## 1 Introduction  
The CyberKnife(r) M6(tm) Series (Accuray) system enables the delivery of highly precise radiation dose to the prostate [1; 2; 3]. Monitoring of bladder volume (BV) is crucial because changes in bladder size can alter the position of the target volume and other surrounding normal organs [4; 5]. Ensuring consistent BV helps maintain the correct positioning of the prostate and, thus, maximizes the accuracy of radiation delivery and minimizes exposure to surrounding normal tissues. Since this system has limited imaging capabilities, positioning of the prostate gland is performed using fiducial markers and two-dimensional X-ray images. This system does not have volumetric imaging capabilities; bladder and rectum filling cannot be monitored. Changes in BV during radiotherapy in patients with prostate cancer do not affect the position of the prostate as a target but may influence the position of the seminal vesicles. Significantly higher bladder toxicity is observed when the real BV (rBV) is less than the planned BV (pBV) [6]. Some studies have shown that BV less than 180 mL is correlated with a higher amount of bladder tissue within the higher dose range. In such cases, a trend toward prolonged acute genitourinary toxicity appears [6; 7]. Due to this, rBV is monitored by cone beam CT (CBCT) prior to each RT session and matched as closely as possible to pBV (this refers to classical linacs). Volumetric imaging is not available for the CyberKnife system. It has been observed that during radiation therapy (RTH), the time it takes for the patient to achieve a similar rBV is changing (increasing) [8; 9]. Introducing ultrasound imaging instead of CBCT during CyberKnife stereotactic radiosurgery to monitor bladder filling seems to be accurate, fast, and comfortable for patients, consequently increasing the reproducibility of the pBV during RTH sessions. Ultrasound imaging plays a significant role in the assessment of bladder volume, particularly in medical fields such as urology and radiation therapy. There are two main types of ultrasound techniques for bladder evaluation: transabdominal ultrasound (TAUS) and transrectal ultrasound (TRUS). These methods differ in terms of their approach and utility, especially when it comes to precision and patient comfort. Transabdominal ultrasound (TAUS) is a non-invasive and widely used method for bladder volume assessment. The probe is placed on the abdomen, and the bladder is visualized in two dimensions, which allows for the calculation of its volume. TAUS is generally sufficient for routine measurements of bladder volume, particularly when high precision is not critical, as it provides a fast, painless, and effective way to estimate bladder content. This makes it a practical choice in daily clinical settings, including radiotherapy, where consistent bladder filling is important for precise targeting. In contrast, transrectal ultrasound (TRUS) offers a more detailed and accurate view of the bladder and surrounding tissues. By inserting the probe into the rectum, the TRUS provides visualization closer proximity to the bladder, making it suitable for detecting abnormalities and providing more precise volume measurements. However, this method is more invasive and less comfortable for the patient, limiting its use primarily to specialized diagnostic situations [10; 11]. When comparing ultrasound with computed tomography (CT) and magnetic resonance imaging (MRI), it is clear that ultrasound offers several advantages. It is noninvasive, portable, and does not expose patients to ionizing radiation, which is a concern with CT scans. Magnetic resonance imaging, while offering highly detailed images, is time consuming and expensive, making it less practical for frequent bladder volume assessments. However, both CT and MRI provide more precise volumetric data, especially in cases where the shape or position of the bladder needs to be considered in greater detail [12].