Challenges and opportunities to minimize the dose in the neurovascular bundles during prostate radiotherapy.

Objective: Radiation damage to the neurovascular bundles (NVB) has been linked to erectile dysfunction after prostate cancer radiotherapy (PCa). NVB sparing using coplanar and non-coplanar automated treatment planning is presented here in two settings: (1) without compromising target coverage, (2) allowing target coverage compromise.

Methods: 20 previously treated patients with localized PCa. Based on a MRI-CT match, the NVB were retrospectively delineated. All treatment plans (5 × 7.25 Gy) were automatically generated using Erasmus-iCycle (in-house automated treatment planning algorithm). Non-NVB sparing (non-NVBsparing) plans and NVB sparing plans in two settings were generated: (1) uncompromised NVB sparing (u-NVBsparing; maintaining target coverage) (2) and compromised NVB sparing (c-NVBsparing; allowing target underdosage). Coplanar and non-coplanar beam arrangements were compared. U-NVBsparing was compared to non-NVBsparing. C-NVBsparing plans were visualized in Pareto fronts. Statistical significance (p-value < 0.05) was determined by Wilcoxon signed-rank test.

Results: u-NVBsparing compared to non-NVBsparing plans showed statistically significant median reductions in NVB D0.1 cc (38.9 vs 42.6 Gy for coplanar; 38.9 vs 43.3 Gy for non-coplanar) and Dmean (25.6 vs 30.0 Gy for coplanar; 24.7 vs 30.2 Gy for noncoplanar). Further lowering NVB D0.1 cc in c-NVBsparing plans clearly correlated to lower target coverage. Non-coplanar c-NVBsparing plans maintained significantly higher target coverages for similar NVB D0.1 cc values, compared to coplanar plans.

Conclusions: NVB sparing without compromising target coverage is feasible. No clinically relevant benefit was found for non-coplanar compared to coplanar NVB sparing plans, although overall statistically superior. Further sparing of the NVB comes at the cost of target coverage, for which a Pareto front could be used as a tool in clinical practise.