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Background and purpose: Robustness evaluation (RE) is vital for proton treatment planning, but lacks international consensus or guidelines, with clinics using varied, self-developed methods focused on selected uncertainties. This ESTRO project surveys expert opinions on clinical RE methods to inform future treatment planning system (TPS) development.

Materials and methods: A study within the European Particle Therapy Network (EPTN) involved 24 European proton therapy centres, with one radiation oncologist and one medical physicist per centre. The goal was to reach a consensus on transitioning from Planning Target Volume (PTV)-based planning to robustly optimized planning, including uncertainties, methods, and reporting of robustness evaluations. An internal committee drafted 39 statements, reviewed by an independent committee. Following a two-round Delphi procedure, consensus was set at a 75% agreement threshold.

Results: Twenty of 24 contacted centers (83.0%) responded to both questionnaire rounds. Consensus was reached on 26 of 39 statements (66.7%), with 5 being high-priority. Strong agreement emerged regarding which uncertainties to include in RE (range, setup, intra-fraction, anatomy changes), methodologies (e.g., for moving targets, combining setup and range), and how to report RE results clinically. Disagreement was found on using the PTV for both planning and dose reporting. The results also offer important implications for TPS vendors and future software development.

Conclusions: The ESTRO Delphi consensus may serve as practical guidance on points where a clear consensus was achieved. For remaining points, the development of guidelines is recommended to standardize methodologies and reporting. Furthermore, TPS vendors are encouraged to align their developments with the community's articulated requirements.