Background and purpose: This study compares two different strategies for maintaining a constant bladder volume during a course of postoperative radiotherapy in prostate cancer. In addition, we studied how changes in bladder filling affect the clinical target volume (CTV) and the coverage hereof.
Material and methods: Twenty-nine patients with PSA-relapse after radical prostatectomy were divided into two groups: voiding and drinking 300 ml 1 hour before treatment (Group 1); and maintained a comfortably filled bladder (Group 2). The bladder volumes were calculated based on the planning CT (pCT) and a weekly Cone Beam CT (CBCT) during the treatment period. Furthermore, the variability of bladder extension was analyzed and correlated to the volume of the bladder covered with the 95% of the dose (V95%,bladder).
Results: The estimated median bladder volumes were 120 ml (95% CI: (93, 154)) and 123 ml (95% CI: (98, 155)) in groups 1 and 2, respectively. The intra-individual variation in bladder volume, assessed as the standard deviation, was 64 ml (95% CI: (46, 105)) in Group 1 and 61 (95% CI: (45, 94)) ml in Group 2. Increasing the bladder volume extended the bladder cranially while the caudal extension was almost constant. The correlation between bladder volume and V95%,bladder was 3.5 ml per 100 ml in group 1 and 1.2 ml per 100 ml in group 2 with no significant difference.
Conclusions: The intention to maintain a constant volume for the bladder is not fulfilled with either of the protocols in this study, and changes in bladder volumes does not seem to affect the position, or the coverage of the CTV.