A robustness optimization is disclosed for a broad beam proton therapy plan. A nominal dose distribution (62) is computed, to be delivered to a volume by performing proton therapy on the volume according to a proton therapy plan (50) having parameters (52) defining a range compensator shape and a range band. The parameters of the proton therapy plan are adjusted to reduce a difference between the nominal dose distribution (62) and a perturbed dose distribution calculated to be delivered to the volume modified by an error scenario (64) by performing proton therapy on the volume modified by the error scenario in accordance with the proton therapy plan with the parameters adjusted by the adjusting operation. The adjusting may be repeated serially for each error scenario of a set of error scenarios (44) to produce a proton therapy plan that is robust for any of these error scenarios.