BV, the US-based rBV assessment is recommended before CT (such an approach reduces the radiation dose for the patient and saves time in the CT scan room). The US-based rBV assessment should be introduced as a standard procedure prior to each fraction of CyberKnife stereotactic radiosurgery in prostate cancer patients and prior to CBCT in preparation for the radiotherapy session. US monitoring of bladder filling during RTH is also important due to the fact that the obtained results, as well as other published studies, indicate variability in bladder filling during the course of RTH. Studies in which daily or weekly bladder ultrasound was performed showed a decrease in BV of 16-50% compared to its initial value [6]. According to some studies, the time required to fill the bladder to the same volumes also changes during the course of RTH (with increasing RT treatment time, the time needed to fill the bladder to the same volume also increases). The bladder filling time in the seventh week of RT may even increase by up to 40 min compared to the time needed in the first week. This requires individual modifications to the patient's preparation protocol for each subsequent RT session and is time-consuming [7]. According to [PERSON] et al., the greater the difference in bladder filling compared to the treatment plan, specifically in terms of decreased BV, the greater the probability of increased toxicity of treatment [6]. In this study, the conformity between ultrasound measurement and radiological measurement was not assessed. While the study compares bladder volume measured by ultrasound (PINIT), no direct volumetric imaging (e.g., CBCT, CT) was used during treatment to validate ultrasound measurements against real-time CT or CBCT-based measurements. This is a significant limitation because the ultrasound device used for bladder volume assessment has a tolerable measurement error of +/-15%, which may introduce variability, particularly in cases where precise bladder volume control is critical to the treatment plan. Although the study found a high degree of reliability in US measurements between different operators, there is still the possibility of operator-dependent variability, particularly with less experienced operators, which could affect the accuracy of BV assessments. Despite the consistency of both measurements, it is possible that both independent operators performed the ultrasound examination incorrectly. This means that both measurements, despite their consistency, may indicate an incorrect bladder volume. In this study, the time between the ultrasound measurement and the start of the radiotherapy session was also not checked. It was assumed that the treatment took place immediately after the ultrasound, in accordance with the protocol. Changes in bladder filling during the radiotherapy session were also not monitored. Since one session lasts more than half an hour, in some cases, the BV may have changed significantly. There are several potential future research directions in the area of monitoring bladder volume during ultrasound-based radiotherapy, particularly with the CyberKnife system. Since the CyberKnife system lacks volumetric imaging capabilities, future research could focus on comparing the accuracy of ultrasound (US) measurements with more advanced imaging techniques such as cone beam computed tomography (CBCT). This would help determine discrepancies between these methods and improve the precision of the bladder volume assessment. Future research could aim to refine and personalize bladder filling protocols to improve consistency between real bladder volume (rBV) and pBV, ensuring more precise treatment delivery.