SU‐E‐P‐42: Benefit of Equivalent Uniform Dose in Prostate IMRT Planning to Reduce Bladder Toxicity
Purpose: To assess the benefit of bladder wall sub-volume equivalent uniform dose (EUD) constrains in prostate cancer IMRT planning Methods: The bladder wall was defined by the volume between the external manually delineated wall and a contraction of 7 mm from this external wall. This bladder wall was then separated into two parts: the internal-bladder wall (bla-in) represented by the portion of the bladder wall that intersected with the planning target volume (PTV) plus 5 mm extension; the external-bladder wall (bla-ex) represented by the remaining part of the bladder wall. Two IMRT plans, with and without using EUD constraints, were generated and compared for 53 prostate cancer patients, to deliver 80Gy to the prostate PTV (V95>95%). In the plan with EUD constraints, the values of the “a” parameter of the EUD models were: 10.0 for bla-in and 2.3 for bla-ex., and 5 for the rectum. Results: The use of bladder EUD objectives increased significantly the conformal index (0.73±0.04 vs. 0.93±0.02) and decreased both the doses in the bladder wall (V70: 22.66% vs. 18.88%, Dmean: 39.40Gy vs. 35.04Gy) and the bladder wall NTCP values (NTCP at 3 years bladder toxicity >= LENT/SOMA Grade2: 16.68% vs. 14.48%, NTCP at 3 years bladder bleeding: 7.16% vs. 5.88%, NTCP at 5 years bladder toxicity >= LENT/SOMA Grade2: 20.15% vs. 17.75%, NTCP at 5 years bladder bleeding: 10.38% vs. 8.36%). The use of bladder EUD objectives, although slightly decreasing the dose to the rectum wall (Dmax: 75.19Gy vs. 75.05Gy, V72: 12.95Gy vs. 12.72Gy) as well, increased however the dose in the femoral heads (V55-femoral head left: 0.06% vs. 0.30%, V55-femoral head right: 0.04% vs. 0.21%). Conclusion: Separating bladder wall into two parts with appropriate bladder EUD objectives may reduce bladder toxicity, while keeping high dose to the prostate.