Impact of additional PRV constraint to Direct OAR robust optimization in IMPT for targets near serial organs at risk

In intensity-modulated proton therapy (IMPT), the benefit of adding planning organ-at-risk volume (PRV)-like nominal constraints to direct robust optimization for serial organs at risk remains uncertain when target-OAR separation is minimal. Using paraspinal chordoma as a model, we evaluated whether an additional nominal canal/thecal sac (PRV-like) constraint improves OAR sparing or compromises target coverage.

Ten patients with paraspinal chordomas were planned using two IMPT strategies: direct cord robust optimization alone (Cord-RO) and direct cord optimization with additional nominal canal/thecal sac constraint (Canal-RO). PTV-based helical tomotherapy (HT) was generated as a secondary benchmark. Prespecified institutional criteria were used for planning, prioritizing OAR constraints over coverage; when standard coverage goals were not achieved, GTV D98 ≥ 59 Gy(RBE) was accepted as the fallback criterion. Endpoints included target coverage, cord/canal doses, and robustness. Statistical comparisons used Wilcoxon signed-rank, Friedman, and Cochran's Q tests, two-sided, p < 0.05.

All plans met plan-specific OAR constraints. Cord-RO achieved superior target coverage (median HR-CTV D98 [Gy(RBE)]: 63.05 vs. 57.38 vs. 59.46; p ≤ 0.002) and met the fallback objective (GTV D98 ≥59 Gy[RBE]) in 10/10 cases versus 4/10 and 7/10 (p=0.011) in Canal-RO and HT respectively. Robustness favored Cord-RO (median worst-case CTV D95: 90% vs 85%, p=0.018). Nominal cord D0.03cc was similar; worst-case cord D0.03cc was slightly lower with Canal-RO.

When targets abut serial OARs, our results suggest that direct OAR-based robust optimization without additional PRV-like constraints improve target coverage and robustness while maintaining acceptable OAR doses. PRV constraints may be reserved for re-irradiation or when target-OAR separation is adequate.