Purpose: To compare the dosimetric performances of small-spot 3D and 4D robustly optimized intensity-modulated proton (IMPT) plans in the presence of uncertainties and interplay effect simultaneously for distal esophageal carcinoma.
Methods: Thirteen (13) patients were selected and re-planned with small-spot (σ ~2-6mm) 3D and 4D robust optimization in IMPT respectively. The internal clinical target volumes (CTVhigh, CTVlow) were used in 3D robust optimization. Different CTVs (CTVhigh4d, CTVlow4d) were generated by subtracting an inner margin of the motion amplitudes in 3 cardinal directions from the internal CTVs and used in 4D robust optimization. All patients were prescribed the same dose to CTVs (50 Gy[RBE] for CTVhigh/CTVhigh4D and 45 Gy[RBE] for CTVlow/CTVlow4d). Dose-volume-histogram (DVH) indices were calculated to assess plan quality. Comprehensive plan robustness evaluations that consisted of 300 perturbed scenarios (10 different motion patterns to consider irregular motion and 30 different uncertainties scenarios combined, both were randomly sampled from the corresponding Gaussian distributions) were done to quantify robustness to uncertainties and interplay effect simultaneously. Wilcoxon signed-rank test was used for statistical analysis.
Results: Compared to 3D robustly optimized plans, 4D robustly optimized plans had statistically improved target coverage and better sparing of lungs and heart (Dmean: P = 0.001; V30Gy[RBE]: P = 0.001). 4D robustly optimized plans had better heart protection (Dmean: P = 0.010; V30Gy[RBE]: P = 0.017; V40Gy[RBE]: P = 0.010) when uncertainties and interplay effect were considered.
Conclusion: Even with small spots in IMPT, 4D robust optimization outperformed 3D robust optimization in terms of normal tissues protection and robustness to uncertainties and interplay effect simultaneously. Our findings support the use of 4D robust optimization to treat distal esophageal carcinoma with small spots in IMPT.
Funding Support, Disclosures, and Conflict of Interest: Funding: National Cancer Institute (NCI) Career Developmental Award K25CA168984, Arizona Biomedical Research Commission Investigator Award, the Lawrence W. and Marilyn W. Matteson Fund for Cancer Research, the Kemper Marley Foundation. Dr. Sio, is a member of the Advisory Board and speaker for Novocure, Inc, but is unrelated to this abstract.