Superiority in robustness of multi-field optimization over single-field optimization for pencil-beam proton therapy for oropharynx carcinoma: an enhanced robustness analysis

Purpose: To compare the difference in robustness of single-field (SFO) and robust multi-field optimized (rMFO) proton plans for oropharynx carcinoma patients by improved robustness analysis.
Methods: rMFO proton plans were generated for 11 oropharynx patients treated with SFO intensity modulated proton therapy (IMPT) with simultaneous integrated boost prescription. Doses from both planning approaches are compared for the initial plans and the worst cases from 20 optimization scenarios of setup errors (SE) and range uncertainties (RU). Expected average dose distributions per RU are obtained by adding the contributions from the respective scenarios with a weight according to their expected SE probability, and thus allowing quantification of the RU-related spread of dose parameters. Boundary dose distributions created from 56 combined SE and RU scenarios and considering the vanishing influence of SE after 30 fractions are used to approximate realistic worst case values for the total treatment course. Error bar distributions are derived from these boundary doses and error bar metrics are reported for the clinical target volumes (CTV) and organs at risk (OAR).
Results: rMFO-plans show improved CTV coverage and homogeneity while simultaneously reducing the average mean dose to the constrictor muscles, larynx and ipsilateral middle ear by 5.6Gy(RBE), 2.0Gy(RBE) and 3.9Gy(RBE), respectively. This is also observed by the different robustness evaluation methods, where additionally the average maximum brainstem and mean ipsilateral parotid dose significantly lower. For rMFO-plans, the RU-related spread in OAR dose parameters is smaller and many error bar metrics are found to be superior. SFO-plans show lower global maximum dose for single-scenario worst cases and slightly lower mean oral cavity dose throughout.
Conclusion: The benefit of better CTV coverage and OAR dose sparing by rMFO compared to SFO proton plans is preserved under considerations of SE and RU.