BACKGROUND: It is generally agreed that patients with constipation are at risk of urinary retention and poorer bladder emptying. In bladder cancer radiotherapy planning, wider CTV-PTV margins are required to account for daily variation in bladder volume. The aim of this was project to look at the relationship between rectal and bladder volumes in patients undergoing radical radiotherapy for bladder cancer at Aberdeen Royal Infirmary.
METHOD: 33 such patients were identified who were treated over a 2 year period from 01/01/14 to 31/12/15. Patients had a planning CT scan then cone beam CT scans for image guidance on days 1-3 of treatment and weekly throughout their 4 week course of radiotherapy. Some patients had additional cone beam CT scans and these were also included in the data. 22 had 3D radiotherapy and 11 had volumetric modulated arc therapy after this was routinely introduced in early 2015. Bladder and rectal volumes were contoured on all scans using Prosoma contouring software.
RESULTS: 241 scans were used for bladder and rectal volumes including planning and cone beam scans. Patients had between 6 and 10 scans in total. Bladder volumes ranged from 49.4 to 438.5ml. Rectal volumes ranged from 32.8 to 246.8ml. Average bladder volume was 134.8ml, average rectal volume was 84.9ml. The scatter plot shows some correlation with low rectal volume equating to low bladder volume, but with outliers in each direction. However significant patient-to-patient and scan-to-scan variation was noted. Overall rectal volume was seen to remain relatively consistent, with some outliers, throughout the treatment course although in some cases there was an overall reduction which may be attributable to enteritis. Bladder volumes were more variable throughout the treatment course.
CONCLUSIONS: Rectal volumes remained relatively consistent throughout the treatment course, but without a consistent correlation to bladder volumes. The reduction in rectal volume between first and last scans may be attributable to enteritis. A future approach to better understand the relationship between bladder and rectum volume would be to identify patients with larger rectums at planning scan for micro enemas to assess if this improves bladder emptying. From the data shown in this study, rectal volumes do not correlate consistently with bladder volumes in these patients.
