3039 - An Improved Method for Optimization of the Overlap Region Between Planning Target Volume and Bladder for Knowledge-Based Planning of Prostate Stereotactic Body Radiation Therapy

Purpose/Objective(s): Knowledge-based planning (KBP) for prostate stereotactic body radiation therapy (SBRT) can be compromised when significant bladder-PTV overlap limits achievement of bladder dose constraints without sacrificing target coverage. Existing KBP models do not explicitly optimize this overlap region. We developed a novel overlap optimization structure (OOS) strategy and performed a feasibility study to determine whether OOS-guided reoptimization improves bladder sparing and plan robustness in anatomically challenging prostate SBRT cases.

Materials/Methods: Ten previously generated prostate SBRT plans with substantial bladder-PTV overlap (= 4.8 cc) that failed to meet ideal institutional bladder constraints were retrospectively reoptimized in Eclipse v16.1 using two OOS pseudo-structures: (1) the geometric intersection of bladder and PTV (bladder–PTV overlap) and (2) a 1.5-cm partial ring cropped 3 mm from the PTV and limited to axial planes containing the PTV. Prescription was 40 Gy in 5 fractions, with optional simultaneous integrated boost to 45 Gy to GTV. OOS-based plans were compared with original plans and with selected institutional ideal plans. Paired t-tests were used for statistical comparison.

Results: For the non-ideal plans, the mean (±SD) total PTV, bladder, and overlap volumes, were 89.0 ± 21.6 cc, 245.2 ± 150.1 cc, and 3.6 ± 1.2 cc, respectively. For equivalent PTV coverage (mean ?V100% = 1.2 cc), OOS-based reoptimization significantly reduced bladder Dmean (by up to 5.4%) and Dmax (49.5 ± 30.8 cGy, p < 0.001). High-dose bladder volumes (V40Gy, V38Gy, V35Gy) were significantly reduced (all p = 0.01), with additional reductions in intermediate-dose-metrics. Urethral and rectal dose-volume parameters were unchanged. Compared with institutional ideal plans, OOS plans achieved similar bladder Dmax and V38Gy, lower V40Gy (0.04 cc, p = 0.02), but had modestly higher intermediate-dose volumes (V35Gy 0.9 cc, p = 0.003, and V25Gy 3.6 cc, p < 0.001).

Conclusion: This proof-of-concept study demonstrates that explicit optimization of the bladder-PTV overlap region using OOS improves bladder sparing in high-overlap prostate SBRT without compromising target coverage or other organs at risk. All the reoptimized plans achieved better bladder dose-volume constraints when using OOS. Incorporation of OOS enhances KBP robustness in anatomically unfavorable cases and may reduce planner-dependent variability, supporting broader clinical implementation.