Effect of bladder filling on doses to prostate and organs at risk: a treatment planning study

Abstract

The objective of this study was to evaluate effects of bladder filling on dose-volume distributions for bladder, rectum, planning target volume (PTV) and prostate in radiation therapy of prostate cancer. Twenty one patients were first scanned with full bladder, and second with empty bladder having been allowed one hour to void. Radiotherapy plans were generated using a four field box technique and dose of 70Gy/35fx. Doses to the prostate (equivalent uniform dose - EUD), bladder and rectum (effective dose - Deff) and normal tissue complication probability (NTCP) were compared. This comparison was performed for empty vs. full bladder scans and also for MLC patterns applied in reverse, i.e., empty bladder pattern to full bladder scan and vice versa. Dose to small bowels was examined. Mean full bladder volume was 354.3cc and empty bladder volume was 118.2cc. Median prostate EUD was 70Gy for both the full and empty bladder scans. The median rectal Deff for the full bladder scans was 55.6Gy for the full vs. 56.8Gy for the empty bladder scans; while the bladder Deff was 29.0Gy for the full vs. 49.3Gy for the empty bladder scans. One patient had part of small bowel receiving >50Gy with a full bladder (7.5cc), and 6 with an empty bladder (2.5cc- 30cc). Bladder filling had no significant impact on the prostate EUD or the rectal Deff. A minimal volume of small bowels received >50Gy in both groups which is below dose tolerance. The bladder Deff was higher with an empty bladder, however, the predicted complication rates were clinically insignificant. When MLC pattern was applied in reverse, substantial PTV underdosing was observed, particularly for patients with prostate shifts in excess of 0.5cm in one of directions. However, prostate shifts did not correlate with bladder filling, therefore this PTV underdosing cannot be related to bladder filling either. For some patients, bladder dose-volume constraints were not fulfilled in the worst case scenario, i.e., when a patient planned with full bladder arrives for treatment with empty bladder throughout the treatment.