KEY POINTS
- This dosimetric comparative planning study included 10 patients with mobile-spine paraspinal chordoma; in 8 cases, the target directly abutted the spinal cord or thecal sac.
- Three strategies were compared: direct cord robust optimization intensity-modulated proton therapy, the same approach plus an additional nominal canal/thecal sac constraint, and planning-target-volume-based helical tomotherapy as a photon benchmark.
- The prescription was 70.2 Gy(RBE) in 32 fractions to the high-risk clinical target volume. When standard coverage was not achievable, gross tumor volume D98 ≥59 Gy(RBE) was the prespecified fallback objective.
- Direct cord robust optimization achieved higher target coverage: median high-risk clinical target volume D98 was 63.05 Gy(RBE) versus 57.38 Gy(RBE) with the added canal constraint and 59.46 Gy(RBE) with helical tomotherapy (P = .001).
- Robustness also favored direct cord robust optimization, with median worst-case clinical target volume D95 of 90% versus 85% (P = .018). Worst-case spinal cord D0.03cc was slightly lower with the added canal constraint (51.69 versus 54.12 Gy(RBE)), but both remained within the prespecified <55 Gy(RBE) robust constraint.
CLINICAL TAKEAWAY
When paraspinal chordoma targets directly abut the spinal cord or thecal sac, adding a nominal planning-organ-at-risk-volume-like canal constraint may compromise intensity-modulated proton therapy coverage and robustness more than it improves spinal cord sparing. This supports direct organ-at-risk robust optimization as a planning strategy in close-abutment scenarios, but the evidence is dosimetric, single-institution, and limited to 10 cases without toxicity or local-control outcomes.