This chapter studies a robust counterpart with interval range uncertainty for LQ dose-response parameters in the fractionation problem with two modalities. As in previous chapters, the robust formulation is inevitably infinite-dimensional. The chapter describes its equivalent reformulation that calls for solving a finite set of subproblems. Each of these subproblems can be solved using the two analytical solution methods described in the previous chapter. Clinical insights into the resulting treatment plans are derived via numerical experiments.