Dosimetric benefits of robust treatment planning for intensity modulated proton therapy for base-of-skull cancers

Purpose: The clinical advantage of intensity modulated proton therapy (IMPT) may be diminished by range and patient setup uncertainties. We evaluated the effectiveness of robust optimization that incorporates uncertainties into the treatment planning optimization algorithm for treatment of base of skull cancers. Methods and materials: We compared 2 IMPT planning methods for 10 patients with base of skull chordomas and chondrosarcomas: (1) conventional optimization, in which uncertainties are dealt with by creating a planning target volume (PTV); and (2) robust optimization, in which uncertainties are dealt with by optimizing individual spot weights without a PTV. We calculated root-mean-square deviation doses (RMSDs) for every voxel to generate RMSD volume histograms (RVHs). The area under the RVH curve was used for relative comparison of the 2 methods' plan robustness. Potential benefits of robust planning, in terms of target dose coverage and homogeneity and sparing of organs at risk (OARs) were evaluated using established clinical metrics. Then the plan evaluation metrics were averaged and compared with 2-sided paired t tests. The impact of tumor volume on the effectiveness of robust optimization was also analyzed. Results: Relative to conventionally optimized plans, robustly optimized plans were less sensitive for both targets and OARs. In the nominal scenario, robust and conventional optimization resulted in similar D_95% doses (D_95% clinical target volume [CTV]: 63.3 and 64.8 Gy relative biologic effectiveness [RBE]), P <.01]) and D_5%-D_95% (D_5%-D_95% CTV: 8.0 and 7.1 Gy[RBE], [P <.01); irradiation of OARs was less with robust optimization (brainstem V₆₀: 0.076 vs 0.26 cm³ [P <.01], left temporal lobe V₇₀: 0.22 vs 0.41 cm³, [P = .068], right temporal lobe V₇₀: 0.016 vs 0.11 cm³, [P = .096], left cochlea D_mean: 28.1 vs 30.1 Gy[RBE], [P = .023], right cochlea D_mean: 23.7 vs 25.2 Gy[RBE], [P = .059]). Results in the worst-case scenario were analogous. Conclusions: Robust optimization is effective for creating clinically feasible IMPT plans for tumors of the base of skull.