Purpose: Manual HDR prostate planning can be time-consuming and user-dependent. Anatomy-based inverse optimization is an effective way to achieve a HDR prostate plan with desirable dose distribution. This study evaluated the optimum planning ring structure for HDR prostate using BrachyVision.
Methods: Five patients treated with single fraction HDR prostate brachytherapy were selected. Five HDR plans were created for each patient by varying the planning ring structure during volume optimization. The planning ring structures were 4 cm wide with 0, 1, 2, 3, and 4 mm gap from the high-risk CTV, respectively. Other planning parameters were kept constant, including the step size, max dwell time, source dwell positions, and dose constraints. Each plan was evaluated for target coverage (V100, V125, V150), urethra, rectum, bladder sparing (D0.1cc), and conformity index (CI=ratio of prescription isodose volume to target volume receiving prescription dose). One-way ANOVA tests were performed.
Results: All plans achieved over 97% target coverage. V100 significantly increased with the planning ring structures with increasing gap from the CTV (p<0.001). The mean V100 values were 97.2%, 98.0%, 98.3%, 98.5% and 98.6% for ring structures with 0, 1, 2, 3, and 4 mm gap. No significant differences were found in V125, V150, D0.1cc rectum, D0.1cc bladder, and D0.1cc urethra. The CI significantly increased with planning ring structures with increasing gap from the CTV (p = 0.024), indicting less conformal plans. The mean CI were 1.32, 1.41, 1.46, 1.52, and 1.54 for ring structures with 0, 1, 2, 3, and 4 mm gap.
Conclusion: When using planning ring structures with increasing gap from the CTV, the target coverage significantly increased at the cost of losing conformity. Since the ring structure with 0 mm gap achieved the most conformal plans with target coverage over 97%, we recommend using it as a starting point for volume optimization.