Feasibility of Satisfying New Dose Volume Constraints of Bladder and Rectum during Treatment Planning of Prostate Cyberknife SBRT
Abstract
Purpose
To examine whether new recently reported dose volume constraints of bladder and rectum can be satisfied in clinical prostate CyberKnife SBRTs without sacrificing target coverage. Also, to use a normal tissue complication (NTCP) model to determine the expected outcome in the clinical and re-optimized treatment plans.
Methods
74 consecutive patients, who underwent prostate CyberKnife SBRT receiving 36-38 Gy in 4 fractions were used to determine dose volume constraints for bladder and rectum. 5 of those patients who significantly violated those constraints were selected for re-optimization. For bladder, V10 < 80% and V20 < 30% were used and for rectum, V12 < 80% and V8 < 90%. The NTCP model parameters for frequency (TD50=21.8Gy, m=0.25 and n=1.0) and rectal urgency (TD50=21.5Gy, m=0.36 and n=1.0) were used to estimate the clinical outcome for the different plans.
Results
The values of bladder V10 and V20 of the clinical plans were 93.7% (90.0-99.4) and 45.8% (37.9-72.0), whereas those of rectum V12 and V8 were 83.8% (77.6-95.7) and 97.6% (92.8-99.9), respectively. The corresponding values of the re-optimized plans were 60.6% (50.5-78.9) and 24.4% (22.3-25.2) for bladder and 45.1% (31.9-58.9) and 65.8% (49.3-82.0) for rectum, respectively. The NTCP values of bladder for the symptom of frequency were 40.8% (29.9-66.8) for the clinical plans and 10.4% (4.9-19.1) for the re-optimized plans. Similarly, the NTCP values of rectum for the symptom of urgency were 30.3% (24.5-38.4) for the clinical plans and 13.9% (7.3-21.0) for the re-optimized plans.
Conclusion
The new dose volume constraints could be satisfied in all the cases examined without affecting target coverage. The NTCP analysis indicated a significant reduction in the risk for urinary and rectal symptoms. These dose volume constraints were found to be feasible, and they may help in reducing the rates of urinary and rectal complications in patients receiving prostate SBRT.