The bladder filling volume greatly varies during RT for prostate cancer (6, 13, 14). Some studies also reported that the bladder filling volume decreases during RT (4, 14). This variation is also correlated with the motion of the target prostate (13, 14). For this reason, it is important to keep the bladder filling volume as constant as possible, and patients are often instructed to control urination and water intake to ensure bladder filling before CT. For planning CT, our patients are required to drink 500 mL of water immediately after urination and CT is performed 30 min after the start of water intake.
Previous studies have only compared bladder volume on CT images before and during RT. The inter-patient variation ranged from 70 to 509 cm3 (13); the average volume ranged from 94 to 317 cm3 (4), and the intra-patient variation showed a mean decrease of 124 cm3 after treatment (14). To the best of our knowledge, the present study is the first to confirm that the intra- and inter-patient bladder filling volume variations were extremely large even during 2 consecutive days of controlling urination and water intake in patients with the bladder unaffected by RT. One study reported that the bladder volume measured on CT images ranged from 41.0 to 1501.3 cm3 in 419 patients without bladder disease in whom neither urination nor water intake was controlled (15). Although a large inter-patient variation was observed, no association between the volume and patient factors was investigated. The present study is the first to examine the correlation between the bladder filling volume variation and patient factors. Our results revealed that the variation was much larger in patients with a higher eGFR, or with better renal function.
Regarding renal function, SCr levels are dependent on the muscle mass and dietary intake required for creatinine generation (16). GFR is the most accurate index to evaluate overall renal function and is estimated based on age, sex, and race, among other factors. GFR more accurately reflects the renal function than SCr levels alone and is widely used (17, 18). In the present study, we used the conversion formula developed from SCr levels and other data in 763 Japanese patients (10). Some previous reports indicated that the bladder filling volume greatly varied during RT in patients with the larger bladder filling volume at the time of planning CT (4, 6, 19, 20). For example, in one study, there was a significantly larger bladder filling volume variation during RT in the 1080 mL water intake arm compared with the 540 mL arm (19). In cases in which the bladder filling volume at the time of planning CT was larger, the mean relative bladder volume (bladder volume on treatment cone beam CT [mL]/bladder volume on planning CT [mL] × 100%) was smaller (20). In the present study, we observed larger intra-patient variation in patients in which the larger bladder filling volume of the 2 consecutive days was larger. This result was comparable with the results of the previous reports.
Our results show that the bladder filling volume greatly varies in patients with a higher eGFR. When RT is planned in such patients, caution should be exercised. When RT planning CT was performed in cervical cancer patients with an empty bladder after urination, bladder volume variations were small, and exposure doses to the small bowel were within the acceptable range (21). For patients with a higher eGFR, the performance of RT planning CT with an empty bladder may also be considered. In the future, we will conduct additional investigations to evaluate variations in the bladder volume measured by RT planning CT with an empty bladder.